Urea nitrogen, creatinine and symmetric dimethylarginine (SDMA) are indirect measurements of glomerular filtration rate (GFR). More accurate measurements of GFR require administration of compounds that are ideally freely filtered and not reabsorbed by the tubules. Examples include exogenous creatinine, radiolabeled compounds, inulin and the contrast medium, iohexol. Typically a bolus is given intravenously, followed by sequential measurements of the compound in blood to evaluate clearance (see review by von Hendy-Willson and Pressler 2011 on merits of each, Heiene and Lefebrve 2013 [International Renal Interest Society, IRIS]).
Iohexol
Reported normal values for clearance are:
- Dogs: Weighted mean of 3.1 mL/kg/min, with 95% confidence intervals of the mean of 2.5-3.6 mL/kg/min (von Hendy-Willson and Pressler review).
- Cats: Weighted mean of 2.5 ml?kg/min, with 95% confidence intervals of 2.2-2.9 L/kg/min (von Hendy-Willson and Pressler review).
- Horses: Results vary depending on the study, ranging from 2-3.3 mL/kg/min (Wilson et al 2009, Meucci et al 2015, Lippe et al 2019). In one study, horses 20-23 years of age had similar clearance (1.9-2.5 mL/kg/min) to horses 14 years old or younger (2-2.8 mL/kg/min), all with normal creatinine concentrations (Lippe et al 2019).
- Donkeys: Appear to have slightly lower GFR than horses (but n=8 per group), ranging from 1.3-2.0 mL/kg/min (Meucci et al 2015)
Decreased GFR is generally considered to be <1.5 mL/kg/min (Heiene and Lefebrve 2013) (a lower cut-off may be applicable to donkeys)
