Development of an estimated glomerular filtration rate formula in cats

BACKGROUND: Estimated glomerular filtration rate (eGFR) formulas are routinely used in human patients to provide a more accurate evaluation of GFR compared to serum creatinine concentration alone. Similar formulas do not exist for cats. OBJECTIVES: To validate a prediction formula for eGFR in cats based on adjusting serum creatinine concentration. ANIMALS: Client‐owned cats with various levels of renal function. METHODS: The study was cross‐sectional. Glomerular filtration rate was determined by iohexol clearance. Variables including signalment, biochemical markers, and noninvasive measurements considered to represent surrogate markers of muscle mass were evaluated with the reciprocal of serum creatinine concentration in a multivariable regression model. The derived eGFR formula was subsequently tested in another group of cats and agreement with GFR assessed. RESULTS: The formula was developed in 55 cats. Only a single morphometric measurement (pelvic circumference) along with the reciprocal of serum creatinine concentration (creatinine(−1)) independently predicted GFR in the final multivariate model. The derived eGFR formula was 0.408 + (243.11 × creatinine(−1) [μmol/L]) ‐ (0.014 × pelvic circumference [cm]). When the formula was tested in another 25 cats it was not found to offer any advantage over creatinine(−1) alone in its relationship with GFR (eGFR, R (2) = 0.44, P < .001 vs reciprocal of creatinine, R (2) = 0.45, P < .001). Furthermore, agreement between eGFR and GFR was poor. CONCLUSIONS AND CLINICAL IMPORTANCE: An eGFR formula for cats that adjusted serum creatinine concentration for a marker of muscle mass was developed. The formula did not provide a reliable estimate of GFR, and therefore, its routine use cannot be recommended.