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

Abstract

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.

Comparison of cystocentesis versus home sampling to determine urinary protein: Creatinine ratio and urine specific gravity in cats

“…In 25% of study cats, a higher proteinuria substage was assigned to the same cat when cystocentesis was performed. It is unlikely that protein binding to the nonabsorbent cat litter caused a decrease in UPC in voided urine, because studies have shown that UPC is not affected by contact with nonabsorbent cat litter 21,22 . The increased storage time (1‐15 hours) of voided urine compared to cystocentesis samples also is not thought to be the cause of lower UPC results in voided urine, because a study in cats has shown that the UPC remains stable after storage of urine for 1 day, even at room temperature 23 .…”

A review of Bland–Altman difference plot analysis in the veterinary clinical pathology laboratory

“…Current ASCVP guidelines for method comparison recommend pairing regression evaluation with BA‐plot analysis 9 . The method is commonly used in the veterinary clinical pathology laboratory and published in Veterinary Clinical Pathology manuscripts 10–14 . Others have noted that the composition and interpretation of BA‐plots are not always appropriate 7,8,15 .…”

“…9 The method is commonly used in the veterinary clinical pathology laboratory and published in Veterinary Clinical Pathology manuscripts. [10][11][12][13][14] Others have noted that the composition and interpretation of BA-plots are not always appropriate. 7,8,15 This work seeks to review the common…”

Biological variation of urinary protein: Creatinine ratio and urine specific gravity in cats

“…[5][6][7][8][9] Recent studies have investigated preanalytical and analytical factors influencing these urinary variables, but information on biological variation of these biomarkers in cats is lacking. [10][11][12][13][14][15] This situation is in contrast to dogs, for which 1 study reported the RCV to be 35% at high UPC values (near 12) and 80% at low UPC values (near 0.5). 16 Previous articles stated that serial UPC values in cats need to differ by 90% to conclude that the level of proteinuria has increased, and that the goal of treatment in case of proteinuria is to achieve a > 90% decrease in UPC.…”

“…Several studies in cats have focused on the biological variation of serum and plasma analytes, but not on UPC or USG 5‐9 . Recent studies have investigated preanalytical and analytical factors influencing these urinary variables, but information on biological variation of these biomarkers in cats is lacking 10‐15 . This situation is in contrast to dogs, for which 1 study reported the RCV to be 35% at high UPC values (near 12) and 80% at low UPC values (near 0.5) 16 .…”