*Purpose: Polyoma infection due to BK virus remains an important cause of allograft failure following renal transplantation.Intensive monitoring of urine and serum for BK virus (BKV) during the first year post-transplant and pre-emptive reduction of immunosuppressive therapy are associated with resolution of viremia, avoidance of clinical BKV nephropathy (BKVN) and low risk of acute rejection. We investigated factors associated with development of BKVN following BKV screening.

*Methods:

All renal transplant patients in our unit are screened for BKVN by urine cytology, plus serum BKV PCR in those patients with positive urine cytology. Screening is performed fortnightly to 3 months, monthly to 6 months, then bimonthly up to a year. We retrospectively reviewed all patients (n=43) with biopsy-proven BKVN during the period 2005-2017 and a matched cohort (n=55) with BK viremia and no clinical evidence of BKVN.

*Results:

The median age of patients with and without BKVN was 51 and 49 years respectively. Diabetes was significantly associated with BKVN, with 16 (16%) developing BKN compared to 4 (7%) who cleared BKV (p<0.001). BK viruria was detected earlier in the BKVN group, with a median (IQR) time to positive screening of 60 (50-75) days, compared to 82 (56-121) days (p<0.005) for non-BKVN patients. Creatinine was higher in the BKVN group at the time of first positive screening, median (IQR) creatinine 164 (140-223) versus 121 (96-148) for non-BKVN patients (p<0.005). Tacrolimus trough levels were similar, but the median (IQR) immunosuppressive (IS) index at the time of first positivity was 7 (5.9-8.5) in the BKN group versus 5.5 (4.5-6.8) in those who cleared BKV (p<0.005). Median viral load was also higher in the BKVN group, 3125 copies (705-21500) versus 585 copies (134-6260) in non-BKVN patients.

*Conclusions:

BKVN occurred after developing BK viruria/viremia despite reduction in immunosuppression. Diabetes and high IS index at the time of first BKV detection are risk factors for progression from viraemia to clinical BKV nephropathy.

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