Effect of laboratory and sample storage factors on urinary protein:creatinine ratios and clinical decision making in cats

BACKGROUND: Urinary protein:creatinine ratio (UPC) results affect the diagnosis, prognosis, and therapy of chronic kidney disease in cats. OBJECTIVES: To investigate the interlaboratory and intralaboratory variability and the effect of storage on UPC and International Renal Interest Society (IRIS) proteinuria substaging in cats. ANIMALS: Healthy and diseased client‐owned cats. METHODS: Prospective study. Urine of 60 cats was randomly sent to 4 (of 9) participating laboratories (to assess interlaboratory variability) and per cat, 2 laboratories each received 2 aliquots (to determine intralaboratory variability). Samples of 23 cats were analyzed in the same laboratory the day of collection, after preservation at 22°C for 1 day and at 4°C during 1‐7 days (short‐term storage) and at −24°C and −80°C for 6‐12 months (long‐term storage). Storage conditions were compared by equivalence testing. RESULTS: UPCs showed good interclass correlation (ICC‐inter, 0.90) and excellent intraclass correlation (ICC‐intra, 0.99). However, in 30/60 (50%) cats at least 1 of 4 laboratories assigned a different IRIS proteinuria substage. Urinary protein:creatinine ratio remained stable with short‐term storage, but not after 6 months storage at −24°C and after 12 months storage at −24°C or −80°C. Long‐term storage caused a change in IRIS proteinuria substage in 27% of cats, whereas a shift occurred only in 4% of cats during short‐term storage. CONCLUSIONS AND CLINICAL IMPORTANCE: Laboratory choice for UPC measurement can result in different IRIS substaging for the same cat, whereas urine storage at room temperature for 1 day or in the refrigerator for up to 7 days does not clinically affect UPC.