Furosemide concentration in plasma of a large population of race horses at 2.75-3.25 h (pre-race) and 4.5-5 h (post-race) after administration of known doses of furosemide, and the relationship of plasma or urine furosemide level with urine dilution were determined to devise procedures for the regulation of furosemide medication in race horses. This study indicated that (1) HPLC was more suitable than ELISA or PCFIA in quantitation of furosemide in plasma and urine, (2) > 90% of pre-race or post-race plasma samples had furosemide level < 30 ng/ml, (3) > 90% of the post-race urine sample had furosemide level ranging from 2 μg/ml to 20 μg/ml, (4) the horses which excreted dilute urine (creatinine value < 40 mg/100 ml) were the horses whose plasma furosemide levels were > 80 ng/ml, and (5) the urine (furosemide/creatinine) ratio was an important indicator of urine dilution due to furosemide medication in race horses. Furosemide use can be regulated by comparing plasma furosemide level with a pre-race "cut-off" value of 100 ng/ml or a post-race "cut-off" value of 80 ng/ml, and by determining urine creatinine and urine (furosemide/creatinine) ratio. Furosemide level > the "cut-off" value, urine creatinine level < 40 mg/100 ml, and urine (furosemide/creatinine) ratio > 0.15 should be considered violative and administrative action should be taken if all the criterion are met. An alternative "urine dilution" method for furosemide regulation has also been proposed in this study.
- Urine dilution