Estimating the measurement uncertainty in forensic breath-alcohol analysis

The evidentiary weight attributed to forensic breath alcohol results in drunk-driving prosecutions requires that measurement uncertainty be established and shown to be fit-for-purpose. The principal components contributing to breath alcohol measurement uncertainty include: (1) biological/sampling, (2) instrumental, (3) traceability and (4) the water/air partition coefficient for control standards. Employing duplicate breath results from over 92,000 subjects to estimate the biological/sampling component and assuming reasonable forensic values for the other components, the combined and expanded uncertainty is determined for a practical example. The combined uncertainty for an unbiased single determination breath alcohol measurement was: . Employing the expanded uncertainty (k = 2.58), the 99% confidence interval for a mean breath alcohol concentration of 0.0935 g/210 L was 0.0866 to 0.1004 g/210 L. The proportion of combined uncertainty associated with each component was determined to be: biological/sampling 73%, analytical 10%, traceability 13% and water/air partition coefficient 4%. These are forensically acceptable estimates and demonstrate fitness-for-purpose of breath alcohol measurement when employing appropriate elements of quality control.