In many states, a suspect’s blood-alcohol concentration can be determined with urinalysis. This is consistently the least accurate of the three available methods of analysis. The reasons for this are basically two.First, the test is completely dependent on the subject voiding his bladder and then waiting 20 minutes for fresh urine to be secreted into the bladder for a more representative sample. And it is virtually impossible for an individual to completely void his bladder: There will usually be about 10cc of old urine left. This urine will combine with approximately 20cc of fresh urine produced during the wait, resulting in a sample that is one-third old urine — a sample that will contain alcohol from many hours before the subject was driving.
Second, urine often contains a yeast called Candida albicans. This organism has an interesting characteristic: it manufactures alcohol in the urine (caused by the interaction with glucose). This "immaculate conception" of alcohol in the bladder has been confirmed by numerous scientific studies. See, for example, "Bladder Beer — A New Clinical Observation", 95 Transactions of the American Clinical Climatological Association 34. To make things more interesting, Candida albicans is also unaffected by preservatives added by the police to urine specimens. In other words, alcohol will continue to be produced inside the evidence vial for days until it is finally analyzed at the crime lab.