*Purpose: BK Polyomavirus nephropathy (BKVN) is an important cause of dysfunction and failure of renal transplants. BKV usually infects urinary tract retrogradely. The collecting duct and distal convoluted tubules in the renal medulla are often the earliest and the most extensive sites of kidney involvement, and this part of the tubule is related to the concentration function of urine. This study assessed the combined use of specific gravity of first morning urine and BK viruria to predict BKVN.

*Methods: We retrospectively enrolled kidney transplant recipients at the First Affiliated Hospital of Sun Yat-sen University from March 2015 to February 2018 including 24 patients (group A) with biopsy-proved BKVN, 22 patients (group B) with BK viruria but no BKVN, 18 patients (group C) with biopsy-proved T cell mediated rejection (TCMR) and 20 patients (group D) who had stable renal function and no BKV infection. Urine specific gravity was detected by dipstick testing which was carried out on Clinitek Atlas (Bayer Corp, Elkhart, USA). Specific gravity of first morning urine was compared among groups at diagnosis. In addition, the changes in urine specific gravity of each group at 0.5, 1, 3, 6, 9, 12, 15, 18, and 24 months after treatment were analyzed. ROC curve analysis was performed to analyze the predictive value of urine specific gravity for BKVN in patients with BK viruria.

*Results: At diagnosis, specific gravity of first morning urine of group A (1.009±0.003) was significantly lower than that of group B (1.019±0.004, P<0.001), group C (1.011±0.00, P=0.003) and group D (1.014±0.006, P=0.001) respectively. There was no significant difference in specific gravity among group B, group C and group D (P=0.253). In BKVN patients, after switching from tacrolimus to low-dose cyclosporine A, the decreasing trend of urinary viral loads and blood viral loads was consistent with the increasing trend of specific gravity of first morning urine, reaching statistical difference at 3 months after treatment (1.011±0.003, P=0.002) compared with those at diagnosis (figure 1). The specific gravity in group A was still significantly lower than those in group B, C and D at 0.5 and 1 months after treatment respectively, but was not significantly different from those at 3-6 months till 24 months after treatment (figure 2). The receiver operating characteristic analysis for BKVN by combining urine specific gravity and BK viruria (≥5000 copies/ml) reveals that the optimal cut-off is 1.009 to detect BKVN, with high sensitivity (66.7%), specificity (86.4%), and area under the ROC curve (0.844).

*Conclusions: Specific gravity of first morning urine is valuable for distinguishing patients with BKVN, TCMR and those with only BK viruria but no BKVN. Combination of specific gravity and urinary BK viral loads is an useful measurement for predicting BKVN in kidney transplant recipients with BK viruria.

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