*Purpose: To describe the characteristics of patients presenting with late BK viremia (>1 year after transplant) and compare to early BK viremia

*Methods: This study was approved by the Institutional Review Board. Between Jan 2010 and Dec 2016, 523 recipients had a kidney transplant at our institution with a minimum of 1 year follow up. All patients were monitored for BK viremia per center protocol. For patients with positive serum BK PCR, their immunosuppression dose was reduced. We reviewed complete information about recipient’s immunosuppressive medications, type of induction, age, etiology of kidney disease, dialysis modalities, cold and warm ischemic times, gender, ethnicity, donor and recipient CMV serostatus, and biopsy-proven acute rejection. We compared recipients developing BK viremia early [within 12 months after transplant] vs late [more than 12 months after transplant]. We described BK viremia in regard to time to first onset of BK viremia, time to clearance of BK viremia, and duration of ongoing BK viremia.

*Results: The initial analysis included 523 patients, 130 of whom developed BK viremia and 22 who developed BK viruria alone. Viruria alone was excluded from further analysis. Upon analysis of baseline characteristics between the early and late viremia groups, we could not detect any differences in age, race, sex, type of induction, maintenance immunosuppression or donor type between early and late groups. There was no difference between mean serum creatinine, time to clear BK virus nor median peak viral load. Those that developed late BK were less likely to clear the infection: clearance rates were 82% vs 59%(p=0.000175). We noted only one incidence of acute rejection (6%) after reduction of immunosuppression in the late BK viremia group, vs 19 after reduction in the early BK viremia group (16%).

*Conclusions: While we could not detect any differences in characteristics between patients presenting with BK viremia early vs late in their post transplant course, we noted that patients who presented late had a lower incidence of clearing the infection, possibly due to late detection, and a lower incidence of rejection after immunosuppression reduction. Our data suggests that patients should be monitored beyond one year for BK viremia, although the clinical significance of late BK viremia is not fully understood.

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