Evaluation of the analytical variability of urine protein-to-creatinine ratio in cats

BACKGROUND: Proteinuria quantification with the urinary protein-to-creatinine (UPC) ratio is part of the diagnostic process in feline patients suspected of chronic kidney disease (CKD). In affected cats, monitoring and substaging of the UPC according to the International Renal Interest Society (IRIS) guidelines is also necessary for appropriate patient management. No information is available about the possible effects of analytical variability on urinary proteins (UPs) and the UPC ratio in cats. OBJECTIVES: This study aimed to determine whether imprecision and method-dependent differences due to the two dye-binding methods, pyrogallol red-molybdate (PRM) and Coomassie brilliant blue (CBB), could affect IRIS substaging. METHODS: Urine samples were collected from proteinuric and nonproteinuric cats. Intra-assay and inter-assay repeatability were assessed with both the PRM and CBB methods. Urinary supernatants (n = 120) were tested using both methods. Agreement between the methods and concordance with sample classification according to IRIS guidelines were determined. RESULTS: On average, the PRM method yielded a higher CV (UP 8.4 ± 5.2%; UPC 9.5 ± 4.8%) than the CBB method (UP 5.6 ± 2.6%; UPC 7.2 ± 2.6%), but similar rates of misclassification were found in samples with UPC ratios close to the IRIS cut-off. Although the two methods were correlated, the CBB method tended to yield UPs and UPC ratios that were significantly higher (P < 0.0001) than that of the PRM method. The Passing-Bablok test also found constant and proportional errors between the PRM and CBB methods. Concordance in substaging samples according to IRIS was good (k coefficient = 0.62). CONCLUSIONS: The two methods were precise, but the higher UPC ratios obtained with the CBB methods might affect the interpretation using the IRIS guidelines and clinical decisions.