Day-to-day variation of the urine protein: Creatinine ratio in female dogs with stable glomerular proteinuria caused by X-linked hereditary nephropathy

Background: Interpretation of serial urine protein: creatinine (UPC) values is confounded by a lack of data regarding random biologic variation of UPC values in dogs with stable glomerular proteinuria. Hypothesis: That there is minimal day-to-day variability in the UPC of dogs with unchanging proteinuria and the number of measurements needed to reliably estimate UPC varies with the magnitude of proteinuria. Animals: Forty-eight heterozygous (carrier) female dogs with X-linked hereditary nephropathy (XLHN) causing stable proteinuria. Methods: Urine samples were obtained daily by cystocentesis for 3 consecutive days on 183 occasions (549 samples). The UPC was measured for each sample with a single dry-film chemistry auto-analyzer. Data were analyzed retrospectively by a power of the mean model because the variance of UPC values within the 3-day evaluation periods increased as the magnitude of proteinuria increased. Results: To demonstrate a significant difference (P < .05) between serial values in these proteinuric dogs, the UPC must change by at least 35% at high UPC values (near 12) and 80% at low UPC values (near 0.5). One measurement is adequate to reliably estimate the UPC when UPC <4, but 2-5 determinations are necessary at higher UPC values. Conclusions and Clinical Importance: These guidelines for interpretation of serial UPC values in female dogs with XLHN may also be helpful for interpretation of UPC values in dogs with other glomerulopathies.