screening is early diagnosis and treatment of a particular condition, effective therapy must be available for asymptomatic individuals. In addition, there must be a test or measurement that distinguishes those individuals who are likely to have the condition from those who are unlikely to have it.3 Also to be considered is the availability of resources and facilities to collect and process test specimens, as well as to provide the range of follow-up services for affected individuals identified through screening. Provisions for evaluation must be incorporated into the initial design and development of the program. (See the discussion in Chapter 7 of the elements of a screening program.) Finally, the benefits of identifying and treating individuals early in the course of disease must be carefully weighed against the costs—both personal (e.g., clinical, psychological, and social ramifications for the individual) and societal (e.g., the actual costs of screening and subsequent medical evaluation and treatment, potential consumption or diversion of resources from other public health or social programs).

Technical Characteristics of a Screening Test and the HIV Testing Algorithm

Among the characteristics that must be considered in assessing the performance and utility of a test, particularly for screening purposes, are its sensitivity, specificity, and predictive value. Sensitivity is the ability of a test to detect the condition of interest in those individuals who truly have that condition. Specificity, on the other hand, reflects the ability of a test to exclude those who do not the condition. In the case of HIV infection, sensitivity refers to the probability that the test will be positive if infection is present; specificity indicates the probability that the test will be negative if infection is absent The predictive value of a test reflects false-positive and false-negative results. The predictive value of a positive test is the probability that an individual is infected, given that the test result is positive; the predictive value of a negative test is the probability that an individual is not infected, given that the test is negative. The predictive value of any test will vary according to the prevalence of the condition in the population being studied (Tables 3-1a and 3-1b). For example, the predictive value of a positive test diminishes with declining

3  

 A screening tool identifies individuals who are at increased risk of having a particular condition. A positive screening test result, therefore, does not necessarily signify the presence of the condition. Rather, it indicates the need for further evaluation of the individual through available diagnostic techniques.



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