Calculation and Verification of Blood Ethanol Measurement Uncertainty for Headspace Gas Chromatography

Abstract

An estimate was made of the measurement uncertainty for blood ethanol testing by headspace gas chromatography. While uncertainty often focuses on compliance to a single threshold level (0.08 g/100 mL), the existence of multiple thresholds, related to enhanced sentencing, subject age, or commercial vehicle licensure, necessitate the use of an estimate with validity across multiple specification levels. The uncertainty sources, in order of decreasing magnitude, were method reproducibility, linear calibration, recovery, calibrator preparation, reference material, and sample preparation. A large set of reproducibility data was evaluated (n = 15,433) in order to encompass measurement variability across multiple conditions, operators, instruments, concentrations and timeframes. The relative, combined standard uncertainty was calculated as ±2.7%, with an expanded uncertainty of ±8.2% (99.7% level of confidence, k = 3). Bias was separately evaluated through a recovery study using standard reference material from a national metrology institute. The uncertainty estimate was verified through the use of proficiency test (PT) results. Assigned values for PT results and their associated uncertainties were calculated as robust means (x*) and standard deviations (s*) of participant values. Performance scores demonstrated that the uncertainty estimate was appropriate across the full range of PT concentrations (0.010-0.370 g/100 mL). The use of PT data as an empirical estimate of uncertainty was not examined. Until providers of blood ethanol PT samples include details on how an assigned value is obtained along with its uncertainty and traceability, the use of PT data should be restricted to the role of verification of uncertainty estimates.

Measurement Uncertainty in Forensic Toxicology: Its Estimation, Reporting and Interpretation

“…Indeed, there are surely other components to be considered. (Sklerov and Couper, 2011) These must be identified by the forensic toxicologist considering their particular laboratory, protocol, instruments, customers and the required fitness-forpurpose.…”

Variability of blood alcohol content (BAC) determinations: The role of measurement uncertainty, significant figures, and decision rules for compliance assessment in the frame of a multiple BAC threshold law

“…[19][20][21][22] This study is not strictly based on the ISO Guide to the expression of uncertainty in measurement (GUM), [23] which describes the so-called bottom-up approach (combining individual sources of variances), but rather, a holistic view of the measurement process was maintained, investigating sources of combined variances including pre-analytical as well analytical processes, and quantifying their combined contribution to the overall measurement uncertainty. [19][20][21][22] This study is not strictly based on the ISO Guide to the expression of uncertainty in measurement (GUM), [23] which describes the so-called bottom-up approach (combining individual sources of variances), but rather, a holistic view of the measurement process was maintained, investigating sources of combined variances including pre-analytical as well analytical processes, and quantifying their combined contribution to the overall measurement uncertainty.…”

“…[19][20][21][22] This study is not strictly based on the ISO Guide to the expression of uncertainty in measurement (GUM), [23] which describes the so-called bottom-up approach (combining individual sources of variances), but rather, a holistic view of the measurement process was maintained, investigating sources of combined variances including pre-analytical as well analytical processes, and quantifying their combined contribution to the overall measurement uncertainty. [19] Moreover, when assessing compliance to specification limits for forensic purposes, significant figures and rounding errors become relevant topics as they directly affect the final result. [24][25][26] In addition, multiple BAC thresholds may require individual uncertainty estimates at each level, as these tend to vary with concentration.…”

“…Several authors have previously discussed with different approaches the issue of measurement uncertainty in BAC determinations. [19][20][21][22] This study is not strictly based on the ISO Guide to the expression of uncertainty in measurement (GUM), [23] which describes the so-called bottom-up approach (combining individual sources of variances), but rather, a holistic view of the measurement process was maintained, investigating sources of combined variances including pre-analytical as well analytical processes, and quantifying their combined contribution to the overall measurement uncertainty. Similar approaches are mainly covered in the third edition of the EURACHEM/CITAC Guide Quantifying Uncertainty in Analytical Measurement and other handbooks.…”

Comments on: Uncertainty of Blood Alcohol Concentration (BAC) Results as Related to Instrumental Conditions: Optimization and Robustness of BAC Analysis Headspace Parameters. Chromatography 2015, 2, 691–708

“…Insufficient empirical data is provided to appropriately estimate the uncertainty of measurement (UOM) for a quantitative measurement, although the authors referenced a publication in which an appropriate approach is described [2]. Historically, an UOM was not routinely calculated and/or reported in a forensic toxicology laboratory as stated by the authors, but currently many laboratories have been accredited to the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) 17025 and American Society of Crime Laboratory Directors/Laboratory Accreditation Board (ASCLD/LAB) Supplemental standards which require an estimation of the UOM to be performed for BAC determinations and weight measurements near statutory limits [3].…”