Three chemical tests are most frequently used. These are the estimation of the alcohol content of the (1) blood, (2) urine, and (3) breath. Each of these shows a reasonably accurate estimation of the degree of intoxication provided certain precautions are observed. Unfortunately, under some circumstances the results of these tests may be misleading and be invalid in evidence. In order to appreciate these limitations it is necessary to understand the physiology of the absorption and excretion of alcohol in the body.
Philip Jones, Medical Aspects of Chemical Tests for Intoxication, 8 Clev.-Marshall L. Rev. 228 (1959)