Atypical Hemolytic Uremic Syndrome (aHUS): Making the Diagnosis

Jeffrey Laurence, MD

Volume 10, Issue 10, Supplement 17 October 2012

Atypical Hemolytic Uremic Syndrome (aHUS): Making the Diagnosis

Plus: A Review of Case Studies

Jeffrey Laurence, MD

Professor of Medicine
Division of Hematology and Medical Oncology New York Presbyterian Hospital and
Weill Cornell Medical College
New York, New York


Atypical Hemolytic Uremic Syndrome (aHUS): Making the Diagnosis

Jeffrey Laurence, MD

Professor of Medicine
Division of Hematology and Medical Oncology
New York Presbyterian Hospital and Weill Cornell Medical College New York, New York

Abstract: Atypical hemolytic uremic syndrome (aHUS) is a major thrombotic microangiopathy (TMA). A TMA is recognized by the laboratory signs of microangiopathic hemolysis, as indicated by schistocytes, elevated lactate dehydrogenase, low haptoglobin, and low hemoglobin, plus thrombocytopenia and accompanying signs and symptoms of organ system involve- ment. aHUS results from chronic, uncontrolled activity of the alternative complement pathway. In most patients, this defect is related to a genetic deficiency in one or more soluble and/or membrane-bound complement regulatory proteins. Complement factor H is most frequently implicated. Clinically, aHUS is often indistinguishable from the other TMAs: Shiga toxin–producing Escherichia coli (STEC) hemolytic uremic syndrome and thrombotic thrombocytopenic purpura (TTP). TTP and aHUS are associated with high morbidity and mortality. aHUS has a distinct pathology from TTP. In nearly all patients, aHUS can be distinguished from TTP on the basis of an ADAMTS13 (a disintegrin and metalloproteinase with a thrombo- spondin type 1 motif, member 13) enzyme activity measurement. It is essential that aHUS and TTP be differentiated quickly, as they require markedly divergent treatments. The standard treatment for TTP is plasma exchange, a therapy that has no role for patients with a diagnosis of aHUS established by ADAMTS13 activity levels.


Since the initial descriptions of the 2 principle categories of systemic micro- vascular clotting disorders—throm- botic thrombocytopenic purpura (TTP) by Moschcowitz1 in 1924 and hemolytic uremic syndrome (HUS) by Gasser in 19552—there have been but a handful of critical diagnostic and therapeutic breakthroughs. Each of these key discoveries, however,

has dramatically altered the course of TTP and HUS. First, there was identification of the utility of plasma exchange (PE) in the treatment of TTP. This was followed by isolation of Shiga toxin–producing Escherichia coli (STEC) as the etiologic agent of many cases of diarrhea-associated [D+] HUS, now known as STEC- HUS. More recently, the means to distinguish between TTP and “atypical” (a)-HUS, using assays for a

specific protease activity, was reported (the latter formerly grouped into a heterogeneous syndrome referred to as non–diarrhea-associated [D-] HUS). Finally, genetic defects in regulation of the alternative comple- ment pathway have been uncovered in aHUS, leading to an effective treat- ment for that disorder. This review focuses on aHUS, a rare, chronic, life-threatening, systemic disease. Frequently unrecognized, it has a high


degree of morbidity and mortality within the first year of presentation unless appropriately treated.

The three conditions noted above—TTP, STEC-HUS, and aHUS—are collectively referred to as the major thrombotic microangiopa- thies (TMAs). Clinically, they are often indistinguishable.3 Indeed, the distinc- tion between TTP and HUS had been vague from the outset: Gasser included a case of “Moschowitz’s disease” in his HUS series, and HUS was originally referred to as “TTP of children.”4

The fact that the TMAs are rare is a further impediment to an accurate diagnosis. Yet the high morbidity and mortality associated with TTP and aHUS, and the markedly divergent treatments they require, mandate swift recognition of a TMA, as well as the ability to distinguish the two con- ditions. This review highlights the dif- ferences in pathophysiologic and diag- nostic criteria among TMA subtypes. An article appearing on pages 9–11 of this supplement examines case stud- ies to elucidate important diagnostic issues in patients with aHUS.

Incidence of the Major Thrombotic Microangiopathies

The annual incidence of STEC-HUS is about 2 per 100,000 in adults and 6.1 per 100,000 in children younger than 5 years.5 The incidence of aHUS is thought to be much lower, about 2 per million for adults and 3.3 per million in children younger than 18 years.6 The latter aHUS figures are similar to those recorded for TTP in the general US population. But the reported fre- quency of the TMAs has varied greatly over the past 3 decades, and the num- ber of adults diagnosed and treated for TTP has risen seven-fold during that period.7 Why? According to one review, documentation of the efficacy of PE for TTP, leading to a decline in mortality from more than 90% to less than 10%,8 “created an urgency for diagnosis which has resulted in

decreased stringency of diagnostic criteria.”3 In turn, that led to the appli- cation of prolonged plasma-infusion based therapies for aHUS, and TMAs erroneously bundled as “TTP/HUS,” despite the lack of any controlled trial indicating that such therapy was effec- tive in influencing disease progression or mortality in aHUS.

Distinguishing Among the TMAs: Primary Considerations

Development of both TTP and aHUS appears to require two conditions: (1) pre-existing susceptibility factors, which may be familial (i.e., genetic) or acquired, and are capable of promot- ing endothelial cell activation, platelet aggregation, or both; and (2) modu- lating factors, encompassing a variety of conditions that can be infectious, inflammatory, or related to pregnancy, stress, or drugs, and are linked epidemi- ologically to both TTP and aHUS. The latter would account for the sporadic development of overt clinical signs and symptoms of disease. For example, the first clinical manifestation of familial TTP (Upshaw-Schulman syndrome) may not occur until late in life, and relapses are infrequent. Similarly, the first overt signs of aHUS may not be recognized until adulthood, despite the fact that the genetic predisposition is present at birth. In reality, however, in between the often dramatic episodes of clinically apparent aHUS which are often initiated by heightened activation of the alternative complement pathway by modulating factors, tissue damage continues to occur. This is evidenced by persistent platelet activation and renal injury, as discussed below.

