There is a perceived tradeoff between the ease of measuring alcohol in the body and the accuracy of the result. Direct tests of blood alcohol concentrations are considered the most accurate, desktop stationary breath testers based on electro-chemical infra-red technology are slightly less accurate, but accepted for evidentiary purposes in most jurisdictions, and quick portable breath testers based on fuel-cell technology are the easiest to administer but not acceptable in many courts. This study compared the accuracy of an evidentiary portable breath tester and an evidentiary desktop breath tester relative to blood alcohol concentrations. Inverse regressions were used to obtain confidence limits for the alcohol levels as read by the breath testers that would provide tradeoffs of false positives and false negatives for three levels of confidence: 95%, 96%, and 98%; corresponding to false positive values of 2.5%, 2%, and 1%, respectively. A decision tree model is offered for the optimal use of the three measures, so that portable breath testers can be sufficient for high level BrAC, stationary breath testers can be sufficient for medium level BrAC, and blood tests are recommended for still lower BrACs. The model provides quantitative BrAC threshold levels for the two most common BAC levels used to imply DWI: 50mg/dl and 80mg/dl.