A Single Sample Method for Estimating Glomerular Filtration Rate in Cats

Background: Validated methods of estimating glomerular filtration rate (GFR) in cats requiring only a limited number of samples are desirable. Hypothesis/Objectives: To test a single sample method of determining GFR in cats. Animals: The validation population (group 1) consisted of 89 client-owned cats (73 nonazotemic and 16 azotemic). A separate population of 18 healthy nonazotemic cats (group 2) was used to test the methods. Methods: Glomerular filtration rate was determined in group 1 using corrected slope-intercept iohexol clearance. Single sample clearance was determined using the Jacobsson and modified Jacobsson methods and validated against slope-intercept clearance. Extracellular fluid volume (ECFV) was determined from slope-intercept clearance with correction for the 1 compartment assumption and by deriving a prediction formula for ECFV (ECFVPredicted) based on the body weight. The optimal single sample method was tested in group 2. Results: A blood sample at 180 minutes and ECFVPredicted were optimal for single sample clearance. Mean +/- SD GFR in group 1 determined using the Jacobsson and modified Jacobsson formulae was 1.78 +/- 0.70 and 1.65 +/- 0.60 mL/min/kg, respectively. When tested in group 2, the Jacobsson method overestimated multisample clearance. The modified Jacobsson method (mean +/- SD 2.22 +/- 0.34 mL/min/kg) was in agreement with multisample clearance (mean +/- SD 2.19 +/- 0.34 mL/min/kg). Conclusions and Clinical Importance: The modified Jacobsson method provides accurate estimation of iohexol clearance in cats, from a single sample collected at 180 minutes postinjection and using a formula based on the body weight to predict ECFV. Further validation of the formula in patients with very high or very low GFR is required.