Urinary protein:creatinine ratio (UPC) results affect the diagnosis, prognosis and therapy of chronic kidney disease in cats. This study aimed to investigate the interlaboratory and intralaboratory variability and the effect of storage on UPC and International Renal Interest Society (IRIS) proteinuria substaging in cats.
Researchers enrolled healthy and diseased client-owned cats in the study. The urine of 60 cats was randomly sent to four (of nine) participating laboratories (to assess interlaboratory variability). Per cat, two laboratories each received two aliquots to determine intralaboratory variability.
Researchers analyzed the samples of 23 cats in the same laboratory the day of collection, after preservation at 22°C for one day and at 4°C during one to seven days (short-term storage) and at -24°C and -80°C for six to 12 months (long-term storage). Storage conditions were compared by equivalence testing.
UPCs showed good interclass correlation (ICC-inter, 0.90) and excellent intraclass correlation (ICC-intra, 0.99). However, in 30 of 60 (50%) cats at least one of four laboratories assigned a different IRIS proteinuria substage.
Urinary protein:creatinine ratio remained stable with short-term storage, but not after six 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.
In conclusion, laboratory choice for UPC measurement can result in different IRIS substaging for the same cat, whereas urine storage at room temperature for one day or in the refrigerator for up to seven days does not clinically affect UPC.
Femke Mortier, et al. “Effect of laboratory and sample storage factors on urinary protein:creatinine ratios and clinical decision making in cats.” J Vet Intern Med. 2023 Apr 4. doi: 10.1111/jvim.16696.
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