Snapshot A 45-year-old man presents to the emergency room for blood in his urine. He reports that he had a renal transplant 3 months ago and is currently on many immunosuppressive medications that he cannot name at the moment. He also reports dysuria and suprapubic pain. He denies having any fevers or chills. A urinalysis is positive for blood but is negative for nitrates or leukocyte esterase. He is started on intravenous fluids and admitted to the inpatient ward. A nucleic acid amplification test of his urine is positive for a polyomavirus. attempts are made to reach his transplant physicians to alter his immunosuppressive medications. Introduction Classification BK virus a non-enveloped, circular, double-stranded DNA virus a polyomavirus causes BK virus nephropathy hemorrhagic cystitis Epidemiology risk factors BK virus nephropathy kidney transplant hemorrhagic cystitis hematopoeitic stem cell transplant Pathogenesis latent in uroepithelial cell and renal tubular cells immunocompromised causes reactivation of the BK virus Prognosis in severe cases, renal graft may fail Studies Labs elevated creatinine nucleic acid amplification testing (NAAT) in blood or urine Biopsy renal biopsy definitive diagnosis of BK virus nephropathy interstitial inflammation, intranuclear inclusion bodies in tubules Differential Acute renal allograft rejection distinguishing factor elevated creatinine, new proteinuria, and oliguria Treatment Management approach reduction of immunosuppression medications if possible some medications may be helpful, though there is no clear evidence for its use cidofovir leflunomide Conservative supportive care modalities hydration bladder irrigation Complications Renal graft failure