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A Delayed Haemolytic Transfusion Reaction (DHTR) is an immune complication that typically develops several days to weeks after a blood transfusion. It occurs when a patient has previously been exposed to a red blood cell antigen through transfusion or pregnancy and has developed antibodies that were initially too weak to detect. Following re-exposure, these antibodies increase and gradually destroy transfused red blood cells.

Commonly involved antibodies include those against Rh, Kidd, Duffy, Kell, and other blood group antigens. Symptoms are often mild and may include unexplained fatigue, jaundice, dark urine, low-grade fever, or a fall in haemoglobin after an apparently successful transfusion. Some patients may have no obvious symptoms, with the reaction detected through laboratory testing.

Diagnosis involves comparing pre- and post-transfusion blood samples, antibody screening, direct antiglobulin testing, bilirubin measurement, lactate dehydrogenase, haptoglobin, and haemoglobin levels. Identification of the responsible antibody is important for future transfusion planning.

Management is usually supportive because haemolysis is often self-limited. Further transfusions should be avoided unless clinically necessary. If transfusion is required, antigen-negative, crossmatch-compatible red blood cells should be selected. Accurate documentation of identified antibodies helps prevent recurrent reactions during future transfusions.