COVID-19 vaccination and Atypical hemolytic uremic syndrome.

Introduction: COVID-19 vaccination has been associated with rare but severe complications characterized by thrombosis and thrombocytopenia. Methods and Results: Here we present three patients who developed de novo or relapse atypical hemolytic uremic syndrome (aHUS) in native kidneys, a median of 3 days (range 2-15) after mRNA-based (Pfizer/BioNTech's, BNT162b2) or adenoviral (AstraZeneca, ChAdOx1 nCoV-19) COVID-19 vaccination. All three patients presented with evident hematological signs of TMA and AKI, and other aHUS triggering or explanatory events were absent. After eculizumab treatment, kidney function fully recovered in 2/3 patients. In addition, we describe two patients with dubious aHUS relapse after COVID-19 vaccination. To assess the risks of vaccination, we retrospectively evaluated 29 aHUS patients (n=8 with native kidneys) without complement-inhibitory treatment, who received a total of 73 COVID-19 vaccinations. None developed aHUS relapse after vaccination. Conclusion: In conclusion, aHUS should be included in the differential diagnosis of patients with vaccine-induced thrombocytopenia, especially if co-occuring with mechanical hemolytic anemia (MAHA) and acute kidney injury (AKI). Still, the overall risk is limited and we clearly advise continuation of COVID-19 vaccination in patients with a previous episode of aHUS, yet conditional upon clear patient instruction on how to recognize symptoms of recurrence. At last, we suggest monitoring serum creatinine (sCr), proteinuria, MAHA parameters, and blood pressure days after vaccination.

Atypical Hemolytic Uremic Syndrome After Kidney Transplantation: Lessons Learned From the Good, the Bad, and the Ugly. A Case Series With Literature Review.

Atypical hemolytic uremic syndrome (aHUS) after kidney transplantation is rare and carries a grave outcome. We present a single-center experience of all aHUS cases since the program's inception. Six patients were diagnosed with aHUS, all after kidney transplants, except for 1 patient. All had nonreactive crossmatches. Delayed graft function (DGF) occurred in 2 patients. Five patients developed aHUS after transplant; 4 (80%) of these patients manifested aHUS ≤ 14 days. All were confirmed by allograft biopsy. Genetic testing was abnormal in all patients except for 1 patient. Actual patient and graft survival during the first year was 100% and 83.3%, respectively. A single graft was lost early in the study secondary to aHUS (eculizumab was not used in the treatment process). Prophylactic and therapeutic use of eculizumab salvaged all other cases. At 1 year, mean creatinine level was 1.9 mg/dL (range, 1.3-2.5). After 6 months of eculizumab treatment (halted in 2 cases) 1 patient had recurrence 2 months later and eculizumab was restarted. However, graft function continued to worsen, and the graft was ultimately lost at 20 months after kidney transplantation. High index of suspicion, prompt diagnosis, and utilization of eculizumab are key to successful salvage of allografts in cases of aHUS after kidney transplantation. aHUS can be prevented by prophylactic use of eculizumab. It still needs to be determined when and if eculizumab therapy can be safely discontinued.

Steroid Responsive Atypical Hemolytic Uremic Syndrome Triggered by Influenza B Infection.

Atypical hemolytic uremic syndrome (aHUS) is characterized by uncontrolled complement activation leading to thrombotic microangiopathy and severe end-organ damage. The most common trigger for an episode of aHUS in the background of genetic deregulation of the alternative complement pathway is systemic infection. There are only 4 reported cases of aHUS triggered by influenza B thus far. Current accepted therapies for aHUS include plasma exchange and eculizumab. We describe a unique patient with aHUS with a rare membrane cofactor protein mutation triggered by influenza B infection, who achieved complete remission with treatment with high-dose corticosteroids after failure of plasmapheresis.

Haploidentical Hematopoietic Stem Cell Transplant Complicated by Atypical Hemolytic Uremic Syndrome and Kidney Transplant From the Same Donor With No Immunosuppression but C5 Inhibition.

