BK Virus-Associated Haemorrhagic Cystitis Following Allogeneic Haematopoietic Stem Cell Transplantation in a Paediatric Patient with β-Thalassaemia Major: A Case Report.

BK virus (BKV) can reactivate in immunosuppressed patients after stem cell transplantation. We report an 8-year-old boy with β-thalassemia major who developed Grade III BKV-associated haemorrhagic cystitis just 17 days post-transplant. Bladder ultrasound showed significant wall thickening with clot formation, and urine BKV Polymerase Chain Reaction(PCR) was markedly elevated, though blood PCR remained negative, meaning infection stayed localised to the urinary tract. He recovered fully with IV fluids, platelet support, and levofloxacin; no antivirals needed. This case highlights that urine BKV PCR is essential in post-transplant haematuria, and that viremia-negative Grade III disease can resolve with supportive care alone.