The vast majority of cases of TTP are idiopathic, and disease susceptibil- ity usually results from an autoanti- body-mediated deficiency of the von Willebrand factor (vWF) cleaving protease ADAMTS13 (a disintegrin and metalloproteinase with a throm- bospondin type 1 motif, member 13). This leads to propagation of platelet

aggregates, related to the inability to cleave long tethers of platelets bound to vWF in ultra-high molecular weight multimers, an activity which requires an intact enzyme. The subsequent uncontrolled microthrombus forma- tion is clinically devastating.9 This acquired condition parallels the sus- ceptibility of individuals with loss of function mutations in ADAMTS13 to development of familial TTP.10

By contrast, in the vast majority of aHUS cases, susceptibility factors are familial, not acquired. They are genetic defects in complement and comple- ment regulatory proteins or related factors which, as discussed below, per- mit uncontrolled amplification of the alternative complement pathway. This potentiates platelet activation, platelet aggregation, and complement-mediated endothelial cell injury throughout the microvasculature.5,11,12 (These processes may also play a role in some cases of STEC-HUS13 and in TMAs occurring in the setting of pregnancy, organ and tissue transplantation, autoimmune disease, and malignancy.14 Those topics are outside the range of this review.)

Given these pathophysiologic dif- ferences between aHUS and TTP, one might think that diagnostic criteria for the TMAs would be relatively simple to apply. Often they are, and this is critical clinically, as it will guide treat- ment decisions. Just as the mortality from TTP declined from more than 90% to less than 10% with institution of appropriate treatment—plasma exchange8—outcome is highly unfa- vorable in aHUS treated with PE. Up to 50% of patients progress to end- stage renal failure within a year, and 25% die during the acute phase.15

Four standard initial steps can be used to reach a specific TMA diagnosis:

1. As outlined in Figure 1, a TMA must first be recognized. This involves examining the principle laboratory criteria for microangiopathic hemo- lysis: schistocytes on peripheral blood smear, low haptoglobin levels, and a

decline in baseline hemoglobin. These changes are usually accompanied by thrombocytopenia, although it should be recognized that while severe decreases in platelet counts are typical of TTP, TMAs such as aHUS, charac- terized by ADAMTS13 levels >5–10% (depending upon the assay used), have significantly higher platelet counts,16 which can be near normal at first presentation. Schistocytes are the sine qua non of a TMA, although they may be infrequent on initial presentation

(fewer than 1 per high-power micro- scopic field). An elevated level of lac- tate dehydrogenase (LDH)—virtually always higher than 600 IU/L—is also characteristic of a TMA. But though an elevated LDH is often attributed to ongoing hemolysis, LDH levels may be far out of proportion to the degree of red cell destruction evinced by changes in indirect bilirubin and hemoglobin. LDH isoenzyme analysis has shown that a substantial portion of LDH elevation in a TMA is instead due to

its release from tissues damaged as a result of microthrombosis-associated systemic ischemia.17

2. These laboratory changes must be accompanied by involvement of at least one organ system. Figure 1 lists the three most common sites: neurologic, renal, and gastrointestinal. But both aHUS and TTP can affect any tissue, as microvessel thromboses lead to tissue ischemia and infarction. In some 20% of initial aHUS presentations in children and adults,

renal function is preserved, despite the word “uremic” in the name of the dis- ease.18 These patients may have some proteinuria and hematuria, but their serum creatinine is in the normal range.19 Arterial hypertension is also frequent and often severe in later stages of aHUS, due to volume overload in cases of oliguria and to hyperreninemia secondary to renal thrombotic microangiopathy.18 Proteinuria and hematuria are also common in classic TTP, although acute renal failure at presentation is unusual, affecting less than 5% of patients (4 of 115 individuals with TMA and severe ADAMTS13 deficiency followed in four different studies).20 The one possible exception to the universality of tissue involvement in the TMAs is the lung, which is virtually never directly involved in TTP, while pulmonary pathology is frequent in untreated aHUS.21

3. aHUS and TTP can be distinguished from STEC-HUS by PCR, or culture- based assays for the Shiga-toxin produc- ing E. coli, using stool or a rectal swab. Gastrointestinal signs and symptoms cannot be relied upon to distinguish among the TMAs, however. Prodromic diarrhea, particularly bloody diarrhea, had been considered a classic sign of a STEC-HUS, but up to one-third of aHUS cases involve diarrhea, which can be bloody, and it is also not uncommon in TTP.6 Because of this, the terms D (diarrhea)+ HUS and D- HUS should no longer be used. Indeed, diarrhea—and the infectious pathogens responsible for it—is one of the most potent activators of the alternative complement pathway. It may itself unmask overt clinical mani- festations of aHUS, as breaching of intes- tinal epithelial barriers leads to microbial translocation and a positive feedback loop for complement activation.

4. Finally, in the majority of cases, TTP typically responsive to PE can be distin- guished from aHUS, for which plasma therapy does not have a role, on the basis of ADAMTS13 activity levels. The utility of ADAMTS13 testing is detailed

below. In most settings, it takes from 48 hours to 1 week to obtain results from the tests, which are generally performed by only specialty laboratories.

