The concentration of drugs and drug metabolites in urine samples of racing horses is strongly influenced by urine pH(Tobin, 1981), depending on whether the drugs are weak acids or weak bases. Drugs that are weak acids tend to concentrate in besic urine. In contrast, drugs that are weak bases tend to concentrate in acidic urine. These relationships have a well-established theoretical basis (the Henderson-Hasselbalch relationship) and have been demonstrated repeatedly in experimental animals and man (Tobin, 1981). More recently, evidence suggests that these relationships also occur with clinically and forensically significant agents in equine urine (Wood, et al. 1990; Gerken et al.1991.)

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Supported by grants entitled 'Development of a test for procaine in horses' and 'Thresholds and clearance times for therapeutic medications in horses' funded by the Equine Drug Council and the Kentucky Racing Commission, and by research support from the Horseman's Benevolent and Protective Association.

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