Serum and urine nucleic acid screening tests for BK polyomavirus-associated nephropathy in kidney and kidney-pancreas transplant recipients

Background: BK polyomavirus-associated nephropathy (BKPyVAN) occurs when BK polyomavirus (BKPyV) affects a transplanted kidney, leading to an initial injury characterised by cytopathic damage, inflammation, and fibrosis. BKPyVAN may cause permanent loss of graft function and premature graft loss. Early detection gives clinicians an opportunity to intervene by timely reduction in immunosuppression to reduce adverse graft outcomes. Quantitative nucleic acid testing (QNAT) for detection of BKPyV DNA in blood and urine is increasingly used as a screening test as diagnosis of BKPyVAN by kidney biopsy is invasive and associated with procedural risks. In this review, we assessed the sensitivity and specificity of QNAT tests in patients with BKPyVAN.

Objectives: We assessed the diagnostic test accuracy of blood/plasma/serum BKPyV QNAT and urine BKPyV QNAT for the diagnosis of BKPyVAN after transplantation. We also investigated the following sources of heterogeneity: types and quality of studies, era of publication, various thresholds of BKPyV-DNAemia/BKPyV viruria and variability between assays as secondary objectives.

Search methods: We searched MEDLINE (OvidSP), EMBASE (OvidSP), and BIOSIS, and requested a search of the Cochrane Register of diagnostic test accuracy studies from inception to 13 June 2023. We also searched ClinicalTrials.com and the WHO International Clinical Trials Registry Platform for ongoing trials.

Selection criteria: We included cross-sectional or cohort studies assessing the diagnostic accuracy of two index tests (blood/plasma/serum BKPyV QNAT or urine BKPyV QNAT) for the diagnosis of BKPyVAN, as verified by the reference standard (histopathology). Both retrospective and prospective cohort studies were included. We did not include case reports and case control studies.

Data collection and analysis: Two authors independently carried out data extraction from each study. We assessed the methodological quality of the included studies by using Quality Assessment of Diagnostic-Accuracy Studies (QUADAS-2) assessment criteria. We used the bivariate random-effects model to obtain summary estimates of sensitivity and specificity for the QNAT test with one positivity threshold. In cases where meta-analyses were not possible due to the small number of studies available, we detailed the descriptive evidence and used a summative approach. We explored possible sources of heterogeneity by adding covariates to meta-regression models.

Main results: We included 31 relevant studies with a total of 6559 participants in this review. Twenty-six studies included kidney transplant recipients, four studies included kidney and kidney-pancreas transplant recipients, and one study included kidney, kidney-pancreas and kidney-liver transplant recipients. Studies were carried out in South Asia and the Asia-Pacific region (12 studies), North America (9 studies), Europe (8 studies), and South America (2 studies).

Index test: blood/serum/plasma BKPyV QNAT The diagnostic performance of blood BKPyV QNAT using a common viral load threshold of 10,000 copies/mL was reported in 18 studies (3434 participants). Summary estimates at 10,000 copies/mL as a cut-off indicated that the pooled sensitivity was 0.86 (95% confidence interval (CI) 0.78 to 0.93) while the pooled specificity was 0.95 (95% CI 0.91 to 0.97). A limited number of studies were available to analyse the summary estimates for individual viral load thresholds other than 10,000 copies/mL. Indirect comparison of thresholds of the three different cut-off values of 1000 copies/mL (9 studies), 5000 copies/mL (6 studies), and 10,000 copies/mL (18 studies), the higher cut-off value at 10,000 copies/mL corresponded to higher specificity with lower sensitivity. The summary estimates of indirect comparison of thresholds above 10,000 copies/mL were uncertain, primarily due to a limited number of studies with wide CIs contributed to the analysis. Nonetheless, these indirect comparisons should be interpreted cautiously since differences in study design, patient populations, and methodological variations among the included studies can introduce biases. Analysis of all blood BKPyV QNAT studies, including various blood viral load thresholds (30 studies, 5658 participants, 7 thresholds), indicated that test performance remains robust, pooled sensitivity 0.90 (95% CI 0.85 to 0.94) and specificity 0.93 (95% CI 0.91 to 0.95). In the multiple cut-off model, including the various thresholds generating a single curve, the optimal cut-off was around 2000 copies/mL, sensitivity of 0.89 (95% CI 0.66 to 0.97) and specificity of 0.88 (95% CI 0.80 to 0.93). However, as most of the included studies were retrospective, and not all participants underwent the reference standard tests, this may result in a high risk of selection and verification bias.

Index test: urine BKPyV QNAT There was insufficient data to thoroughly investigate both accuracy and thresholds of urine BKPyV QNAT resulting in an imprecise estimation of its accuracy based on the available evidence.

Authors' conclusions: There is insufficient evidence to suggest the use of urine BKPyV QNAT as the primary screening tool for BKPyVAN. The summary estimates of the test sensitivity and specificity of blood/serum/plasma BKPyV QNAT test at a threshold of 10,000 copies/mL for BKPyVAN were 0.86 (95% CI 0.78 to 0.93) and 0.95 (95% CI 0.91 to 0.97), respectively. The multiple cut-off model showed that the optimal cut-off was around 2000 copies/mL, with test sensitivity of 0.89 (95% CI 0.66 to 0.97) and specificity of 0.88 (95% CI 0.80 to 0.93). While 10,000 copies/mL is the most commonly used cut-off, with good test performance characteristics and supports the current recommendations, it is important to interpret the results with caution because of low-certainty evidence.