Errors of converting a urine alcohol value into a blood alcohol level.

Abstract

A blood alcohol determination is one of the more frequently requested analyses in a toxicology or forensic chemical laboratory. There are many reliable methods for determining the concentration of alcohol in the blood. It is the purpose of this communication to show that it is, however, not reliable to determine the concentration of alcohol in the urine and report it as a blood alcohol level. This is not reliable even using the best of average factors of equivalence. These factors used are an average o f many determinations, some of which show very wide ranges from the mean. Random specimens of urine and blood were collected from 148 cases examined for alcohol content. An average urine-blood alcohol ratio of 1.28:1, with a range of 0.21 to 2.66, was obtained. The blood alcohol level was calculated in each case from the corresponding urine alcohol determination by means of the average ratio obtained from our data. In 32 (21.5 %) of the cases, the blood figures calculated from the urine value exceeded the actual level determined in blood. In 51 cases (34.5 %) the calculated blood alcohol concentration was below the determined value. In 65 cases (44 %) the values corresponded. This procedure was repeated using the conversion factor (1.25:1) employed in some communities. In this instance, the calculated blood alcohol concentration exceeded the actual value in 39 cases (26.5%). In 49 cases (33%) the calculated value was below the observed level, and in 60 cases (40.5%) the values corresponded. In view of the wide ranges in the individual urine-blood alcohol ratios found in most published reports, we find it hard to understand how so many investigators can conclude that it is a satisfactory procedure to calculate the alcoholic content of blood, to the second decimal place, from a selected specimen of urine. Our data clearly confirm what other investigators2,7–10 have claimed: that the relationship (ratiorange) between the concentrations of alcohol in urine and in blood may vary widely. This renders it unreliable to use an average conversion factor in medicolegal cases.