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Screening versus Anonymous Testing

Screening high-risk groups is a useful tool in disease control if it is combined with treatment and contact tracing. Such screening will help identify truly high-risk groups and provide information about the patterns of spread of the disease. Anonymous testing of volunteers does not accomplish these goals; it does not give prevalence or incidence data because there is no way to know the numerator or the denominator. If there are 100 tests done and 10 are positive, one does not know if 100 people were tested and 10 of them were positive or if 10 people were tested 10 times and only 1 of them was positive.

Calculating the costs of screening for HIV infection must include the value of information about the spread of a new disease. With an established disease such as syphilis, which is at equilibrium in the population, the probability of finding the disease in a given population is known. This allows a meaningful calculation of the cost-benefit ratio for case finding. The prevalence of HIV is unknown in almost all populations. While the cost of finding a case in a given population may be high, this may be offset by the value of knowing the prevalence in that population.

The problem of the delay between initial infection and the presence of detectable antibodies has been greatly overstated. The development of antibodies appears to be an exponential process. Most persons develop detectable antibodies within six months, but a small number will go for years without testing positive on antibody-based tests.[148] This is termed the window period. Screening a given population will not ensure that all HIV carriers are detected, but it will detect much in excess of 90 percent of carriers. Shifting to direct tests for viral components such as the polymerase chain reaction will allow detection within a short period of infection.[149] This will narrow the window period to the point where screening will identify over 99 percent of infected persons.[150]

[148]Imagawa DT et al.: Human immunodeficiency virus type 1 infection in homosexual men who remain seronegative for prolonged periods. N Engl J Med 1989; 320(22):1458.

[149]Eisenstein BI: The polymerase chain reaction: A new method of using molecular genetics for medical diagnosis. N Engl J Med 1990; 322(3):178.

[150]Daar ES; Moudgil T; Meyer RD; Ho DD: Transient high levels of viremia in patients with primary human immunodeficiency virus type 1 infection. N Engl J Med 1991; 324(14):961.

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