LAUSR

Arsenic can be easily found in our surrounding environment. Due to its ubiquitous nature, horse urine and blood invariably contain low levels of arsenic. Nevertheless, inorganic arsenic, despite its general use as a tonic for horses, is an effective doping agent having a deleterious effect due to its ability to induce gastroenteritis. The misuse of arsenic in horseracing has been controlled by an international urinary threshold of total arsenic at 0.3 μg/mL. However, an equivalent threshold for total arsenic in plasma is yet to be established. In this paper, an inductively coupled plasma-mass spectrometry method has been developed for quantifying total arsenic in equine plasma. Statistical analysis determined that the data from a population study of 1552 post-race and out-of-competition plasma samples fits a Gaussian mixture model with two Gaussian components. A rounded-up provisional threshold for plasma total arsenic at 2.5 ng/mL was subsequently established. Results from administration trials with a sodium arsanilate-containing supplement showed that both urinary and plasma arsenic were significantly elevated after administration. The maximum urinary detection time was around 22 hours based on the international threshold. However, the maximum plasma detection time would be longer than 73 hours if the provisional threshold of 2.5 ng/mL adopted. In view of the high discrepancy between the urine and plasma detection times, a revised plasma threshold of 15 ng/mL is proposed to afford a comparable detection time in both matrices. The risk of a normal sample exceeding the proposed plasma total arsenic threshold is practically zero.