Acute kidney injury is one of the most frequent and clinically important manifestations of thrombotic microangiopathy (TMA), occurring in up to 80% of patients with secondary TMA. Renal involvement ranges from mild impairment to dialysis-dependent kidney failure and is a major determinant of both short- and long-term outcomes. The etiologic spectrum of TMA is broad, encompassing primary and secondary disorders that require different therapeutic approaches. So, systematic identification of the underlying cause is essential for appropriate management. However, a proportion of patients remain without a definitive etiologic diagnosis. In these situations, kidney biopsy provides value beyond establishing the diagnosis of TMA. Histopathologic evaluation can define disease activity and chronicity, identify kidney pathology, and provide prognostic information regarding the likelihood of kidney recovery and clinical management.

Clinical data, laboratory findings, kidney biopsy, and targeted etiologic investigations were retrospectively reviewed to establish the diagnosis and assess the patient's clinical course.

A 71-year-old woman presented with a two-week history of oliguria, dark-colored urine, fatigue, and nausea. Laboratory investigations demonstrated severe acute kidney injury, microangiopathic hemolytic anemia, thrombocytopenia, elevated lactate dehydrogenase, undetectable haptoglobin, and schistocytosis. A PLASMIC score of 5 prompted urgent plasma exchange while ADAMTS13 testing was pending. ADAMTS13 activity was 36%, excluding severe ADAMTS13 deficiency-mediated thrombotic thrombocytopenic purpura. Extensive evaluation for autoimmune disease, antiphospholipid syndrome, infection, monoclonal gammopathy, malignancy, drug-induced TMA, and available complement investigations were unrevealing. Kidney biopsy demonstrated chronic active renal TMA with mesangiolysis, glomerular basement membrane duplication, concentric fibrointimal hyperplasia, chronic ischemic glomerulosclerosis, and no immune-complex deposition. Following plasma exchange, hemodialysis, and supportive care, hematologic parameters improved; however, kidney recovery remained limited, and the patient remained dialysis-dependent.

Kidney biopsy remains clinically valuable even when the underlying etiology of TMA cannot be definitively established. In this patient, histopathologic evaluation explained the persistent dialysis dependence, clarified disease chronicity, and refined prognostic assessment