BACKGROUND: Atypical hemolytic uremic syndrome (aHUS) is life-threatening condition particularly when complicating allograft hematopoietic stem cell transplant (HSCT). In the past, the outcome was very poor with the majority of patients reaching end-stage renal disease or dying with little or no chances of kidney transplant (KTx) due to the high risk of relapse. The availability of C5 inhibition has opened up significant therapeutic opportunities and has improved the outcome particularly if complement dysregulation (CD) is the underlying pathogenetic mechanism. METHODS: We describe a peculiar case of a girl with aHUS complicating HSCT and her subsequent successful KTx received from the same donor thus performed without immunosuppression but anti-C5 inhibition. RESULTS: Soon after HSCT performed for acute lymphoblastic leukemia, the patient developed a TMA due to CD and reached end-stage renal disease. After 2 years on dialysis, the patient received a KTx from her father who was already the HSCT donor. Given the full chimerism, no immunosuppressive agent was prescribed except a short (2 days) course of steroids and eculizumab to prevent aHUS relapse. Nine months after the KTx, the patient is well with normal renal function, no immunosuppression and continues eculizumab prevention of aHUS (1 infusion every 21 days). CONCLUSIONS: All patients with transplant-associated thrombotic microangiopathy should be screened for the causes of CD. C5 inhibition with eculizumab is an important therapeutic resource to manage this complication. When KTx is necessary, immunosuppression can be safely withhold in case of same donor for both grafts and documented full chimerism.

Favorable course of previously undiagnosed Methylmalonic Aciduria with Homocystinuria (cblC type) presenting with pulmonary hypertension and aHUS in a young child: a case report.

BACKGROUND: Cobalamin C (cblC) defect is the most common inborn error of Vitamin B12 metabolism often causing severe neurological, renal, gastrointestinal and hematological symptoms. Onset with pulmonary hypertension (PAH) and atypical hemolytic-uremic syndrome (aHUS) is rare. CASE PRESENTATION: We describe the case of a 2-years old child, previously in good health, admitted to the hospital with severe respiratory symptoms, rapid worsening of clinical conditions, O2 desaturation and palmo-plantar edema. The patient showed PAH and laboratory findings compatible with aHUS. cblC defect, an inborn error of metabolism, was identified as the cause of all the symptoms described (cardiac, respiratory and renal involvement). Results of neonatal screening for inborn errors of metabolism had been negative. Administration of IM OHCbl (intramuscular hydroxocobalamin), oral betaine and symptomatic treatment with diuretics and anti-hypertensive systemic and pulmonary drugs induced dramatic improvement of both cardiac and systemic symptoms. CONCLUSIONS: In this case of cblC defect the metabolic treatment completely reverted symptoms of aHUS and PAH. The course was favorable, and the prognosis is what we foresee for the future.

Síndrome hemolítico urémico atípico / Atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is a rare thrombotic microangiopathy, characterized by microangiopathic hemolytic anemia, thrombocytopenia and renal involvement. It causes end stage renal disease requiring dialysis in most affected patients. It mainly affects young adults (contrary to what was thought years ago). When aHUS is primary, the cause is a genetic mutation in the alternative complement pathway. Instead, secondary aHUS is caused by external factors that trigger the disease by themselves or in combination with a genetic vulnerability. The type of mutation determines the severity of the disease, prognosis, response to therapy and renal transplantation. Advances in the understanding of renal diseases associated with complement defects and the development of specific biologic therapies changed the course of this disease. Eculizumab is internationally approved for the treatment of primary aHUS. Its inhibitory action on the complement cascade leads to hematologic remission and restoration of renal function. We present a review of aHUS detailing its etiology, pathogenesis, clinical presentation, diagnosis and treatment.

Síndrome urémico hemolítico atípico recurrente post-trasplante renal: tratamiento con eculizumab / Recurrent atypical hemolytic uremic syndrome after renal transplantation: treatment with eculizumab

El síndrome urémico hemolítico atípico (SUHa) es una entidad rara que se presenta como una microangiopatía trombótica (anemia hemolítica no inmune, trombocitopenia e insuficiencia renal aguda), cuyas lesiones anatomopatológicas típicas son el engrosamiento de las paredes de capilares y arteriolas con trombosis obstructiva del lumen vascular. Se produce por desregulación de la vía alterna del complemento en la superficie celular, debido a causas genéticas o adquiridas, con una alta tasa de mortalidad, enfermedad renal crónica terminal y recurrencia post-trasplante renal. Las mutaciones de peor pronóstico son las asociadas a factor H, factor B y fracción C3 del complemento. La terapia plasmática resulta útil solo en algunos casos, mientras que el uso de eculizumab es altamente eficaz tanto para el tratamiento agudo como para prevenir las recurrencias en el post-trasplante. Comunicamos el caso de una mujer adulta con diagnóstico de SUHa congénito (mutación de C3) en tratamiento preventivo con eculizumab posterior al trasplante renal, sin recurrencia de la enfermedad, ni efectos adversos relacionados al medicamento a los 36 meses de seguimiento post-trasplante.

