The accuracy of drug tests for transit and other transportation workers is an important subject. Concepts of accuracy that are most relevant are applied to laboratory proficiency and transportation worker drug-usage data, and accuracy levels that could occur in transit drug testing programs are estimated. Seemingly accurate tests for abused drugs can sometimes be inaccurate to a disturbingly high degree. Therefore, a methodology by which decision makers can correctly set desired accuracy standards for their organizations is suggested to avoid potential inaccuracies.