A 35-year-old man who underwent renal transplantation eight months ago is found to have a serum creatinine level of 1.6 mg/dL (normal: 0.6-1.2), during routine follow-up. He is completely asymptomatic, and has maintained a stable weight for the last half-year. His two most recent clinic visits were six-and-a-half and seven months ago, during which serum creatinine levels were 0.95 mg/dL and 1.0 mg/dL respectively. He was diagnosed with IgA nephropathy at the age of 23 years, with subsequent progression into bilateral chronic kidney disease and end-stage renal-disease, necessitating deceased-donor kidney transplantation. There were no complications following surgery. He is currently on maintenance immunotherapy with tacrolimus, mycophenolate mofetil, and prednisone, with excellent compliance. His medical, surgical, family, and allergic histories are otherwise unremarkable, and he does not smoke, drink, or use recreational drugs. A complete blood count, electrolyte profile, fasting plasma glucose, and HbA1c are all within normal parameters. Blood urea nitrogen is 13 mg/dL (normal: 7-20). A urinalysis is also unremarkable, with dipstick testing for both nitrites and leukocyte estate being negative. However, urine cytology demonstrates multiple cells with large homogenous basophilic nuclear inclusions.
The ultrasound scan shows clear resolution between the cortex and medulla of the transplanted kidney. Color doppler studies show the renal artery and vein to both be patent, with homogeneous blood flow inside the kidney. There is no evidence of urinary tract obstruction.
Serologic tests for cytomegalovirus and Ebstein-Barr virus are negative. Plasma PCR for BK virus: 20,000 copies/mL (the 93% specific threshold for BK virus nephropathy: 10,000 copies/mL)
Tacrolimus level: 10 µg/L (therapeutic range: 5-15)
Histological examination reveals mild interstitial inflammation, but no evidence of tubular necrosis, atrophy, or fibrosis. Cytological analysis demonstrates multiple urothelial cells with intranuclear basophilic viral inclusions. Immunohistochemical stains are positive for cross-reacting SV40 large T antigen.