Distinguishing Between aHUS and TTP: Additional Considerations

A new patient presenting with labora- tory and clinical signs of a TMA, as recognized by fulfilling the criteria in the first two rows of the algorithm of Figure 1, is usually begun on plasma therapy. Plasma exchange rather than plasma infusion is the initial standard of care for an undifferentiated TMA.8 If an apheresis station is not immediately available, and renal function permits, fresh frozen plasma infusions may instead be initiated, awaiting eventual pheresis. In cases involving diarrhea, the possibility of STEC-HUS should be ruled out, which takes less than a day.

PE is continued pending ADAMTS13 activity results. Blood for such testing must be drawn prior to initiating plasma therapy. Based on those results, there are three possibili- ties; two are straight-forward and one is more complex:

1. If the ADAMTS13 activity is less than 5% of normal control levels by the most commonly available fluorescence resonance energy transfer (FRET)- based assays (or <10% in some gel- based assays), the diagnosis is TTP.22 In this case, PE should be continued. In addition, acquired inhibitors of ADAMTS13, usually immunoglobu- lin G autoantibodies, are detectable in 80–90% of TTP patients with severe ADAMTS13 deficiency.22 If, after 1–3 treatments, each representing replace- ment of 1–1.5 plasma volumes, there are only minor responses in laboratory and clinical parameters—the meaning of “response” is discussed below— other treatment modalities shown to be effective in recalcitrant TTP, and involving various immune suppressive protocols, should be considered.

2. If the ADAMTS13 activity is >5–10%, depending on the assay used, the diagnosis is aHUS.22 PE should be stopped, and specific, complement- based treatment instituted. Comple- ment-based assays have been used by some groups in an attempt to confirm the diagnosis of aHUS, but they have no relationship to treatment response and, as discussed below, are often of little utility.

3. But what if an ADAMTS13 assay was not initially drawn, and the patient has now had multiple cycles of PE, rendering suspect any subsequent ADAMTS13 determination? This is an important consideration, as the major effect of PE in TTP is thought to be restoration of a functional ADAMTS13 enzyme and, after several cycles of PE, the patient will have exogenous enzyme, which repre- sents replenishment from donor plasma. Secondly, what if the treating physician believes that the diagnosis of TTP is firm, despite ADAMTS13 levels in the normal range, based on his or her inter- pretation of certain published studies? That is, in one large series, ADAMTS13 activity of less than 5% was seen in only 33% of patients with “idiopathic TTP.”9 A parallel study was a bit clearer, but it was not absolute: 29% of patients diag- nosed as having idiopathic or secondary TTP, responsive to PE, did not have an ADAMTS13 deficiency.23 This issue is confounded by the fact that it cannot always be established that ADAMTS13 levels were obtained prior to initiation of plasma therapy. In addition, misdiagno- sis may be common amongst individuals labeled as having “TTP” but who have an ADAMTS13 activity >10%, as we have discovered searching for comple- ment regulatory protein mutations in archived samples from such TMA cases only partially responsive to PE (unpub- lished observations). More problematic, and in apparent support of the conten- tion, by some, that diagnosis dependent upon an ADAMTS13 level alone may not be definitive, are instances in which the patient appears to be responding to

PE, at least in terms of some laboratory and/or clinical parameters. Where does one go from here?

An additional source of delay in diagnosis may be failure to consider that a variety of other disorders, par- ticularly autoimmune diseases such as systemic lupus erythematosus (SLE) and malignant hypertension, can pres- ent with signs and symptoms similar to aHUS or TTP. In turn, aHUS and TTP can co-exist with SLE and related conditions.19 Unless a primary TMA is considered, and appropriate diagnostic procedures undertaken, accurate treat- ment decisions will not be made.

These are critical considerations. In the majority of cases, delays in confirming a diagnosis of aHUS and institution of effective therapy result in the need for renal dialysis or renal transplantation, or in death within a year.15 Laboratory evidence for ongoing disease in aHUS despite intensive plasma therapy has been well documented, and includes progressive decline in the glomerular filtration rate and continued platelet activation.24

Defining a Response to Plasma Exchange in the Context of Distinguishing aHUS From TTP

As a general rule, response in the context of idiopathic TTP involves improvement or complete correction in the TMA parameters of Table 1.25 If a plasma taper or 2 plasma exchanges are completed after normalization of these measures, and the patient remains off PE for at least 1 month, a “complete response” has been achieved. TMA recurrence beyond a month is considered a relapse in TTP. Recurrence within 1 month is evidence of inadequate treatment.

Response may also be defined in terms of the amount of plasma required. The first randomized study of PE versus plasma infusion in TTP, defined clinically and without the benefit of ADAMTS13 testing, dem- onstrated the superiority of PE over

plasma infusion.8 Forty-seven percent of those receiving PE had a complete response after the first cycle, which involved an average of 21.5 ± 7.8 liters of fresh frozen plasma exchanged over 9 days. In an additional 31% of patients, 1 or 2 further cycles of PE were required to effect a complete response.8 This is similar to many later trials of PE, where remissions were obtained with a mean number of plasma exchanges of 19 ± 17 in one study,26 and a median of 9 exchanges (mean cumulative infused plasma of 43 ± 77 liters) in another.27

By contrast, upwards of 80% of patients with classic aHUS have partial responses to short-term PE that are limited to increases in platelet count and hemoglobin and a decline, but usually not a normalization, in LDH.5 Tissue damage persists, and mainte- nance of even those partial responses is dependent upon continued PE. Recall that aHUS is a chronic, genetic disease, leading to high morbidity and mortal- ity despite plasma therapy. Therefore, if a TMA patient experiences such a limited response to PE, or is requiring quantities of plasma exceeding those outlined above for TTP, it is prudent to re-evaluate the diagnosis.