Atypical hemolytic uremic syndrome (aHUS) is a rare entity. It is characterized by a thrombotic microangiopathy (nonimmune hemolytic anemia, thrombocytopenia, and acute renal failure), with a typical histopathology of thickening of capillary and arteriolar walls and an obstructive thrombosis of the vascular lumen. The syndrome is produced by a genetic or acquired deregulation of the alternative pathway of the complement system, with high rates of end stage renal disease, post-transplant recurrence, and high mortality. Mutations associated with factor H, factor B and complement C3 show the worst prognosis. Even though plasma therapy is occasionally useful, eculizumab is effective both for treatment and prevention of post-transplant recurrence. We describe here an adult case of congenital aHUS (C3 mutation) under preventive treatment with eculizumab after renal transplantation, with neither disease recurrence nor drug-related adverse events after a 36-months follow-up.

Hemolytic uremic syndrome with dual caution in an infant: cobalamin C defect and complement dysregulation successfully treated with eculizumab.

BACKGROUND: Hemolytic uremic syndrome (HUS) is a clinical syndrome characterized by hemolytic anemia, thrombocytopenia, and acute kidney injury. Atypical hemolytic uremic syndrome (aHUS) is a devastating disease with significant mortality and high risk of progression to end-stage kidney disease. It is mostly caused by dysregulation of the alternative complement pathway. Cobalamin C (Cbl C) defect is a genetic disorder of cobalamin metabolism and is a rare cause of HUS. CASE-DIAGNOSIS/TREATMENT: We present a 6-month-old male infant who was admitted to the pediatric intensive care unit (PICU) due to restlessness, severe hypertension, anemia, respiratory distress, and acute kidney injury. Metabolic screening revealed elevated plasma homocysteine levels, low methionine levels, and methylmalonic aciduria, and the patient was diagnosed as having HUS secondary to Cbl C defect. Additionally, complement factor H (CFH) and complement C3 levels were decreased. The infant was treated with betaine, hydroxycobalamin, and folic acid. After treatment, the homocysteine and methylmalonic acid levels were normalized but hemolysis and acute kidney failure persisted. He required continued renal replacement treatment (CRRT) and plasma exchange due to thrombotic microangiopathy (TMA). Therefore, we considered a second mechanism in the pathogenesis as complement dysregulation and gave eculizumab to the patient. After eculizumab treatment, the renal and hematologic indices improved and he was free of dialysis. CONCLUSIONS: To the best of our knowledge, our patient is the first to have Cbl C defect-HUS accompanied by complement dysregulation, who responded well to eculizumab therapy.

Genetic testing of complement and coagulation pathways in patients with severe hypertension and renal microangiopathy.

A diagnosis of thrombotic microangiopathy on kidney biopsy in a patient presenting with hypertensive emergency has historically elicited the diagnosis of malignant hypertension-associated thrombotic microangiopathy. Recent studies, however, have raised awareness that a number of these patients may actually represent atypical hemolytic uremic syndrome. To further investigate this premise, we performed next-generation sequencing to interrogate the coding regions of 29 complement and coagulation cascade genes associated with atypical hemolytic uremic syndrome in 100 non-elderly patients presenting with severe hypertension, renal failure and a kidney biopsy showing microangiopathic changes limited to the classic accelerated hypertension-associated lesion of arterial intimal edema ('mucoid intimal hyperplasia') in isolation and without accompanying glomerular microthrombi. No pathogenic or likely pathogenic variants were identified in any of the genes analyzed, although 13 patients had rare variants of uncertain significance predicted to be deleterious by all in-silico prediction methods utilized. Accordingly, this large patient cohort showed no definitive burden of disease secondary to genetic variants involving complement or coagulation pathways, which contrasts sharply with the high frequency of similar mutational events reported for atypical hemolytic uremic syndrome. Our results also inform recent data by suggesting that patients who present with severe or malignant hypertension and renal thrombotic microangiopathy may be at higher risk for atypical hemolytic uremic syndrome only if the biopsy shows more active disease that includes glomerular fibrin thrombi.

