Control of the misuse of bromide in horses

Abstract

Bromide is a sedative hypnotic. Due to its potential use as a sedative or calmative agent in competition horses, a method to control bromide is needed. Colorimetric method had been employed in the authors' laboratory from 2003 for the semi-quantification of bromide in equine plasma samples. However, the method was found to be highly susceptible to matrix interference, and was replaced in 2008 with a more reliable inductively coupled plasma-mass spectrometry (ICP/MS) method. Equine plasma was protein-precipitated using trichloroacetic acid, diluted with nitric acid, and then submitted directly to ICP/MS analysis. Since bromide is naturally occurring in equine plasma, a threshold is necessary to control its misuse in horses. Based on population studies (n = 325), a threshold of 90 µg/mL was proposed (with a risk factor of less than 1 in 10 000). Using the ICP/MS screening method, equine plasma samples with bromide greater than 85 µg/mL would be further quantified using the more accurate ICP/MS standard addition method. Confirmation of bromide was achieved by gas chromatography-mass spectrometry (GC-MS), with the bromide detected as its pentafluorobenzyl derivative. A sample is considered positive if its plasma bromide concentration exceeds the threshold (90 µg/mL) plus the measurement uncertainty of the quantification method (8 µg/mL at 99% 1-tailed confidence level) and its presence is confirmed using the GC-MS method. Following oral administration of potassium bromide (60 g each) to two geldings, plasma bromide levels peaked after approximately 2 hours at about 300 µg/mL, and then remained above the threshold for 8 and 13 days respectively.