Chemical Education Today
Letters Conversion of Serum-Alcohol Concentrations to Corresponding Blood-Alcohol Concentrations In a timely and pedagogically useful article in this Journal on the determination of blood-alcohol concentration (BAC), Zabzdyr and Lillard state that conversion of a serum-alcohol concentration (SAC) into the corresponding BAC requires the use of a conversion factor (1). The factor they cited, 1.14, is ¯ attributed to Payne apparently a mean SAC-to-BAC ratio ( R) et al. (2). Therefore, as noted by the authors, BAC = SAC/1.14. I would like to clarify and expand upon the significance of the conversion factor cited by Zabzdyr and Lillard (1). Ac¯ reported by Payne et al. (2) in their tually, the value of R study of 20 subjects is 1.18 (standard deviation [SD]: 0.057) and not 1.14, and it denotes the mean ratio of the plasmaalcohol concentration (PAC) to the BAC. Nevertheless, since the PAC and SAC are essentially identical (3), the two ratios can be used interchangeably. More important, however, is that ¯ to effect SAC-to-BAC conversions is inapthe sole use of R propriate for most subjects, given that R is variable (3, 4 ). For the data of Payne et al., the statistical range for 99% ¯ ± 2.58 SD) would be 1.03 to 1.33 (exof the population (R perimental range: 1.10–1.35). This is consistent with data having a normal distribution, which I have verified via the application of the Kolmogorov–Smirnov normality test to that data using StatView 4.5 (5). More recent studies involving larger population samples further confirm the variability of R. For example, Rainey (3) analyzed data obtained from 211 emergency department patients in a busy clinical laboratory setting. His results, derived from lognormal statistical analysis of the raw data, ¯ of 1.16 and a range of 0.90 to 1.49 for 99% of indicate an R the population (experimental range: 0.88–1.59). With regard to the lower limit of either of these ranges, Rainey notes that R values 99% is a well-accepted standard for scientific evidence.” Given the above arguments, I would point out that, in many instances involving DWI-induced motor vehicle accidents, the only relevant SAC data available stem from clinical laboratories. Therefore, use of the results of Rainey (3) would be appropriate in such cases if, as Rainey states, “the probability of a false allegation of legal intoxication” is to be reduced. On the other hand, if a police or other forensic laboratory is the source of SAC data in a particular case, then the results of Winek and Carfagna (6 ) and of Charlebois et al. (7 ), obtained under more controlled conditions than those of Rainey (3), would be more appropriate. In either situation, SAC-to-BAC conversions should not be restricted to determinations of mean BACs but should also include the upper and lower limits of the associated ranges for 99% of the population. Literature Cited 1. Zabzdyr, J. L.; Lillard, S. J. J. Chem. Educ. 2001, 78, 1225–1227. 2. Payne, J. P.; Hill, D. W.; Wood, D. G. L. Nature 1968, 217, 963–964. 3. Rainey, P. M. Clin. Chem. 1993, 39, 2288–2292. 4. Fitzgerald, E. F.; Labianca, D. A. In Fitzgerald, E. F. Intoxication Test Evidence, 2nd ed.; West Group: St. Paul, MN, 1999; Chapter 19. 5. StatView 4.5 for the Macintosh; Abacus Concepts, Inc.: Berkeley, CA 94704. 6. Winek, C. L.; Carfagna, M. J. Anal. Toxicol. 1987, 11, 267–268. 7. Charlebois, R. C.; Corbett, M. R.; Wigmore, J. G. J. Anal. Toxicol. 1996, 20, 171–178. Dominick A. Labianca Department of Chemistry Brooklyn College of The City University of New York Brooklyn, NY 11210
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JChemEd.chem.wisc.edu • Vol. 79 No. 7 July 2002 • Journal of Chemical Education
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