Variability of UAC to BAC Ratio

Even if urine could be tested immediately upon leaving the kidneys, the concentration of alcohol in the urine would not be equal to the concentration of alcohol in blood. The two concentrations are only proportional, not equal. In order to derive BAC from UAC, a ratio must be determined and used (i.e., UAC * ratio = BAC). Most experts agree that the UAC-to-BAC ratio for most persons is between .67 and .83, although the ratio for some persons is outside of this range. It is apparent that even if the effects of urine pooling in the bladder are lessened by double or triple voiding, individual variations in the UAC-to-BAC ratio make urine testing an unreliable indicator of BAC. It should be noted that the ratios expressed above are in terms of BAC/UAC. Some statutes and commentators express the UAC-to-BAC ratio in terms of UAC/BAC, in which case the range of ratios for most persons varies from 1.2 to 1.5.

Because there is no generally-accepted UAC-to-BAC ratio, different states have adopted different ratios for purposes of determining BAC from urine testing. In California and West Virginia, for example, the UAC-to-BAC ratio is .77 (or 1/1.3). In Montana the ratio is .753, while in Minnesota the ratio is .67. Suppose that a person is subjected to urine testing and is found to have a UAC of .14 percent. In California and West Virginia the person would be found to have a BAC of .11 percent (.14 * .77), while in Minnesota the same person would be found to have a BAC of .09 percent (.14 * .67). This example illustrates how disagreement about the variability of the UAC-to-BAC ratio, even among the states, relegates urine testing to be an unreliable indicator of BAC.

Dr. Kurt Dubowski, for three decades the nation’s foremost authority on chemical testing, in DUI-DWI cases, has opined that blood alcohol concentration estimates from pooled urine are unreliable.

Copyright 2015. William C. Head. All Rights Reserved.