Within the racing industry, there has been a large push to establish model rules for drug testing. The objective is to establish uniform testing within drug testing labs and to establish uniform regulatory thresholds and withdrawal time guidelines.

At the American Association of Equine Practitioners (AAEP) Convention held in Salt Lake City, Utah Dec. 6-10, 2014, Heather Knych, DVM, PhD, DACVCP, Associate Professor at the University of California’s School of Veterinary Medicine discussed experimental data used to establish thresholds and withdrawal times. The current ARCI (Association of Racing Commissioners International) recommended withdrawal times for intra-articular (IA) administration of corticosteroids as seven days for triamcinolone acetonide (TA), betamethasone and isoflupredone, and 21 days for methylprednisolone acetate.

These withdrawal times are based on previous pharmacokinetic (absorption, distribution, metabolism, and excretion of drugs) studies. For example, the seven day withdrawal time was based on a dose of 9 mg of triamcinolone acetonide administered into a single joint. It is important to note that withdrawal time recommendations apply to a specific route of administration. For instance, following IM administration of 0.1 mg/kg, TA can be detected for upwards of 40 days while concentrations fall below the accepted detection point by about four days following 9 mg of IA administration. In another study following administration of higher doses (20-30 mg) of IA triamcinolone, serum levels dropped below detection threshold within 5-6 days. In yet another study, 6 mg TA + 22 mg hyaluronic acid was injected into each tarsometatarsal hock joint; by 50 hours, triamcinolone dropped below the recommended regulatory threshold. In conclusion, Knych said, “The 7-day withdrawal time is acceptable when using triamcinolone under these conditions.”

In one study, methylprednisolone acetate administered at 100 mg into a single joint drops below the detection threshold level by 7-10 days in the blood but can remain in synovial fluid for up to 70 days in some horses. Another study administered methylprednisolone acetate at 100 mg into two joints. Following a washout period, three joints were injected with 200 mg of methylprednisolone acetate. The results: Methylprednisolone fell below the recommended regulatory threshold concentration within 5-7 days after administration in multiple joints