WHY URINE TESTING IS PREFERRED TO BLOOD TESTING
Tobin, T. (1983). Pre-race testing and its role in equine medication control. Equine Veterinary Journal, 15, 54-55.
"In general, blood is the only material on which a practical pre-race testing scheme can be based. Blood testing is not as sensitive as urine testing and detects only about 66 per cent of the drugs detectable in urine. Therefore, pre-race blood testing is always performed in conjunction with post-race urine testing. Because blood is easily and rapidly drawn, the use of blood samples in all post race testing schemes is recommended. Pre-race testing is also a relatively expensive proposition, but it is the only method which actually prevents the running of an illegally medicated horse".
Implication. Urine reveals more drugs than does blood.