Severe Intravascular Hemolysis and Acute Kidney Injury Triggered by Mycoplasma-Associated Cold Agglutinin Disease: A Case Report.

Introduction: Mycoplasma pneumoniae infection commonly causes mild respiratory illness, but cold agglutinin syndrome (CAS) is an uncommon extrapulmonary complication. CAS typically results in mild, predominantly extravascular hemolysis; however, severe intravascular hemolysis and acute kidney injury (AKI) are rare. Case presentation: A 59-year-old woman presented with profound weakness, jaundice, dark urine, and anuria following a mild upper respiratory illness. Laboratory evaluation revealed severe intravascular hemolysis, markedly elevated LDH activity, undetectable hemoglobin (Hb) level, hemoglobinuria, and AKI. Direct antiglobulin testing was positive for C3d only, and the cold agglutinin titer was markedly elevated (1:2048). Mycoplasma pneumoniae IgM serology was positive. She required warmed blood transfusions, antibiotics, thermal protection, and continuous renal replacement therapy. Over 10 days, hemolysis-related laboratory parameters and serum creatinine level improved significantly. Conclusion: This case illustrates a rare and severe form of Mycoplasma pneumoniae-associated CAS presenting with massive intravascular hemolysis and AKI. Early recognition and supportive interventions-particularly warmed transfusions and renal support-were critical to recovery. Clinicians should consider cold agglutinin-mediated hemolysis in patients with dark urine and AKI following respiratory infection.