*Purpose: BK infection remains a risk factor for kidney transplant dysfunction and allograft loss. While many risk factors are reported in the literature, many are from historical cohorts and have never been validated in the contemporary era. The aim of this study was to explore risk factors for BK virus infection in the contemporary era of kidney transplantation and immunosuppression using a well-phenotyped clinical cohort.

*Methods: In this single-centre observational study, data was extracted from hospital informatics systems for all kidney allograft recipients transplanted between January 1^st2007 and June 30^th2018. BK virus was checked in the context of any transplant dysfunction (indication-based) aligned with UK guidance. Positive BK virus results were defined as >200 copies/ml. Clinical outcomes extracted from electronic hospital records were linked with national datasets. Univariate analysis was undertaken to investigate the association between donor-, recipient- and transplant-related variables and risk for BK virus reactivation. Any variable with a p-value <0.15 in univariate analysis was subsequently entered into a multivariate logistic regression model.

*Results: Data was analysed for 1,770 kidney transplant recipients with median follow up 5.3 years (IQR 2.7 to 8.7 years). BK virus was associated with (versus without); male sex (8.3% versus 5.3% respectively, p=0.010), ABO-incompatible transplantation (12.7% versus 5.9% respectively, p=0.021) and delayed graft function (9.0% versus 6.3% respectively, p=0.032). In a multivariate analysis, including all variables in univariate analysis with p-value <0.15, the only independent variables associated with BK infection were recipient male sex (OR 2.051 [95% CI 1.196-3.519], p=0.009 and ABO-incompatible transplantation (OR 4.087 {95% CI 1.847-9.042], p=0.001). From an outcome perspective, recipients with BK viraemia had worse graft function at 1-, 3- and 5-year post kidney transplantation but no increased risk for death-censored graft loss.

*Conclusions: In the contemporary era, only recipient male sex and ABO-incompatible kidney transplantation are independently associated with risk for BK infection. Study limitations include lack of data regarding novel risk factors (e.g. BK virus serostatus). Association does not imply causality and we suggest further work is warranted to validate contemporary risk factors for BK infection in different kidney transplant population cohorts.

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