Successful Treatment of Transplantation-associated Atypical Hemolytic Uremic Syndrome With Eculizumab.

We herein reported a 4-month-old boy with transplantation-associated atypical hemolytic uremic syndrome (TA-aHUS) who was successfully treated with eculizumab. The patient diagnosed with type 3 of familial hemophagocytic lymphohistiocytosis underwent cord blood transplantation. After transplantation, he developed TA-aHUS, but plasma exchanges were unsuccessful. We identified deletions in CFH-related gene 1 (del-CFHR1) by the multiplex ligation-dependent probe amplification testing procedure and CFH autoantibodies. Eculizumab has been administered to the patient, with a marked improvement being achieved in thrombocytopenia. He has been well except for the persistent microhematuria for a year after transplantation. Uncontrolled complement activation might be involved in the pathophysiology of TA-aHUS.

Kidney Diseases Associated With Alternative Complement Pathway Dysregulation and Potential Treatment Options.

Atypical hemolytic uremic syndrome and C3 glomerulopathy (dense deposit disease and C3 glomerulonephritis) are characterized as inappropriate activation of the alternative complement pathway. Genetic mutations affecting the alternative complement pathway regulating proteins (complement factor H, I, membrane cofactor protein and complement factor H-related proteins) and triggers (such as infection, surgery, pregnancy and autoimmune disease flares) result in the clinical manifestation of these diseases. A decade ago, prognosis of these disease states was quite poor, with most patients developing end-stage renal disease. Furthermore, renal transplantation in these conditions was associated with poor outcomes due to graft loss to recurrent disease. Recent advances in targeted complement inhibitor therapy resulted in significant improvement in disease remission, renal recovery, health-related quality of life and allograft survival.

Atypical hemolytic uremic syndrome induced by CblC subtype of methylmalonic academia: A case report and literature review.

RATIONALE: Methylmalonic acidemia (MMA) is a common organic acidemia, mainly due to methylmalonyl-CoA mutase (MCM) or its coenzyme cobalamin (VitB12) metabolic disorders. Cobalamin C (CblC) type is the most frequent inborn error of cobalamin metabolism; it can develop symptoms in childhood and often combine multisystem damage, which leads to methylmalonic acid, propionic acid, methyl citrate, and other metabolites abnormal accumulation, causing nerve, liver, kidney, bone marrow, and other organ damage. PATIENT CONCERNS: A 4-year-old girl presented with paleness, fatigue, severe normochromic anemia, and acute kidney injury. DIAGNOSIS: Based on severe normochromic anemia and acute kidney injury, renal biopsy showed membranous proliferative glomerular lesions and thrombotic microvascular disease, supporting the diagnosis of aHUS. Although the serum vitamin B12 was normal, further investigation found the concentration of urinary methylmalonic acid and serum homocysteine increased obviously, genetic analysis revealed a heterozygous MMACHC mutation (exonl: c. 80A >G, c. 609G >A). The final diagnosis was aHUS induced by inherited methylmalonic acidemia (MMACHC heterozygous mutation exonl: c. 80A >G, c. 609G >A). INTERVENTIONS: The patient was treated with a 1mg vitamin B12 intramuscular injection daily for 4 days after which the dose was then adjusted to a 1mg intramuscular injection twice a week. At the same time, the girl was given levocarnitine, betaine, folic acid, along with supportive treatment. OUTCOMES: After treated by vitamin B12 for 10 days, the patient condition significantly improved, Follow-up results showed complete recovery of hemoglobin and renal function. LESSONS: Although the majority of MMA onset from neurological damage, our case illustrates that partial CblC-type MMA can onset with severe metabolic aHUS. On the basis of chronic thrombotic microangiopathy (TMA)-induced renal damage, it can be complicated by acute hemolytic lesions. MMA should be considered in those patients with unclear microangiopathic hemolytic anemia accompany significant megaloblastic degeneration in bone marrow. We should pay attention to the causes and adopt a reasonable treatment strategy.

Aortic Valve Replacement as a Trigger of Atypical Hemolytic Uremic Syndrome.

Mechanical hemolysis is a frequent but usually harmless complication of aortic valve replacement. The most common reason is valvular leakage. This report presents atypical hemolytic uremic syndrome (aHUS) as an alternative cause of mechanical hemolysis after this procedure. aHUS is a complement-mediated disease characterized by microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. It necessitates immediate specific treatment including plasma exchange or complement inhibition to avoid an adverse outcome. The present case identifies aortic valve replacement as a trigger of aHUS and shows that this disease must be taken into account in the differential diagnosis of hemolysis after valve surgery.

