Estimation of glomerular filtration rate via 2-and 4-sample plasma clearance of iohexol and creatinine in clinically normal cats

Objective-To compare 2 methods for estimation of glomerular filtration rate (GFR), study the effects of age and body size on GFR estimates, and provide a reference range for estimated GFR in clinically normal cats. Animals-57 cats. Procedures-In each cat, GFR was estimated via plasma clearance of iohexol and creatinine. Results of a 1-compartmental model (CL(1comp)) were calibrated to a trapezoidal method estimate (CL(trap)) by use of a correction formula applicable to dogs or humans and standardized to body weight; for iohexol clearance, data were also standardized to extracellular fluid volume (ECFV). For all 57 cats, method comparison was performed via agreement analysis. Reference ranges for GFR derived by the different methods were established by use of data from a subset of 51 cats after exclusion of 6 cats that were azotemic, Birman, or both. Results-In 57 cats, mean CL(trap) of creatinine was 0.29 mL/min/kg (13%) higher than CL(trap) of iohexol. In 51 nonazotemic cats, mean CL(trap) was 2.26 mL/min/kg for iohexol (reference range, 1.02 to 3.50 mL/min/kg) and 2.55 mL/min/kg for creatinine (reference range, 1.27 to 3.83 mL/min/kg). Values of GFR/kg or GFR standardized to liters of ECFV did not decrease with increasing age. A negative linear relationship was detected between body weight and estimated GFR/kg or GFR standardized to liters of ECFV. Conclusions and Clinical Relevance-Reference ranges for estimated GFR via plasma clearance of iohexol and creatinine should facilitate early detection of impaired renal function in cats, although body weight should be taken into account. (Am J Vet Res 2009;70:176-185)