Noninvasive tool for the diagnosis of polyomavirus BK-associated nephropathy in renal transplant recipients.

Noninvasive methods can facilitate early diagnosis of BK virus (BKV) replication and guide the evaluation of BKV-associated nephropathy (BKVAN). We developed 3 noninvasive methods for BKVAN screening including quantitative polymerase chain reaction (PCR) assay for BKV DNA load in urine and plasma, and quantitative assay of urine cytology by light microscopy or electron microscopy, and used these assays concurrently with renal transplant biopsies for the evaluation of 338 patients. BKVAN was diagnosed in 24 (7.1%) of 338 renal recipients. The median level of the 3 methods was the highest in pattern B of BKVAN (P < 0.05). Using these 3 methods for pattern B of BKVAN yielded a high sensitivity of 100%. Using decoy cells without quantitation had a sensitivity of 95.8% and a specificity of 83.1% for BKVAN. The amount of decoy cells in urine samples was related to BKV DNAuria, BKV DNAemia, and the pattern of BKVAN. Using a decoy cell threshold of >5 per 10 high-power fields (HPF) had an ideal sensitivity and specificity for high-risk BKVAN and BKVAN. Using a decoy cell threshold of >20 per 10 HPF for BKVAN had a specificity of 99.7%. Quantitative assay of urine cytology is a very convenient and sensitive method for diagnosis of BKVAN, which can be deemed as an additional diagnostic method for quantitative PCR screening with increased accuracy.