Evaluation of iohexol clearance to estimate glomerular filtration rate in normal horses

In adult horses and foals, renal dysfunction can occur as a secondary complication to gastrointestinal disorders, dehydration, septicemia, endotoxemia and nephrotoxic drug administration. Measurement of renal function is an important feature not only in the diagnosis, but also in the prognosis and management of renal disease. Commonly used drugs such as phenylbutazone and gentamicin can be highly nephrotoxic under certain conditions.

Estimation of the glomerular filtration rate (GFR), accepted as one of the earliest and most sensitive assessments of renal function, can be determined in horses using standard techniques such as endogenous or exogenous renal creatinine clearance. These techniques can be time consuming, dangerous to perform on fractious patients, require trained personnel and are subject to errors most often associated with improper or incomplete urine collection. Recently, tests using iohexol, a radiographic contrast agent, have been developed to estimate the GFR in human beings, pigs, sheep, dogs, cats and horse foals with results that have been validated by traditional standards.

Serum clearance of a substance that is freely filtered by the kidneys without tubular secretion or reabsorption, that is not protein bound, and that is not metabolized, is a measurement of glomerular filtration rate. Iohexol meets all of these requirements and thus its clearance from serum should accurately estimate GFR. Utilization of serum clearance studies for estimation of GFR provides a clinically feasible and reproducible method in order to measure GFR in horses. Other commonly used methods to assess renal function in horses are fraught with inherent and operator error. Serum clearance of iohexol does not necessitate collection of urine and has been shown to be a safe, reproducible method using collection of timed blood samples to assess renal function in humans and animals. The objectives of this project were 1) to determine a method of estimation of GFR based on serum clearance of a substance that meets the requirements of a marker for GFR, and 2) to make the method clinically applicable by developing a method using two blood samples to derive clearance and thus GFR in normal adult horses.

Results of this study showed good agreement between GFR derived by exogenous creatinine clearance and serum clearance of iohexol. In addition, GFR values for all horses using either method were within published reference ranges for this species. The results of this study indicate that a single intravenous injection of iohexol at a dose of 150 mg/kg, followed by collection of 2 serum samples at 3 and 4 hours post injection can be used to estimate the GFR in healthy horses.