Autoimmune hemolytic anaemia is rare in the paediatric population. Differentiation of the underlying aetiology is complicated by heterogeneity in diagnostic criteria and testing strategies. Paroxysmal cold hemoglobinuria (PCH) is an uncommon form of paediatric autoimmune hemolytic anaemia. Identification of the causative biphasic hemolysin requires clinical recognition and access to the Donath-Landsteiner (DL) test.
We report a young paediatric patient with no significant past medical history who presented with severe anaemia, jaundice, and dark urine following a respiratory illness. Initial laboratory evaluation showed a haemoglobin of 3.6 g/dL with plasma free haemoglobin 170 mg/dL (reference range <5 mg/dL), 3+ hemoglobinuria (reference range = 0), and direct antiglobulin testing (DAT) positive for complement component 3 (C3) only. Haemoglobin continued to decline following RBC transfusions using a blood warmer for presumed cold agglutinin syndrome. Subsequent testing at the reference laboratory revealed a DAT positive for C3 and immunoglobulin isotype G (IgG) and an eluate pan-agglutinin most consistent with a warm autoantibody, but the patient's anaemia was non-responsive to glucocorticoids and blood warmer cessation. However, a maximum cold agglutinin titre of 4 and absent thermal amplitude substantially weakened the evidence for the clinical significance of the cold autoantibodies. Consultation with the institutional transfusion medicine specialist prompted collection for the DL test, which demonstrated a definitive biphasic hemolysin consistent with PCH.
Conflicting clinical and immunohematologic evidence can obscure the aetiology of autoimmune hemolysis, including concurrent warm and/or cold autoantibodies. Clinical correlation, consultation with the institutional transfusion service, and access to specialised testing are essential to establish the correct diagnosis.

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