Correlation Between Urine Anion Gap and Urine Ammonia-Creatinine Ratio in Healthy and Kidney Disease Cats

Abstract: Background
Ammonium excretion decreases as kidney function declines in several species including cats, and may have predictive or prognostic significance related to chronic kidney disease (CKD). Urine ammonia measurement is not readily available in clinical practice, so urine anion gap (UAG) has been proposed as a surrogate test.

Objectives
The primary aim was to determine degree of correlation between urine ammonia to creatinine ratio (UACR) and UAG in healthy cats and CKD cats. A secondary aim was to evaluate if there was a significant difference between the UAG of healthy and CKD cats.

Animals
Previously collected urine samples from client-owned cats assessed to be healthy (n=59) or to have stable CKD (n=17) were thawed and processed.

Methods
Urine electrolytes were measured using a commercial chemistry analyzer. UAG was calculated as sodium + potassium - chloride. Urine ammonia and creatinine were previously measured using commercially available enzymatic assays and used to calculate UACR. Coefficient of determination (square of Pearson correlation coefficient) between UAG and UACR was calculated for both groups. Unpaired Student’s T-test was used to compare mean UAG of healthy versus CKD cats (p < 0.05).

Results
UAG had a very weak correlation with UACR in healthy (R² = 0.18) and CKD (R² = 0.07) cats. There was a significant difference between UAG in healthy and CKD cats (p< 0.002).

Conclusions and Clinical Importance
This calculation for UAG cannot be used as a substitute for UACR in cats. The clinical significance of the difference between healthy and CKD cats remains unknown.