Breath alcohol measurement variability associated with different instrumentation and protocols.

Breath alcohol measurement has variability resulting from instrumental, procedural and biological components. Reliable estimates of the standard deviation (S.D.) are necessary for calculating uncertainty in the form of confidence intervals. These estimates are concentration dependant and can be obtained from models derived from duplicate breath test data. Duplicate data from four state jurisdictions (Alabama using the Drager 7110, Minnesota using the Intoxilyzer 5000, Washington using the BAC Datamaster and Wisconsin using the Intoximeter EC/IR) were analyzed to derive predictive models for standard deviation as a function of concentration. All jurisdiction/instrument combinations yielded forensically acceptable variation while showing a general linear increase in standard deviation with concentration. This is consistent with a multiplicative error model. Jurisdictions using the same instruments but not performing duplicate analyses could also employ the straightforward models derived here to estimate standard deviations for their measurement results.