After alcohol is absorbed into the bloodstream of a given individual, it is distributed throughout all body fluids and tissues according to the water content of those fluids or tissues. At any given point in time the UAC will be considerably different from the BAC.
After the cessation of drinking, the BAC may rise for a period of time. At this point, the UAC will be less than the BAC because of absorption and distribution considerations. Thereafter, the BAC and UAC curves will cross. For some period of time the UAC will continue to rise, whereas the BAC will remain constant (plateau) or begin to decrease. In the fully post-absorptive state, the UAC will always exceed the BAC because of the higher water content for urine compared to that for blood. The peak BAC is generally reached within 30 to 90 minutes after the peak BAC is achieved.
Blood analysis is a direct method for the determination of a blood alcohol level and urine analysis is an indirect method. In other words, a blood sample analysis is the most accurate means to predict a blood alcohol level. To some extent a UAC can corroborate a BAC, but difficulties can arise with the use of a single urine sample because of pooled urine in the bladder. In living subjects, more than one urine sample should be obtained over a known time interval. Obviously in cases where death has occurred, the only sample that is available for analysis is the urine sample found in the bladder, which was formed prior to death.
Urine alcohol levels are far more reliable when two urine samples are collected about 0.5-1.0 hours apart and the bladder is completely emptied at the first void. This ensures that the urine sample collected at second void was formed within the period of time between the first and second void samples. The difference in UAC values between the first and second void provides information concerning the state of the UAC curve (rising or falling). In addition, the alcohol content of the second void represents the average alcohol concentration of the urine formed between the first and second void. When a urine sample is collected in this manner it is known that the UAC:BAC relationship is approximately 1.33:1. This means that the urine alcohol content will be 1.33 times greater than the blood alcohol content. Under these conditions a UAC of 133 mg% would equate to a BAC of 100 mg%. Because the urine is formed over a period of time, the predicted BAC based upon a UAC result refers to a blood alcohol level at some time prior to the collection of the urine sample.
When single urine samples are analyzed, a far greater range of values are reported in the scientific literature for the UAC:BAC ratio. One scientific study reported that the mean UAC:BAC ratio varied from 1.4-1.7:1, when the BAC of the subjects studied exceeded 50 mg%. When second specimens of urine were obtained approximately 60 minutes after an initial void, the mean UAC:BAC ratio was found to be 1.35:1. This study confirms the highly desirable feature of collecting two urine samples, when urine alcohol levels are used to predict a BAC level, as well as to assess the state of the blood alcohol curve (rising or falling) at a time interval of interest.
Excerpted from: http://www.forcon.ca/learning/alcohol.html