Rethinking the Diagnosis: TTP Versus aHUS

Expanding upon the complement system in the setting of a TMA

Components of the complement system are required to mount an appropriate innate immune response to pathogens. The classic complement pathway is primarily activated follow-

ing formation of immunoglobulin- antigen immune complexes, while the alternative pathway is an amplifying system responsive to groups of mol- ecules present on a virus or bacterium, such as polysaccharide and endotoxin. Both can activate C3, converting it to C5, which is then broken down into C5a (anaphylatoxin) and C5b-9 (membrane attack complex or MAC). The importance of the alterna- tive pathway—and its relevance to aHUS—is that it is always “on,” at low levels, ready to be amplified by certain stressors such as infection, trauma, pregnancy, or surgery. But C5a and MAC can help destroy the membranes of pathogens as well as of normal cells.

To prevent the panoply of activi- ties related to unchecked activation of the alternative pathway—particularly endothelial cell activation and injury, platelet activation and aggregation, and inflammation—most mammalian cells have membrane-bound regulators that block complement activation. They function in concert with soluble complement regulators.18,28 Soluble factors H and I, and the membrane- bound proteins MCP (CD46) and CD55, regulate conversion of C3 to C5. Membrane-bound CD59 regu- lates C5b-9.

In aHUS, identifiable muta- tions—usually heterozygous—in one or more of these proteins, along with related molecules (CFHR1, CFHR3, C3, complement factor B, and throm- bomodulin, which can inactivate C3a and C5a) account for disease sus- ceptibility in up to 60% of cases.11,19 (Ten percent of aHUS cases may have an acquired component, linked to

development of anti-factor H autoan- tibodies leading to decreased factor H function.29 Even in these instances, a C mutation is often present.14) In the absence of these functional regulators, aHUS may develop.28 Reduction in expression of even 50% of normal levels in just one of these regulators appears to leave an individual vulnerable to overt clinical manifestations of aHUS at times of infection, injury, pregnancy, surgery, or other stressors. These details help explain why a complement-based strategy should be effective in treating aHUS. But this information is of far less help in the clinic in distinguishing aHUS from TTP.

Currently, complement and sol- uble complement regulatory protein levels in plasma or serum are unreli- able in the diagnosis of aHUS. A low C3 level accompanied by a normal C4 level would be consistent with selective alternative pathway activation, but this pattern is only occasionally observed— serum C3 is normal in up to 80% of aHUS patients—and is too inconsistent for diagnostic purposes.6,30 Comple- ment pathways can also be transiently activated in classic TTP, leading to ele- vated plasma levels of C3a and C5b-9.31

Similarly, as noted above, genetic mutations of soluble and membrane- linked C regulatory proteins have been documented in only about half of classic aHUS cases, and thus are an unreliable means of excluding an aHUS diagnosis.32 (It is hypothesized that more extensive, genome-wide sequenc- ing will eventually reveal complement- related abnormalities in most aHUS cases. Such analyses will be complicated by the fact that non-synonymous muta- tions involving amino acid substitu- tions in factor H—the most frequently involved protein in aHUS—are present in 5% of healthy controls, with no family history of TMA.33) In addition, such testing takes months, and is not generally available, making genetic test- ing unnecessary for initial assessment and management of aHUS. (A center at the University of Iowa provides such

services, but they are expensive. Genetic testing can be useful, however, in the counseling of family members of an affected proband, as carriers might be closely monitored during conditions triggering marked C activation, such as surgery, trauma, infection, malignancy, and pregnancy.6) Clearly there is room to improve diagnostic associations and testing for complement activation.

Examining tissue
Biopsies are rarely done in the TMAs, although they may serve as a guide in difficult diagnostic situations.34 Gin- gival tissue, skin, or bone marrow are suggested sites to sample, regardless of whether there is an apparent lesion.35 The thrombi of TTP are typically com- posed of platelets and vWF, with only small amounts of fibrin.22 Vascular or perivascular inflammatory cell infiltra- tions are minimal or absent,22 consis- tent with the fact that endothelial cell damage is apoptotic in nature, and such programmed cell death, as opposed to necrosis, typically lacks an inflamma- tory component.21 In contrast, biopsy of similar sites in aHUS typically reveals microthrombi in which fibrin domi- nates, and an inflammatory infiltrate may be seen.22,36 Deposits of terminal components of complement (C5a and C5b-9) may be seen in involved microvessels. It must be emphasized, however, that the sensitivity and speci- ficity of such biopsy results in distin- guishing TTP from aHUS, or from TMAs arising in the setting of other disorders such as SLE, particularly with reference to the potential for a response to plasma-based versus complement- based therapies, have not been authen- ticated in clinical trials. Kidney biopsy is rarely necessary in patients with renal manifestations of a TMA as little diagnostic or prognostic information is added to that derived from more basic laboratory tests.37 (Post-renal transplant syndromes are an exception, where it may be otherwise difficult to distin- guish antibody-mediated rejection from aHUS and other TMAs in the absence

of tissue.37) The use of tissue-based test- ing should not be done routinely, but rather on a case-by-case basis. Larger scale studies are needed to define and standardize such an approach.


• aHUS is a chronic, rare, life-threaten- ing, systemic disease. Its etiology is based upon unregulated activation of the alter- native complement system. Unrecog- nized and inappropriately treated, it has a high degree of morbidity and mortality within the first year of presentation.

• aHUS is one form of thrombotic microangiopathy (TMA). A TMA is recognized by the laboratory signs of microangiopathic hemolysis (schis- tocytes, elevated LDH, low hapto- globin), accompanied by signs and symptoms of organ system involve- ment. Thrombocytopenia is common, although it is much less severe than in TTP, another major TMA, and may be near normal at presentation.

