What is BAC and Pooled Urine

The concentration of alcohol in a person’s blood varies over time, depending on the relative rates of absorption and elimination. Therefore, the concentration of alcohol in urine produced by the kidneys also varies over time. However, because all urine produced by the kidneys is pooled in the bladder, the urine in the bladder at any particular time represents a pooled mixture of all urine produced since the bladder was last voided. Thus, urine produced from high BAC blood is mixed with urine produced from lower BAC blood, and vice versa. It only follows, then, that the longer the period of time since the bladder was last voided, the less likely it is that the urine in the bladder will accurately indicate the person’s true BAC at the time the urine sample is obtained.

Because the urine in the bladder represents a mixture of all urine produced since the bladder was last voided, whether the pooled urine overestimates or underestimates a person’s BAC at the time of sampling depends on when the bladder was last voided and whether the person was in the absorptive or elimination phase of the alcohol distribution process at the time of sampling. If a person has not voided his or her bladder since the commencement of drinking and is in either the absorptive or equilibrium phase at the time of sampling, a sample of pooled urine is likely to underestimate the person’s BAC because the bladder contains urine that was produced when the person had little or no alcohol in his or her blood.

On the other hand, if, as more often happens, a person’s BAC has peaked and he or she is in the elimination phase of the alcohol distribution process at the time of sampling, a sample of pooled urine is likely to overestimate the person’s BAC at the time of sampling, especially if the person has voided his or her bladder since the commencement of drinking but not since attaining peak BAC. The reason a urine sample taken under these conditions is likely to overestimate a person’s BAC is that the sample contains urine that was produced when the person’s BAC was higher than at the time of sampling. The longer the period that sampled urine has been in the bladder since peak BAC, the more likely it is to overestimate the person’s BAC at the time of sampling.

In a reported test, a person consumed alcohol and did not void his bladder for six hours thereafter. A urine sample taken at the expiration of the six-hour period indicated a BAC of .16 percent for the person, while a blood sample taken at the same time indicated a BAC of only .08 percent. In another reported test, a person consumed 250 milliliters of whiskey upon going to bed at 10 P.M. and did not urinate until 8 A.M. the next morning. A urine sample taken at that time indicated a BAC or .11 percent, while a blood sample taken at the same time indicated a BAC of .00 percent. While these tests may represent extreme examples of the effects of urine pooling in the bladder, they clearly indicate the unreliability of testing pooled urine.
To lessen the effects of urine pooling, most experts require a person to completely void his or her bladder prior to sampling and to void again within 30 minutes thereafter for the purpose of obtaining a urine sample. Some experts require two complete voidings prior to sampling. Also, the shorter the period between voidings, the more likely the sampled urine is to indicate the person’s actual BAC at the time of sampling. The administrative rules or regulations in some states contain urine sampling requirements that must be complied with in order for a urine test result to be admissible in evidence. Defense counsel should, of course, be familiar with such requirements because if they were not complied within a particular case the test results are likely to be held inadmissible upon the motion or objection of defense counsel.

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