The withdrawal guidelines do not guarantee that a positive finding will not occur. There is always the possibility of an outlier – a horse that for some reason metabolizes medication more slowly than others – but the statistical analysis used in determining the thresholds is very robust and designed to minimize that risk. Moreover, if dosing guidelines for the medication or administration routes recommended are not followed, there is an increased chance of a positive finding.
Furthermore, the experiments performed to set these thresholds do have limitations. The experiments were performed using a specific dose of each medication and did not account for concurrent administration of other medications or other substances – which could affect the length of time required for the medication or its metabolite to fall below the regulatory threshold. Trainers and veterinarians are encouraged to perform their own risk assessment based upon the characteristics of the medication, its rate of clearance, and the dose and route of administration as well as the treatment of the horse with other substances. The use of compounded medications can also cause unintentional overages – even if the recommended dose is used based on label concentration – because in some cases compounded medications do not contain the label concentration of the active ingredient.
The RMTC withdrawal recommendations use the best information available at the time, but ultimately it is the responsibility of the trainer in consultation with his or her veterinarian to do the appropriate risk analysis based on the numerous factors that can effect drug elimination. Drug formulation, route of administration, multiple drug interactions, horse health, size, exercise activity, fitness and many other factors can impact drug eliminations and need to be considered when drugs are used in racing.