Field-portable test methods may be quantitative, semi-quantitative, or qualitative and screening methods are often used in the field to determine if the concentration of a toxic substance exceeds regulatory or recommended standards or action levels. For on-site analysis, accurate quantitative tests for field measurements may not be available, depending on the analyte(s) or specific field situation. Thus, in lieu of more definitive test methods, screening tests which are based on qualitative or semi-quantitative methods are often used for making immediate decisions in the field, e.g. for compliance or risk assessment. Also, quantitative methods may be used for screening purposes in many instances. To ensure the quality of these screening tests and the decisions that are made based upon their results, screening methods need to be evaluated with sufficient data and should meet basic performance criteria prior to their being employed for decision-making purposes. Although quantitative, semi-quantitative and qualitative methods demonstrate different characteristics, it is desired that the performance criteria for all three method categories be consistent. If there is consistency, then one can have a sound basis for selecting the most appropriate test(s) for a given application. In order to unify the performance criteria for the different types of methods, a performance function is used to characterise both qualitative and semi-quantitative methods; in turn, this performance function is related to that for quantitative methods. False negative rates, false positive rates, sensitivity and specificity are key characteristics of screening methods that can be determined from the pertinent performance curves. The performance characteristics of each method are related to the uncertainty region that is associated with each method and the applicable uncertainty regions can be gleaned from the performance curves. Also, various options for using multiple test results to improve decisions based on test results are provided.