Background: Glomerular filtration rate (GFR) decreases in the aging human kidney, but limited data exist in dogs. Hypothesis: There is an effect of age and body size on estimated GFR in healthy dogs. Animals: One hundred and eighteen healthy dogs of various breeds, ages, and body weights presenting to 3 referral centers. Methods: GFR was estimated in clinically healthy dogs between 1 and 14 years of age. GFR was estimated from the plasma clearance of iohexol, by a compartmental model and an empirical correction formula, normalized to body weight in kilograms or liters of extracellular fluid volume (ECFV). For data analysis, dogs were divided into body weight quartiles 1.8-12.4, 13.2-25.5, 25.7-31.6, and 32.0-70.3 kg.Results: In the complete data set, there was no trend toward lower estimated GFR/kg or GFR/ECFV with increasing age. GFR decreased with age in dogs in the smallest weight quartile only. A significant negative linear relationship was detected between body weight and estimated GFR/kg and GFR/ECFV. Reference ranges in different weight quartiles were 1.54-4.25, 1.29-3.50, 0.95-3.36, and 1.12-3.39 mL/min/kg, respectively. Standardization to ECFV rather than kilogram body weight did not produce substantial changes in the relationships between GFR estimates and age or weight.Conclusions and Clinical Importance: Interpretation of GFR results for early diagnosis of renal failure should take into account the weight and the age of the patient for small dogs.

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Use of iohexol and inulin yielded similar but not identical results for plasma clearance. Accuracy for limited-sample methods would be acceptable for many clinical and research situations.

Plasma iohexol and creatinine clearances can be used interchangeably for screening patients suspected of having chronic kidney disease (ie, low GFR), but large differences may exist for dogs with a GFR within or above the reference range.