*Purpose: BK viremia is endemic among kidney transplant recipients (KTR). Previous work has not compared the incidence, risk factors, and outcomes of any vs. clinically significant BK viremia in KTR in the modern era.

*Methods: This retrospective cohort study examined KTR transplanted between January 1, 2009 and June 30, 2017, with follow-up until June 30, 2018. BK viral load was determined with quantitative PCR of serum, where any positive result was considered any BK viremia and clinically significant viremia was >10³ copies/mL. The cumulative incidence of BK viremia was assessed via the Kaplan-Meier method. Risk factors and outcomes for BK viremia were explored in Cox proportional hazards models. The impact of BK viremia on graft function was examined using mixed linear models. Adjustments to immunosuppression were collected for any first BK viremia event.

*Results: Among 1193 KTR, there were 344 and 244 cases of any and clinically significant BK viremia over 3466.81 and 3815.82 person-years of follow-up, respectively. Significant risk factors for any BK viremia included recipient age (HR 1.02 [95%CI: 1.01, 1.03]) and donor age (HR 1.01 [95%CI: 1.00, 1.02]); for clinically significant BK viremia, risk factors included recipient age (HR 1.02 [95%CI: 1.01, 1.03]), Caucasian recipient race (HR=0.70 [95%CI: 0.52, 0.95]), non-depleting induction therapy (HR 0.61 [95%CI: 0.42, 0.89]), and delayed graft function (HR 0.61 [95%CI: 0.42, 0.88]). The mean estimated glomerular filtration rate was 4.32 mL/min/1.72 m² (95%CI: 2.92, 5.71) and 5.81 mL/min/1.72 m² (95%CI: 1.65, 9.97) lower for KTR with any BK viremia and clinically significant BK viremia, respectively. No immunosuppression was adjusted for almost all (95.9%) cases of viremia <10² copies/mL. The most common adjustment was a reduction in the dose of anti-proliferative agents. Antiviral agent use was limited to cases of viremia >10⁴ copies/mL.

*Conclusions: Recipient age predicts an increased risk of any and clinically significant BK viremia. Caucasian recipient race and delayed graft function were negatively associated with clinically significant BK viremia. BK viremia was linked to lower graft function. While most adjustments for clinically significant BK viremia were to anti-proliferative agents, clinical responses beyond the first positive BK viremia test, and their implications for graft outcomes, merit further investigation.

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