Plasma exogenous creatinine clearance in clinically healthy cats: Comparison with urinary exogenous creatinine clearance, tentative reference intervals and indexation to bodyweight

- Plasma and urine clearance of exogenous creatinine are highly correlated.
- A tentative reference interval for the plasma clearance is provided.
- Concerns about arbitrary indexation of GFR to bodyweight in cats are raised.
- Associations between GFR, plasma creatinine and other covariables are assessed.
- Other relevant pharmacokinetic parameters of creatinine are described.

Glomerular filtration rate (GFR) is considered to be the best indicator of overall kidney function. The major objectives of this study were to compare plasma exogenous creatinine clearance (PECC) with a reference method, to establish reference intervals (RIs) for PECC and to assess the effects of indexation of GFR to bodyweight (BW) in cats. PECC was compared with urinary clearance of exogenous creatinine (UECC) in six clinically healthy domestic shorthair cats (experiment 1). Tentative RIs were determined according to current guidelines and the effects of indexation to BW and of covariables on GFR were assessed in 43 clinically healthy cats of various breeds (experiment 2).PECC was 15% higher than UECC (P <0.01), but the two estimates were strongly correlated (r2=0.97, P = 0.001). RIs for PECC were 6.4-21.3 mL/min or 1.2-4.9 mL/min/kg. The absolute (i.e. non-indexed) GFR value was not dependent on BW. Thus, indexation of GFR to BW in cats would not standardize the GFR value, but could introduce bias in clinical interpretation. Significant effects of breed, plasma protein concentration and plasma albumin concentration on GFR were demonstrated. Plasma concentrations of urea and creatinine, when assessed separately, were also weakly correlated with GFR in healthy cats. These combined findings contribute to a better understanding of renal function assessment in cats.