Estimation of the probability for exceeding thresholds of urine specific gravity and plasma concentration of furosemide at various intervals after intravenous administration of furosemide in horses

Objective - To estimate the probability of concurrently exceeding thresholds for plasma concentration of furosemide and urine specific gravity after IV administration of furosemide in horses. Animals - 12 mature healthy Thoroughbred (n = 6) or Quarter Horse (6) mares. Procedure - Venous blood was collected from each horse prior to and 0.25, 0.5, 0.75, 1, 2, 3, 4, 4.5, 5, and 6 hours after IV administration of 250 mg (first experiment) or 500 mg (second experiment) of furosemide. Urine was collected hourly between 1 and 6 hours after administration of furosemide at both doses. Concentrations of furosemide were determined by use of an ELISA. Concentration of furosemide and urine specific gravity was modeled as a function of time, accounting for inter- and intrahorse variabilities. On the basis of pharmacokinetic and specific gravity data, the probability of exceeding a concentration of 100 ng of furosemide/ml as a function of time was determined, using a semiparametric smooth functional averaging method. A bootstrap approach was used to assess the inherent variation in this estimated probability. Results - The estimated probability of exceeding the threshold of 100 ng of furosemide/ml and urine specific gravity < 1.012 was approximately 0% between 4.0 and 5.5 hours after IV administration of 250 mg of furosemide/horse, and ranged from 0 to 1% between 4 and 5.5 hours after IV administration of 500 mg of furosemide/horse. The probability of a horse being falsely identified as in violation of regulatory concentrations was inversely associated with time. Conclusions and Clinical Relevance - Coupling plasma furosemide concentration with urine specific gravity testing will greatly reduce the chance that some horses are misclassified as being in violation of regulatory concentrations.