BK Virus-Associated Hemorrhagic Cystitis in Hematopoietic Cell Transplant Recipients—What We Understand and What Remains Unclear

BK virus-associated hemorrhagic cystitis (BKV-HC) is a significant complication following alloge-neic hematopoietic cell transplantation (HCT), typically resulting from the reactivation of latent BK polyomavirus in the context of profound immunosuppression. This review addresses three core re-search questions: (1) What are the most accurate and practical diagnostic strategies for identifying BKV-HC in HCT recipients? (2) Which therapeutic interventions demonstrate the greatest efficacy and safety in clinical practice? (3) How can emerging immunological insights inform future indi-vidualized treatment approaches? Current diagnostic approaches rely on clinical assessment combined with quantitative PCR to detect active viral replication in urine or plasma, with exclusion of alternative causes of hematuria and renal injury. BKV-HC presents with a wide spectrum of clinical severity and contributes substan-tially to morbidity, prolonged hospitalization, and healthcare resource utilization. While reduction of immunosuppression remains the primary intervention, antiviral agents such as cidofovir, leflunomide, and fluoroquinolones have shown inconsistent benefits. Supportive strategies, in-cluding bladder irrigation, and novel immunotherapies targeting virus-specific immune responses are under active investigation. Given the disease’s complex pathogenesis and the absence of standardized treatment protocols, future research should prioritize prospective trials to establish validated viral load thresholds, op-timize therapeutic algorithms, and explore the utility of immune monitoring to inform personalized management.