• The clinical presentation of the 3 major TMAs—aHUS, STEC-HUS, and TTP—can be identical. For example, despite the term “uremic” in the disease name, approximately 20% of aHUS cases have preserved renal function at diagnosis, defined by a serum creatinine in the normal range. All of the TMAs can occur at any age. The presence or absence of diarrhea or neurologic symptoms does not reliably distinguish among these TMAs. Any patient presenting with diarrhea, however, should be evaluated for STEC-HUS.

• aHUS is distinct pathologically from TTP. aHUS results from chronic, uncontrolled activity of the alternative complement pathway. In the majority of cases, this defect is related to a genetic deficiency in one or more soluble and/ or membrane-bound complement regulatory proteins. Complement factor H is most frequently implicated.

• In the vast majority of cases, aHUS can be distinguished from TTP on the basis of an ADAMTS13 enzyme activity measurement. It is important to order this test, prior to initiating any plasma therapy, in a patient presenting with the laboratory and clinical signs of a TMA, even though the results may take several days to obtain.

• Assessment of complement levels or currently identifiable complement reg- ulatory protein mutations cannot be used to rule out a diagnosis of aHUS.

• Unlike TTP, plasma therapy has no role in the treatment of aHUS. Short- term, partial responses in hematologic parameters (platelet count, hemoglo- bin, and LDH) have been seen in up to 80% of cases of aHUS treated with PE. This is thought to be related to replace- ment of soluble complement regulatory proteins by plasma infusion. However, ongoing tissue damage persists.

• A specific, FDA-approved, com- plement-based therapy for aHUS is now available. Once an ADAMTS13 activity of >5–10% (depending on the assay used) has been documented in the setting of a TMA, and thus the diagnosis of aHUS has been made, plasma therapy should be discontinued and appropriate treatment instituted.

• If a putative TTP or other TMA patient is not responding to plasma therapy, according to generally accepted measures of hematologic, clinical, and organ system changes, or requires prolonged plasma exchange to effect and/or maintain a par- tial remission, it is necessary to re-evaluate the diagnosis and consider aHUS. If an ADAMTS13 level had not been obtained prior to institution of PE, consideration should be given to stopping plasma infusions and ordering this test, so that a definitive treatment might be applied.


This article was supported by funding from Alexion Pharmaceuticals, Inc. This

article reflects the opinions and views of the author and was developed indepen- dently of Alexion Pharmaceuticals, Inc. Dr. Laurence is a member of the Speakers’ Bureau of Alexion Pharmaceuticals, Inc.


1. Moschcowitz E. Hyaline thrombosis of the terminal arterioles and capillaries: a hitherto undescribed disease. Proc NY Pathol Soc. 1924;24:21-24.
2. Gasser VC, Gautier E, Steck A, Siebenmann RE, Oechslin R. Hamolytisch-uramische syndrom: bilater- ale nierenrindennekrosen bei akuten erworbenen hamolytischen anamien. Schweiz Med Wochenschr. 1955;85:905-909.

3. Roy V, Rizvi MA, Vesely SK, George JN. Thrombotic thrombocytopenic purpura-like syndromes following bone marrow transplantation: an analysis of associated conditions and clinical outcomes. Bone Marrow Trans- plantation. 2001;27:641-646.

4. Rossi EC. Plasma exchange in the thrombotic microangiopathies. In: Rossi EC, Simon TL, Moss GS, Gould SA, eds. Principles of Transfusion Medicine. 2nd ed. Baltimore, MD: Williams and Wilkins; 1996.

5. Noris M, Remuzzi G. Atypical hemolytic-uremic syndrome. N Engl J Med. 2009;361:1676-1687.
6. Ohanian M, Cable C, Halka K. Eculizumab safely reverses neurologic impairment and eliminates need for dialysis in severe atypical hemolytic uremic syndrome. Clin Pharmacol Adv Applications. 2011;3:5-12.

7. Clark WF, Rock GA, Buskard N, et al. Therapeutic plasma exchange: an update from the Canadian Apher- esis Group. Ann Int Med. 1999;131:453-462.
8. Rock GA, Shumak KH, Buskard NA, et al. Com- parison of plasma exchange with plasma infusion in the treatment of thrombotic thrombocytopenic purpura. N Engl J Med. 1991;325:393-397.

9. Vesely SR, George JN, Lammle B, et al. ADAMTS13 activity in thrombotic thrombocytopenic purpura- hemolytic uremic syndrome: relation to presenting features and clinical outcomes in a prospective cohort of 142 patients. Blood. 2003;102:60-68.

10. Levy GG, Nichols WC, Lian EC, et al. Mutations in a member of the ADAMTS gene family cause thrombotic thrombocytopenic purpura. Nature. 2001;413:488-494. 11. Skerka C, Jozsi M, Zipfel PF, Dragon-Durey M-A, Fremeaux-Bacchi V. Autoantibodies in hemolytic-uremic syndrome (HUS). Thromb Hemost. 2009;101:227-232. 12. Roumenina LT, Jablonski M, Hue C, et al. Hyper- functional C3 convertase leads to complement deposition on endothelial cells and contributes to atypical hemolytic uremic syndrome. Blood. 2009;114:2837-2845.

13. Lapeyraque A-L, Malina M, Fremeaux-Bacchi V, et al. Eculizumab in severe shiga-toxin-associated HUS. N Engl J Med. 2011;364:2561-2563.
14. Waters AM, Licht C. aHUS caused by complement dysregulation: new therapies on the horizon. Pediatr Nephrol. 2011;26:41-57.

