The effect of non-absorbent hydrophobic sand litter on the urine protein-to-creatinine ratio in feline urine

Background Proteinuria can be quantified through the measurement of the urine protein-to-creatinine ratio (UPC). Voided urine samples in cats are often exposed to a non-absorbable litter substrate prior to collection and urinalysis. Little is known about the effect exposure to such substrates has on pre-analytical variability of UPC measurements. Objectives The aim of this study was to assess agreement between UPC measurements from urine obtained by cystocentesis before and after exposure to non-absorbent hydrophobic sand for 24 hours. Methods UPCs were measured in 40 urine samples obtained by cystocentesis from 39 cats (baselineUPC). Urine was then exposed to non-absorbent hydrophobic sand litter for 24 hours, recovered, and repeat UPCs were measured (litterUPC). Agreement between paired measurements and the presence of any bias or error was evaluated using Bland-Altman analysis Passing-Bablok regression analysis, respectively. Cohen's kappa was used to measure agreement for the International Renal Interest Society (IRIS) proteinuria classification of samples. Observed total error (TEobs) was calculated for the laboratory analyzer and compared against absolute percentage changes in paired UPC measurements. Results Neither proportional nor constant error was identified using Passing-Bablok regression between baselineUPC and litterUPC. Visual inspection of the Bland-Altman plot revealed good agreement, with 95% of paired measures falling within the limits of agreement (LOA). Cohen's kappa demonstrated almost perfect agreement for the IRIS classification of proteinuria between baselineUPC and litterUPC. Absolute percentage changes of paired UPC measurements outside of the LOAs were lower than the inter-assay TEobs. Conclusions Feline urine exposed to non-absorbent hydrophobic sand litter appears acceptable for UPC measurements.