Background: Atypical Hemolytic Uremic Syndrome (aHUS) remains a diagnostic challenge as it shares overlapping features of Thrombotic Thrombocytopenic Purpura (TTP) and Shiga-toxin mediated HUS (STM-HUS). It is caused by complement activation in the alternative pathway and is mostly related to genetic defects in complement-related factors or acquired autoantibodies. Early recognition and treatment have excellent outcomes.

Case Presentation: A 74-year-old female with Hypertension presented with a 3-week history of non-bloody diarrhea and was found to have acute kidney injury (AKI) with creatinine 5.27 mg/dL (baseline 0.5), anemia (7.5 g/dL), and thrombocytopenia (123×10⁹/L) and normal liver function. CT imaging of the abdomen revealed uncomplicated sigmoid diverticulitis and mesenteric lymphadenopathy. She was treated with intravenous fluids and started on antibiotics with ceftriaxone and metronidazole for diverticulitis. Despite fluid resuscitation, renal function deteriorated, necessitating initiation of intermittent hemodialysis. Due to anemia, thrombocytopenia and AKI further workup with peripheral smear pursued showed schistocytes, helmet cells, and keratocytes. Hemolysis labs showed LDH 1433 U/L, undetectable haptoglobin, and a negative direct Coombs test. Methylmalonic acid level was normal. Homecystine level was elevated. These findings were consistent with microangiopathic hemolytic anemia. Coagulation profile was normal. Given concern for TTP, ADAMTS13 was obtained but returned at 68%, ruling out TTP. Shiga toxin PCR was negative, ruling out STM-HUS. Complement level checked showed low C3 and C4, suggesting complement activation and consumption. Total complement, Factor 8, factor B, monoclonal protein were within normal. A renal biopsy was initially postponed due to uncontrolled hypertension. Eculizumab was started. Renal biopsy later done confirmed aHUS. Following initiation of eculizumab, the patient's renal function progressively improved, and she was able to discontinue hemodialysis. Laboratory markers of hemolysis stabilized, with normalization of LDH, haptoglobin, homocystine and complement. She completed the induction phase of eculizumab and entered a long-term monitoring phase, with labs every 2 weeks for 6 months, then monthly for 36 months. No relapses were noted to date and is off dialysis.

Conclusion: Differentiating aHUS from TTP and STM-HUS is critical, as management strategies differ significantly. Prompt initiation of eculizumab can reverse renal injury and prevent further organ damage.

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2025
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