15. Caprioli J, Noris M, Brioschi S, et al. Genetics of HUS: the impact of MCP, CF, and IF mutations on clinical presentation, response to treatment, and out- come. Blood. 2006;108:1267-1279.

16. Hovinga JAK, Vesely SK, Terrell DR, Lammle B, George JN. Survival and relapse in patients with thrombotic thrombocytopenic purpura. Blood. 2010;115:1500-1511. 17. Cohen JA, Brecher ME, Bandarenko N. Cellular source of serum lactate dehydrogenase elevation in

patients with thrombotic thrombocytopenic purpura. J Clin Apheresis. 1998;13:16-19.
18. Loirat C, Fremeaux-Bacchi V. Atypical hemolytic uremic syndrome. Orphanet J Rare Dis. 2011;6:60-90. 19. Sellier-Leclerc A-L, Fremeaux-Bacchi V, Dragon- Durey M-A, et al. Differential impact of complement mutations on clinical characteristics in atypical hemolytic uremic syndrome. J Am Soc Nephrol. 2007;18:2392-2400. 20. Tsai H-M. The kidney in thrombotic thrombocyto- penic purpura. Minerva Med. 2007;98:731-747.

21. Laurence D, Jaffe EA, Weksler B, Hajjar KA, Soder- land C, Laurence J. Thrombotic thrombocytopenic pur- pura and sporadic hemolytic-uremic syndrome plasmas induce apoptosis in restricted lineages of human micro- vascular endothelial cells. Blood. 1997;89:1224-1234. 22. Tsai H-M. Pathophysiology of thrombotic throm- bocytopenic purpura. Int J Hematol. 2010;91:1-19.

23. Veyradier A, Obert B, Houllier A, Meyer D, Girma J-P. Specific von Willebrand factor-cleaving protease in thrombotic microangiopathies: a study of 111 cases. Blood. 2001;98:1765-1772.

24. Licht C, Pluthero FG, Ling L, et al. Platelet-associated complement factor H in healthy persons and patients with atypical HUS. Blood. 2009;114:4538-4545.
25. Wun T. Thrombotic thrombocytopenic purpura. 2011. Accessed August 21, 2012.

26. Marn Pernat A, Buturovic-Ponikvar J, Kovac J, et al. Membrane plasma exchange for the treatment of thrombotic thrombocytopenic purpura. Ther Apher Dial. 2009;13:318-321.

27. Lara PN Jr, Coe TL, Zhou H, Fernando L, Holland PV, Wun T. Improved survival with plasma exchange in patients with thrombotic thrombocytopenic purpura-hemolytic uremic syndrome. Am J Med. 1999;107:573-579.

28. Atkinson JP, Goodship THJ. Complement factor H and the hemolytic uremic syndrome. J Exp Med. 2007;204:1245-1248.
29. Mache CJ, Acham-Roschitz B, Fremeaux-Bacchi V, et al. Complement inhibitor eculizumab in atypical hemolytic uremic syndrome. Clin J Am Soc Nephrol. 2009;4:1312-1316. 30. Noris M, Caprioli J, Bresin E, et al. Relative role of genetic complement abnormalities in sporadic and familial aHUS and their impact on clinical phenotype. Clin J Am Soc Nephrol. 2010;5:1844-1859.

31. Reti M, Farkas P, Csuka D, et al. Complement activation in thrombotic thrombocytopenic purpura. J Thromb Hemostas. 2012;10:791-798.
32. Noris M, Remuzzi G. Genetics and genetic testing in hemolytic uremic syndrome/thrombotic thrombo- cytopenic purpura. Semin Nephrol. 2010;30:395-408. 33. Neumann HPH, Salzmann M, Bohnert-Iwan B, et al. Hemolytic uremic syndrome and mutations of the factor H gene: a registry-based study of German speak- ing countries. J Med Genet. 2003;40:676-681.

34. Chapin J, Weksler B, Magro C, Laurence J. Eculizumab in the treatment of refractory idiopathic thrombotic thrombocytopenic purpura. Brit J Hematol. 2012;157:772-774.

35. Bell WR, Braine HG, Ness PM, Kickler TS. Improved survival in thrombotic thrombocytopenic purpura-hemolytic uremic syndrome. Clinical experi- ence in 108 patients. N Engl J Med. 1991;325:398-403. 36. Hosler GA, Cusumano AM, Hutchins GM. Throm- botic thrombocytopenic purpura and hemolytic uremic syndrome are distinct pathologic entities. A review of 56 autopsy cases. Arch Pathol Lab Med. 2003;127:834-839. 37. Barbour T, Johnson S, Cohney S, Hughes P. Throm- botic microangiopathy and associated renal disorders. Nephrol Dial Transplant. 2012;27:2673-2685.



Diagnosis of Atypical Hemolytic Uremic Syndrome: A Review of Case Studies

Jeffrey Laurence, MD

Professor of Medicine
Division of Hematology and Medical Oncology
New York Presbyterian Hospital and Weill Cornell Medical College New York, New York

D iagnosis of atypical hemo- lytic uremic syndrome (aHUS) can be difficult, but is of critical importance given the recent approval by the US Food and Drug Administration (FDA) of an effective therapy for this disorder. The following published case studies highlight important points regarding the distinction between the 2 prin- ciple thrombotic microangiopathies, aHUS and thrombotic thrombocyto-

penic purpura (TTP).

aHUS in an Infant

Ariceta and colleagues reported a case of a 28-day-old male infant who pre- sented with the symptoms of a classic thrombotic microangiopathy (TMA).1 Despite multiple blood and plasma infusions for a presumptive diagnosis ofTTP,theinfantexperiencedcontin- ued thrombocytopenia, hemolysis with schistocytes on peripheral blood smear, and rising creatinine. The infant also had other manifestations of systemic microvascular thrombosis, including multiple intestinal perforations and skin necrosis.

