What associated finding is most likely to indicate a delayed transfusion reaction after a blood transfusion?

A delayed transfusion reaction typically occurs days to weeks after a blood transfusion and is often associated with an immune response to mismatched blood antigens. The production of antibodies can lead to the breakdown of transfused red blood cells, ultimately resulting in hemolysis.

Haemosiderinuria, the presence of hemosiderin in the urine, is a key finding in this context. After red blood cell hemolysis, iron released from hemoglobin is stored as hemosiderin, which may then be excreted in urine. This is especially relevant in delayed transfusion reactions, where ongoing hemolysis can occur in the days following the transfusion owing to the immune system's response.

Conjugated hyperbilirubinemia can be seen in both delayed and acute hemolytic reactions but is generally not as specific for the delayed type of reaction as haemosiderinuria. Elevated D-dimer is a marker of fibrin degradation but is not directly related to hemolysis or delayed transfusion reactions. Haemoglobinuria generally suggests a more acute hemolytic reaction as it indicates the presence of free hemoglobin in the urine, typically resulting from rapid hemolysis.

Therefore, the presence of haemosiderinuria is