|
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 apparently a mean SAC-to-BAC ratio (R bar) attributed to Payne 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). Actually, the value of R bar reported by Payne et al.(2) in their study of 20 subjects is 1.18 (standard deviation [SD]: 0.057) and not 1.14, and it denotes the mean ratio of the plasma-alcohol 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 the sole use of R bar to effect SAC-to-BAC conversions is inappropriate for most subjects, given that R bar is variable (3, 4).
For the data of Payne et al., the statistical range for 99% of the population ( R bar ± 2.58 SD) would be 1.03 to 1.33 (experimental 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 bar. 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, indicate an R bar of 1.16 and a range of 0.90 to 1.49 for 99% of the population (experimental range: 0.88–1.59). With regard to the lower limit of either of these ranges, Rainey notes that R bar values <1.0 are inconsistent with the theoretical premise that the SAC always exceeds the BAC. He attributes this anomaly to analytical variation in the SAC and BAC measurements comprising R bar, emphasizing that, where busy clinical laboratories are involved, multiple analysts and rapid turnaround rather than pinpoint precision are often the norm.
Prior to Rainey's work, Winek and Carfagna (6) evaluated 50 subjects, and as noted by Rainey (3), this study "was designed to achieve very high precision." Their data, consistent with a normal distribution (5), reflect an R bar of 1.14 and a range of 1.09 to 1.19 for 99% of the population (experimental range: 1.09–1.18). Moreover, the subsequent work of Charlebois et al. (7), also characterized by a high degree of precision and normally distributed data, involved 235 subjects. The associated R bar and the range for 99% of the population are, respectively, 1.14 and 1.04 to 1.24 (experimental range: 1.04–1.26).
As Zabzdyr and Lillard (1) aptly emphasize, students must "understand the implications of an incorrect determination (a suspect could go to jail)." Rainey (3) stressed a related point when he said, "To fully interpret an [SAC] within a legal context, one also should know the range of possible [BACs] that may correspond to a given [SAC] [where] a certainty of >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
- Zabzdyr, J. L.; Lillard, S. J. J. Chem. Educ. 2001, 78, 1225–1227.
- Payne, J. P.; Hill, D. W.; Wood, D. G. L. Nature 1968, 217, 963–964.
- Rainey, P. M. Clin. Chem. 1993, 39, 2288–2292.
- Fitzgerald, E. F.; Labianca, D. A. In Fitzgerald, E. F. Intoxication Test Evidence, 2nd ed.; West Group: St. Paul, MN, 1999; Chapter 19.
- StatView 4.5 for the Macintosh; Abacus Concepts, Inc.: Berkeley, CA 94704.
- Winek, C. L.; Carfagna, M. J. Anal. Toxicol. 1987, 11, 267–268.
- Charlebois, R. C.; Corbett, M. R.; Wigmore, J. G. J. Anal. Toxicol. 1996, 20, 171–178.
|
Citation