On day 10 of admission, with con- tinued failure of plasma therapy and recognition of complement activation with a low C3 level, eculizumab was initiated. Within 4 days, hemodialysis could be discontinued, and C3 levels as well as platelet counts normalized within 2 weeks. However, signs and symptoms of persistent thrombotic

microangiopathy were manifested 11 days after the first eculizumab dose, related to inadequate pharmacologic levels. An increase in dose led to TMA resolution, and therapy has continued. Levels of ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), which is an enzyme with von Willebrand factor cleaving protease activity that characteristically is severely reduced in TTP (<5% of normal activity) but normal (>5–10% of normal activity, with cut-off values dependent on the assay method) in aHUS, was determined several weeks later and found to be normal (56%). The authors also performed several sophisticated tests for complement regulatory factors including search for mutations in soluble and membrane- bound complement regulatory pro- teins and autoantibodies against the principal such protein, complement factor H (CFH). No abnormalities were documented.

Key Points

This patient shows that TMAs can occur at any age, from very young infants, as in this case, to the elderly. Another important point is that mea- surement of complement abnormali- ties are not reliable indicators of an aHUS diagnosis, nor a reliable way to distinguish among the TMAs. In this patient, a low level of complement protein C3 was found, but this is not often present in aHUS. It is possible

for a patient to have a severe comple- ment problem leading to a TMA and to have normal, low, or high levels of complement proteins C3 and C4. In addition, approximately 50% of patients with aHUS responsive to appropriate therapy with eculizumab lack a documented genetic mutation in the complement regulatory system, as in this case. It is generally believed that more than 90% of aHUS cases have a genetic, complement-based abnormality, but that some 30–50% of these abnormalities are not identified when sought because the assays that are currently available are not global enough to detect the abnormalities. Finally, one clinical pearl: In general, it is not possible to distinguish clini- cally among the TMAs. Some 20% of individuals with aHUS have serum creatinine levels within the normal range at first presentation, and almost 50% have neurologic changes. Diar- rhea, including bloody diarrhea, may be a feature of all of the TMAs. This infant, however, was noted to require mechanical ventilation. Although the authors do not explain why such sup- port was required, the lungs are often involved in late stages of aHUS, but are virtually never involved in classic, ADAMTS13-deficientTTP.

aHUS in a Young Man

Durán and coworkers discussed the case of a 28-year-old man, a known cocaine user, who presented with renal failure and hemolytic anemia.2

Evidence for the latter included a high level of lactate dehydrogenase and a low level of haptoglobin. A microangiopathic process was docu- mented on renal biopsy. The patient also had low levels of complement proteins C3 and C4, although, as I mentioned previously, these values are not reliable indicators of a sys- temic TMA. Levels of complement factor H protein were low-normal. The authors also tested for a muta- tion in this protein, which is the most common complement regulatory factor to be mutated in patients with aHUS. The mutation was found, and the patient was diagnosed with aHUS. The authors instituted plasma exchange, which was administered every other day for 1 month with no response.

Key Points

This patient represents a case of aHUS diagnosed not in the typical fashion, by recognition of classic laboratory and clinical signs of a TMA in the setting of a normal ADAMTS13 level, but via an involved tissue (kidney) biopsy together with identification of a complement factor H mutation. The first teaching point is that although the patient was diagnosed via genetic anal- ysis, this is an unreliable method. The assays are not generally available, and require several months to complete, during which time, in the absence of appropriate anti-complement therapy, tissue damage continues. As noted in the prior case, levels of complement and/or complement regulatory factors can be normal, and complement muta- tions not identifiable, in aHUS.

The second teaching point is that although this patient was diagnosed with aHUS and not TTP, plasma exchange was instituted anyway. There are no clinical trials indicating a role for plasma infusion or plasma exchange in the treatment of aHUS, and in this patient, plasma exchange was contin- ued for 1 month with no response. As a guide, plasma exchange is usually initi-

ated upon first presentation of a TMA, before appropriate ADAMTS13 test- ing can be completed, and these results can take more than 1 week to return. In that period of time, upwards of 60–80% of classic aHUS patients may have a transient partial response to plasma therapy, typically limited to an increase in platelet counts and resolu- tion of hemolysis, but the underlying process of unregulated complement activation persists. The latter may be evidenced by continued decline in renal function, persistent platelet activation, or failure to completely normalize the LDH. As a general guide, the majority of classic TTP patients, with ADAMTS13 activity less than 5–10% (the cut-off depend- ing on the assay method used), will have resolution of thrombocytopenia within 7 days and of creatinine within 2 weeks, and will require an average of about 20 liters of plasma administered over 7–9 days, to get there.3 Failure to reach these milestones, and certainly a requirement for continued plasma therapy for more than 2 weeks in the context of a normal ADAMTS13 activity, should lead one to rethink the diagnosis and consider aHUS. An educational point illustrated by this patient is that he presented with his first manifestation of a TMA at age 28, and yet was documented to have a genetic mutation in complement factor H. The question arises as to why this patient did not manifest a clinically apparent TMA much sooner in his lifetime. The answer is that the genetic mutation in and of itself was insufficient to lead to a clinically recognizable TMA, just as patients with familial TTP and mutations in the ADAMTS13 gene infrequently develop clinical TTP before the age of 6 years, and then may have only a few episodes of TTP over their lifetimes. It is now recognized that there are modulating factors in aHUS that act in concert with this chronic, genetic condition, and these modulating fac- tors are complement-related. Potent

activators of the alternative comple- ment pathway include surgery, trauma, infection, and pregnancy. In this case, the authors suggested that cocaine use was the modulating factor. Cocaine has been shown in animal models to acti- vate complement. It could also offer a “double-hit,” as cocaine can directly injure small blood vessels, which may accentuate the microthrombotic char- acteristic of aHUS.4

