There are now a variety of rapid test methods available to assist in the diagnosis of the three most common infectious diseases seen in ambulatory medicine: pharyngitis, urinary tract infection, and venereal disease. Before choosing to implement any of these methods, several aspects should be carefully considered. The objectives met by onsite testing should be clearly identified and the appropriate test chosen to meet those objectives. For example, if the goal is to establish a diagnosis while the patient is still available in the office, a test method that requires more than 20 to 30 minutes to perform will not fulfill the objective. Similarly, if tests must be batched for maximum efficiency, timeliness of results will be compromised. The prevalence of the specific agent or disease in the patient population should be established, as prevalence will significantly influence the predictive value of a diagnostic test. The lower the prevalence of a disease, the lower the predictive value of a positive result and the higher the predictive value of a negative result. Conversely, the higher the prevalence, the higher the positive predictive value and the lower the negative predictive value. Thus, in one setting a test may serve best as a screen to rule out disease and, in another setting, as a confirmatory test to establish a diagnosis. The impact of false-positive and/or false-negative results must be clearly appreciated to avoid diagnostic errors. The technical features of a test method should be carefully appraised. How difficult is the test to perform and what skill level is necessary to ensure the validity of results? How much quality control is necessary? How much additional equipment is needed to perform the assay, and does the equipment have multiple applications or just a single application? Finally, does the cost-benefit ratio of providing on-site testing exceed that of referring the specimens to a reference laboratory?