Monitoring of BK virus replication in the first year following renal transplantation

Background. BK virus-associated nephropathy (BKVAN) is one of the most common viral diseases affecting renal allografts. Screening for viral replication may allow for earlier intervention with reduced allograft loss. A plasma viral load > 10(4) copies/mL of BKV DNA is recommended for a presumed diagnosis of BKVAN. Methods. We monitored BKV load on serum and urine samples by Real-Time TaqMan PCR in 229 renal transplant recipients in the first year post-transplantation. Overall, 2025 serum and 2025 urine samples were evaluated. A graft biopsy was performed in 47/229 patients to investigate the declining renal function. Operating characteristics [sensitivity, specificity, negative predictive value (NPV), positive predictive value (PPV)] and receiver operating characteristic (ROC) curve analysis at different viral load values were calculated. Results. Serum BKV viral load was > 10(4) in 5/229 patients (2.2%). A histological diagnosis of BKVAN was made in 3/229 patients (1.3%): 3/5 (60.0%) among those with serum viral load > 10(4) and 3/4 (75.0%) in those with > 1.6 x 10(4). Operating characteristics of a serum BK load of 10(4) for the diagnosis of BKVAN were as follows: sensitivity, 100%; specificity, 99.1%; NPV, 100%; PPV, 59.4%. Specificity and PPV rose to 99.6% and 75.0% when using a cut-off level of 1.6 x 10(4) copies/mL. Conclusions. The recommended level of BK viraemia of 10(4) copies/mL is useful to identify patients at risk of BKVAN, although specificity and PPV increase by using a cut-off level of 1.6 x 10(4) copies/mL. BK replication may occur in the first 3 months post-transplantation and subsequently recede. Therefore, the temporal profile of BKV replication has to be accurately evaluated and occasionally elevated values should prompt a closer monitoring.