Severe aHUS in a Woman

Ohanian and colleagues reported a case of a 50-year-old woman with a history of rheumatoid arthritis.5 She presented with fatigue, abdominal pain, bloody diarrhea, sepsis, acute renal failure, and thrombocytopenia. Testing for Shiga toxin was negative. This patient had low complement protein C3 and C4 levels. ADAMTS13 activity was normal, and schistocytes were seen on the peripheral blood smear. Further evidence for microangiopathic hemo- lysis included a haptoglobin level of less than 6 mg/dL and a negative direct Coombs test, to rule out an autoim- mune hemolytic anemia.

The patient underwent total abdominal colectomy and end ileos- tomy, with intestinal pathology show- ing evidence of microthrombotic angi- opathy. During surgery, she received 2 units of packed red blood cells, 2 units of fresh frozen plasma, and 2 units of pooled platelets. Her platelets increased following this intervention, but she developed respiratory failure. Electroen- cephalography (EEG) showed evidence of a cerebral dysfunction, and magnetic resonance imaging (MRI) of the brain showed infarcts. Her renal failure pro- gressed, and she required hemodialysis. aHUS was diagnosed based on her clinical features (severe renal failure, neurologic changes), evidence of TMA, including microangiopathic hemolysis, and normal ADAMTS13 activity. She also had low complement levels, which supports the diagnosis of aHUS, but was not required for it.

Key Points

There are 3 major types of TMAs: Shiga toxin–related STEC-HUS; TTP, which is related to an ADAMTS13 deficiency; and aHUS, which is related to abnor- malities of the alternative pathway of complement. Before there were effective treatments for aHUS, patients who presented with bloody diarrhea were usually diagnosed with D(diarrhea)+ HUS—a term that should no longer be used—of which STEC-HUS is a subset. But bloody diarrhea can present in all 3 of the TMAs, so it is not a useful distin- guishing point. Indeed, diarrhea is one of the most potent causes of activation of the alternate complement pathway. The bacteria and viruses that are typi- cally associated with diarrhea injure the epithelium lining the intestine. This injury can result in relative transparency that allows microbial translocation—the movement of bacterial products directly from injured epithelium into the blood- stream. Microbial translocation is a very potent complement activator.

With her renal failure, mental status changes, and intestinal and respiratory conditions, this patient shows that aHUS is often a multisystem disease. As noted above, renal failure is not necessarily asso- ciated with aHUS—approximately 20% of patients with aHUS do not have mani- festations of acute renal disease on their first presentation—while involvement of other organ systems can predominate.

TMA-related perforations of the colon that can lead to sepsis have been reported. This patient also had small bilateral pleu- ral effusions. As noted earlier, lung pathol- ogy is not uncommon in aHUS, while in classic TTP, the lung is virtually never involved. A key teaching point in terms of evaluating a “response” to therapy in aHUS is the fact that the patient’s plate- let count increased when she received 2 units of packed red blood cells, 2 units of fresh frozen plasma, and 2 units of pooled platelets during surgery. A complete hemolytic response with normalization of the platelet count, normalization of the hemoglobin, and disappearance of schistocytes on peripheral blood smear may be interpreted as evidence that the patient’s disease is responsive to plasma therapy. However, in aHUS, there is no evidence that plasma alters the ultimate course of disease, with persistent decline in glomerular filtration rate, persistent platelet activation, and continuation of other systemic organ pathology.


All three of these cases illustrate the significant morbidity that accompanies aHUS, and consequences of the failure to recognize the condition, distinguish it from other TMAs, and treat it appro- priately. Now that an effective therapy is available, the importance of quickly making the diagnosis of aHUS is clear.


This article was supported by funding from Alexion Pharmaceuticals, Inc. This article reflects the opinions and views of the author and was developed independently of Alexion Pharmaceuti- cals, Inc. Dr. Laurence is a member of the Speakers’ Bureau of Alexion Phar- maceuticals, Inc.

Editor’s Note

Further citations for the information in this review can be found in the article “Atypical Hemolytic Uremic Syndrome (aHUS): Making the Diag- nosis,” which appears on pages 2–8 of this supplement.


1. Ariceta G, Arrizabalaga B, Aguirre M, Morteruel E, Lopez-Trascasa M. Eculizumab in the treatment of atypical hemolytic uremic syndrome in infants. Am J Kidney Dis. 2012;59:707-710.

2. Durán CE, Blasco M, Maduell F, Campistol JM. Rescue therapy with eculizumab in a transplant recipi- ent with atypical haemolytic-uraemic syndrome. Clin Kidney J. 2012;5:28-30.

3. Rock GA, Shumak KH, Buskard NA, et al. Com- parison of plasma exchange with plasma infusion in the treatment of thrombotic thrombocytopenic purpura. N Engl J Med. 1991;325:393-397.

4. Tanhehco EJ, Yasojima K, McGeer PL, Lucchesi BR. Acute cocaine exposure up-regulates comple- ment expression in rabbit heart. J Pharmacol Exp Ther. 2000;292:201-208.

5. Ohanian M, Cable C, Halka K. Eculizumab safely reverses neurologic impairment and eliminates need for dialysis in severe atypical hemolytic uremic syndrome. Clin Pharmacol. 2011;3:5-12.


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