Eculizumab in a child with atypical haemolytic uraemic syndrome and haemophagocytic lymphohistiocytosis triggered by cytomegalovirus infection.

We present the case of a 21-month-old girl with two rare and life-threatening conditions, atypical haemolytic uraemic syndrome (aHUS) and haemophagocytic lymphohistiocytosis (HLH), triggered by a cytomegalovirus (CMV) infection. Soon after admission, the girl became anuric and required continuous venovenous haemodiafiltration.Initial treatments included methylprednisolone, fibrinogen and plasma infusion (for HLH), plasmapheresis (for thrombotic microangiopathy), immunoglobulins (for inflammation), ganciclovir (for CMV infection) and the antibiotic cefotaxime. On day 5, eculizumab (600 mg) was given for aHUS, with rapid improvement in haematological and nephrological parameters. Despite a subsequent isolated episode of right heart thrombosis that resolved with heparin treatment, the patient showed a favourable response to eculizumab (300 mg/15 days), with improved renal function, normal haematological values, and no treatment complications. In conclusion, eculizumab effectively treated aHUS in this case despite a comorbid immunological disease.

Eculizumab in secondary atypical haemolytic uraemic syndrome.

Background: Complement dysregulation occurs in thrombotic microangiopathies (TMAs) other than primary atypical haemolytic uraemic syndrome (aHUS). A few of these patients have been reported previously to be successfully treated with eculizumab. Methods: We identified 29 patients with so-called secondary aHUS who had received eculizumab at 11 Spanish nephrology centres. Primary outcome was TMA resolution, defined by a normalization of platelet count (>150 × 10 9 /L) and haemoglobin, disappearance of all the markers of microangiopathic haemolytic anaemia (MAHA), and improvement of renal function, with a ≥25% reduction of serum creatinine from the onset of eculizumab administration. Results: Twenty-nine patients with secondary aHUS (15 drug-induced, 8 associated with systemic diseases, 2 with postpartum, 2 with cancer-related, 1 associated with acute humoral rejection and 1 with intestinal lymphangiectasia) were included in this study. The reason to initiate eculizumab treatment was worsening of renal function and persistence of TMA despite treatment of the TMA cause and plasmapheresis. All patients showed severe MAHA and renal function impairment (14 requiring dialysis) prior to eculizumab treatment and 11 presented severe extrarenal manifestations. A rapid resolution of the TMA was observed in 20 patients (68%), 15 of them showing a ≥50% serum creatinine reduction at the last follow-up. Comprehensive genetic and molecular studies in 22 patients identified complement pathogenic variants in only 2 patients. With these two exceptions, eculizumab was discontinued, after a median of 8 weeks of treatment, without the occurrence of aHUS relapses. Conclusion: Short treatment with eculizumab can result in a rapid improvement of patients with secondary aHUS in whom TMA has persisted and renal function worsened despite treatment of the TMA-inducing condition.

Transplant-associated thrombotic microangiopathy: opening Pandora's box.

Transplant-associated thrombotic microangiopathy (TA-TMA) is an early complication of hematopoietic cell transplantation (HCT). A high mortality rate is documented in patients who are refractory to calcineurin inhibitor cessation. Estimates of TA-TMA prevalence vary significantly and are higher in allogeneic compared with autologous HCT. Furthermore, our understanding of the pathophysiology that is strongly related to diagnosis and treatment options is limited. Recent evidence has linked TA-TMA with atypical hemolytic uremic syndrome, a disease of excessive activation of the alternative pathway of complement, opening the Pandora's box in treatment options. As conventional treatment management is highly inefficient, detection of complement activation may allow for early recognition of patients who will benefit from complement inhibition. Preliminary clinical results showing successful eculizumab administration in children and adults with TA-TMA need to be carefully evaluated. Therefore, realizing the unmet needs of better understanding TA-TMA in this complex setting, we aimed to summarize current knowledge focusing on (1) critical evaluation of diagnostic criteria, (2) epidemiology and prognosis, (3) recent evidence of complement activation and endothelial damage and (4) treatment options.

Absence of thrombocytopaenia and/or microangiopathic haemolytic anaemia does not reliably exclude recurrence of atypical haemolytic uraemic syndrome after kidney transplantation.

