At the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif., Keith Soring, DVM, presented material (on behalf of Tom Tobin, PhD, of the University of Kentucky's Gluck Equine Research Center) regarding withdrawal times and therapeutic thresholds of medications in horses. A threshold is a defined concentration of a "regulatory analyte" (an analyte being a chemical substance that is the subject of analysis) in blood or urine, and withdrawal timelines are linked to these thresholds. A drug's withdrawal time indicates a suggested timeline by which the last dose can be administered before an event to avoid blood or urine test levels that exceed the allowable regulatory threshold. Using these thresholds encourages standardization of post-competition drug testing and allows optimal use of therapeutic medications without violating the rules.

Soring explained that zero tolerance (no traceable drug allowed in a test sample) is an ever-changing standard because no chemist can test down to zero. He presented a real-life analogy, describing 1 nanogram as comparable to one second if you are 32 years old, while 1 picogram is comparable to one second if you are 32,000 years old.

Giving an example of a commonly used medication, he noted that 2 molecules of phenylbutazone (Bute) are a zeptogram (parts per sextillion) and a 3-gram dose of Bute generates more molecules than there are stars in the known universe. With a half-life (the time required for half the amount to be eliminated by natural processes) of 7.2 hours, it takes 21 days to eliminate all these molecules from the body following a single Bute injection. In contrast, the withdrawal time for Bute is usually 24 hours for racing competition, based on a particular threshold amount allowable in the plasma. Bute retains a therapeutic effect for only a day, although detectable traces remain in the blood for almost three weeks.

Soring recommended that the wide variations in thresholds eventually be brought under one umbrella so everyone is following the same rules and using the same labs with similar techniques for testing.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at or by calling 800/582-5604). She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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