Study: Stall Contamination Affects Metformin Positives

A Thoroughbred racehorse is likely to show a positive test result for metformin if its groom is taking the diabetes medication and urinates in the horse's stall, according to a study done by the School of Veterinary Medicine at the University of California-Davis. The test results reported by the study—at three different metformin concentrations in urine and from samples taken at a half-hour after exposure up to seven days after—appear to support the Horseracing Integrity and Safety Authority's proposed minimum reporting level for metformin to weed out contamination cases and zero in on intentional administration. Metformin is a banned substance, according to HISA's Anti-Doping and Medication Control Program because the medication has no recognized therapeutic use in racehorses. The ADMC program started in 2022 with minimum standards of 0.5 nanograms per milliliter in blood and 1 ng/ml in urine for metformin, so any test result above these levels triggered an adverse analytical finding and a potential penalty. A series of metformin positives in 2024, however, and anecdotal connections in many of these cases to grooms and other handlers around these horses being prescribed this medication led the Horseracing Integrity and Welfare Unit—the enforcement arm of HISA—to put a stay on any enforcement until further scientific review could be conducted. HIWU has nine cases pending from 2024-25. In November 2025, HISA announced a proposed minimum reporting level for metformin of 4 ng/ml in blood to reflect a level more consistent with intentional administration that also aims to minimize the reporting of positives caused by inadvertent exposure. This new standard is currently being reviewed by the ADMC Committee and, if approved, will be sent on to the Federal Trade Commission for final adoption. HISA does not have a minimum testing threshold for metformin in urine testing. "Due to metformin's demonstrated erratic elimination behavior in urine and an inconsistent relationship between urine and blood concentrations, the RMTC's Scientific Advisory Committee recommended to HISA that laboratories perform confirmatory analysis in blood only," HIWU said in a statement regarding the thresholds. The importance of the minimum reporting level is underscored by the UC-Davis study. The university used six exercised Thoroughbred horses in its study, two mares and four geldings of ages 3 to 5. The horses were exposed to urine spiked with metformin that was then spread over shavings in one corner of their stalls. Three different concentrations of metformin were used: 10, 135, and 270 micrograms per milliliter and two horses were exposed to each level of concentration. These concentrations were derived from unpublished data of concentrations in people who are taking 1,000 mg doses of metformin. Researchers then spread 800 ml of the prepared urine over clean shavings. Solid waste was spot-cleaned but the urine-soaked shavings were left. Blood samples were then taken from these horses at intervals starting at a half-hour after exposure and then at one, two, four, six, eight, 12, 24, 30, 36, 48, 72, 96, 120, 144, and 168 hours after exposure. Urine samples were taken from all six horses after four, 24, 48, 72, 96, 120, 144, and 168 hours after exposure. The study used a 0.25 ng/ml level of quantitation for serum and plasma and a 0.3 ng/ml level for whole blood. The level of quantitation for urine was 0.5 mg/ml. Level of quantitation is the lowest level at which a substance cannot only be detected in a sample, but also have the exact amount measured and identified. At the lowest concentration of metformin treated urine (10 micrograms/ml), one horse tested above HISA's previous threshold of 0.5 ng/ml in blood at 30 hours (34.6 ng/ml) and 48 hours (0.99 ng/ml) after exposure. The 34.6 ng/ml was identified by the study as an outlier to all other test results with no apparent explanation behind this spike. The study's conclusion did note that metformin results "pulsed" in blood tests, going from no detection for several hours, to a couple of positive tests, and then back to no detection. Researchers speculated that, as in humans, red blood cells serve as a compartment for metformin that then gets released over time into serum or plasma. In the urine tests, all six horses showed some level of metformin at four hours after exposure and were all above the 0.5 ng/ml level of quantitation after 24 hours. The four horses exposed to either 135 micrograms/ml or 270 micrograms/ml concentrations never had their urine test levels fall below 1.25 ng/ml and most had results of greater than 3.0 ng/ml through seven days after exposure. "Results from the current study demonstrate that exposure to metformin via contaminated bedding can result in detection of the drug in biological samples collected from horses," the study concluded. "Data from this contamination study, combined with concentrations from horses knowingly administered metformin, can be used to establish a screening limit that accounts for inadvertent exposure through contamination, thus reducing the likelihood of AAF (adverse analytical finding) due to inadvertent exposure." One trainer to date as been sanctioned because of a metformin positive. Trainer Jonathan Wong was suspended two years and fined $25,000 after his horse Heaven and Earth tested positive in June 2023 with 0.631 ng/ml of metformin in its blood and 242 ng/ml in its urine, according to HIWU documents. Wong contested the test results as contamination and also challenged the chain of custody procedure used by Industrial Laboratories in Denver. Wong at first claimed he was the source of the contamination because he had been taking metformin, but then retracted that position and took a polygraph exam to prove he did not know the source of the contamination. The results of the polygraph were later dismissed because of questions about the credential of the person conducting the exam. Wong then told investigators he learned from a colleague that a former groom had been taking metformin. According to arbitration panel records, Wong could not provide credible proof that the groom was taking the medication and had been urinating in the horse's stall. The trainer provided what he said was a sworn statement from the groom, but the veracity of the statement could not be verified. Ultimately, the statement was not considered credible and viewed as an 11th hour "contrivance" by the arbitration panel. According to Equibase records, Wong continued to train horses in Louisiana during his suspension. Louisiana is a jurisdiction that does not adhere to HISA rules because of an on-going legal challenge of the authority's constitutionality. Wong's runners earned more than $1.6 million racing throughout 2025. His HISA suspension ended July 1, 2025.