Hemoglobinuria-associated acute kidney injury in hemolytic uremic syndrome without renal thrombotic microangiopathy

Background Hemolytic uremic syndrome (HUS) is defined by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury (AKI), classically attributed to renal thrombotic microangiopathy (TMA). Case presentation: A 64-year-old woman with hypertension treated with an ACE inhibitor presented with asthenia, nausea, dark urine, and oliguria. Laboratory tests showed AKI (serum creatinine 4.5 mg/dL), thrombocytopenia (46 × 10⁹/L), and hemolysis (LDH > 1,800 U/L, schistocytes, low haptoglobin). Procalcitonin was markedly elevated; complement and ADAMTS13 were normal. A positive direct Coombs test suggested pneumococcal-associated or secondary HUS, but extensive microbiological studies were negative. Suspected STEC-HUS was managed with plasma exchange (subsequently discontinued), supportive therapy, and hemodialysis from day 3. Hematologic abnormalities resolved, but renal function worsened, with serum creatinine peaking at 10 mg/dL on day 6. Kidney biopsy on day 7 revealed acute tubular injury with hemoglobin pigment casts and no evidence of TMA. Dialysis was withdrawn, and renal recovery followed. Conclusions This case highlights that AKI in HUS is not invariably due to TMA; hemoglobinuria-induced tubular injury can be the dominant lesion. When hematologic recovery contrasts with persistent renal dysfunction, pigment nephropathy should be suspected and biopsy considered when feasible. To our knowledge, this is the first reported case of HUS with full clinical triad but biopsy-proven tubular injury without renal TMA.