A 54-year-old man was diagnosed with atypical haemolytic uraemic syndrome (aHUS) with confirmed complement H mutation in 2012, requiring ongoing dialysis. He was commenced on eculizumab in 2014 once the pharmaceutical board approved this drug. After 4 months, he received a live unrelated donor renal transplant from his wife and continued eculizumab post-transplant. Three months later, there was a rise in his creatinine with no laboratory features of haemolysis and a kidney biopsy confirmed rejection, which was treated with increased immunosuppression. After completing 12 months of treatment with eculizumab, he opted for close monitoring rather than continuation with therapy. Five months post-cessation of the drug, there was a rise in creatinine, and once again, haematological parameters remained within reference range; however, his kidney biopsy showed features consistent with recurrence of aHUS; hence, eculizumab was recommenced with good effect. While there was no evidence of haemolysis, there was a gradual rise in LDH level and a drop in platelet count, although the parameters remained within the normal range. This suggests that aHUS can recur in the allograft in the absence of haematological abnormalities. Clinicians should have a low threshold for allograft biopsy if haematological parameters are not just outside the reference range, but possibly also if there are changes of at least >25% from baseline in platelet count and LDH levels, particularly in those patients who are no longer eligible for eculizumab.

Small-molecule factor D inhibitors selectively block the alternative pathway of complement in paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome.

Paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome are diseases of excess activation of the alternative pathway of complement that are treated with eculizumab, a humanized monoclonal antibody against the terminal complement component C5. Eculizumab must be administered intravenously, and moreover some patients with paroxysmal nocturnal hemoglobinuria on eculizumab have symptomatic extravascular hemolysis, indicating an unmet need for additional therapeutic approaches. We report the activity of two novel small-molecule inhibitors of the alternative pathway component Factor D using in vitro correlates of both paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome. Both compounds bind human Factor D with high affinity and effectively inhibit its proteolytic activity against purified Factor B in complex with C3b. When tested using the traditional Ham test with cells from paroxysmal nocturnal hemoglobinuria patients, the Factor D inhibitors significantly reduced complement-mediated hemolysis at concentrations as low as 0.01 µM. Additionally the compound ACH-4471 significantly decreased C3 fragment deposition on paroxysmal nocturnal hemoglobinuria erythrocytes, indicating a reduced potential relative to eculizumab for extravascular hemolysis. Using the recently described modified Ham test with serum from patients with atypical hemolytic uremic syndrome, the compounds reduced the alternative pathway-mediated killing of PIGA-null reagent cells, thus establishing their potential utility for this disease of alternative pathway of complement dysregulation and validating the modified Ham test as a system for pre-clinical drug development for atypical hemolytic uremic syndrome. Finally, ACH-4471 blocked alternative pathway activity when administered orally to cynomolgus monkeys. In conclusion, the small-molecule Factor D inhibitors show potential as oral therapeutics for human diseases driven by the alternative pathway of complement, including paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome.

Atypical hemolytic uremic syndrome (aHUS): essential aspects of an accurate diagnosis.

Atypical hemolytic uremic syndrome (aHUS), a thrombotic microangiopathy (TMA), is a rare, life-threatening, systemic disease. When unrecognized or inappropriately treated, aHUS has a high degree of morbidity and mortality. aHUS results from chronic, uncontrolled activity of the alternative complement pathway, which activates platelets and damages the endothelium. Two-thirds of aHUS cases are associated with an identifiable complement-activating condition. aHUS is clinically very similar to the other major TMAs: Shiga toxin-producing Escherichia coli (STEC)-HUS, thrombotic thrombocytopenic purpura (TTP), and disseminated intravascular coagulation (DIC). The signs and symptoms of all the TMAs overlap, complicating the differential diagnosis. Clinical identification of a TMA requires documentation of microangiopathic hemolysis accompanied by thrombocytopenia. DIC must be recognized and treated before it is possible to discriminate among the other 3 major TMAs. STEC-HUS can be excluded through testing for Shiga toxin-producing E. coli. aHUS can be distinguished from TTP on the basis of ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) activity, with a severe decrease characteristic of TTP. This test, as both an activity assay and an inhibitor assay, should be ordered before the initiation of plasma therapy in any patient presenting with a TMA. Finally, it is important to recognize that aHUS remains a clinical diagnosis, but in complex scenarios, tissue biopsy may be a useful adjunct in diagnosis.