Factors Influencing Viral Clearing and Renal Function During Polyomavirus BK-Associated Nephropathy After Renal Transplantation

Background. The course of BK virus nephropathy (BKVN) is difficult to predict. Methods. Between 2008 and 2010, we diagnosed BKVN in 46 (5.5%) of 859 patients with transplant biopsies by simian virus 40 (SV40) staining and routine serum polymerase chain reaction. We measured the influence of different variables on glomerular filtration rate (Delta GFR increasing or decreasing) and the time for viral polymerase chain reaction reduction by 1 log (<= 13 or >13 weeks). At diagnosis, we either reduced calcineurin inhibitor (CNI) and mycophenolate mofetil by 30% to 50% (n=23), or we switched from CNI to mammalian target of rapamycin (mTOR) inhibitor (n=7) or from CNI to mTOR inhibitor as a second step in patients with protracted viral reduction (n=16). Results are the following: GFR stabilized or increased in 61% of patients and decreased in 39% (graft failure, 15%). Viral reduction by 1 log was rapid in 54% (<= 13 weeks) and slow in 46% (>13 weeks). Rapid viral reduction was associated with stable or increasing GFR (84%), compared with slow viral reduction (33%; P=0.0004). High peak viral load, tacrolimus treatment, and late diagnosis (biopsy for cause vs. protocol biopsy) had a negative influence on GFR and viral reduction time. Defining 1-log viral load reduction as an event, tacrolimus compared with cyclosporine was associated with slow viral reduction (P=0.0043). In 88% of patients with slow viral reduction, the secondary switch from CNI to mTOR inhibitor favored viral load decrease. Conclusions. We conclude that peak viral load, tacrolimus treatment, delayed diagnosis, and viral reduction time influence outcomes in patients with BKVN.