Correlation between urine anion gap and urine ammonia-creatinine ratio in healthy cats and cats with kidney disease

Background: Ammonium excretion decreases as kidney function decreases in several species, including cats, and may have predictive or prognostic value in patients with chronic kidney disease (CKD). Urine ammonia measurement is not readily available in clinical practice, and urine anion gap (UAG) has been proposed as a surrogate test. Objectives: Evaluate the correlation between urine ammonia-to-creatinine ratio (UACR) and UAG in healthy cats and those with CKD and determine if a significant difference exists between UAG of healthy cats and cats with CKD. Animals: Urine samples collected from healthy client-owned cats (n = 59) and those with stable CKD (n = 17). Methods: Urine electrolyte concentrations were measured using a commercial chemistry analyzer and UAG was calculated as ([sodium] + [potassium]) - [chloride]. Urine ammonia and creatinine concentrations had been measured previously using commercially available enzymatic assays and used to calculate UACR. Spearman's rank correlation coefficient between UAG and UACR was calculated for both groups. The UAG values of healthy cats and cats with CKD were assessed using the Mann-Whitney test (P < .05). Results: The UAG was inversely correlated with UACR in healthy cats (P < .002, r(0) = -0.40) but not in cats with CKD (P = .55; r(0) = -0.15). A significant difference was found between UAG in healthy cats and those with CKD (P < .001). Conclusions and clinical importance: The UAG calculation cannot be used as a substitute for UACR in cats. The clinical relevance of UAG differences between healthy cats and those with CKD remains unknown.