in five chance that a women with a repeatedly positive ELISA test is truly infected with HIV. Note that these rates apply to repeated ELISA testing only. When the original blood samples are subjected to Western blot confirmatory testing, most of the false positive results would test negative. Some fraction of Western blot results are indeterminate (depending on the testing procedure used and the laboratory), but some of these indicate an early-stage infection (Pins et al., 1997).
There have been numerous economic evaluations of HIV testing and treatment in pregnancy, each with different assumptions and different specific questions. Taken as a group, however, they generally establish the cost-effectiveness of prenatal HIV screening and treatment programs.
Mauskopf and colleagues (1996) have estimated the economic impact of treating pregnant women who are HIV-positive with ZDV (zidovudine), and have found that such treatment is cost saving over a wide range of assumptions. They further find that voluntary prenatal HIV screening programs are cost saving if the prevalence exceeds 4.6 per 1,000 (under certain assumptions). Under the assumption that the prevalence rate is 1.7 per 1,000 (the national average), the cost per case avoided of a voluntary screening program with comprehensive counseling and 100% acceptance is $155,000. The same program with limited pre-test counseling is actually cost saving.
In their base case analysis, assuming a prevalence rate of 1.7 per 1,000 in pregnant women, Gorsky and colleagues (1996) find that implementation of the Public Health Service (PHS) counseling and testing guidelines nationally would prevent 656 pediatric HIV infections annually and would result in a medical care cost saving of $105.6 million. Varying the maternal seroprevalence rate, they find that screening is cost saving as long as the prevalence rate is above 1.1 per 1,000.
Myers and colleagues (1998) have determined the cost-effectiveness of mandatory versus voluntary prenatal HIV screening. They conclude that mandatory screening will prevent more cases of pediatric AIDS, but at a somewhat higher cost than voluntary screening. Under their base assumptions, including a maternal seroprevalence rate of 1.7 per 1,000, the cost per case averted was $255,000 for mandatory screening and $367,000 for voluntary screening. The incremental cost-effectiveness of mandatory compared with voluntary screening was $29,500.
Two additional assumptions are needed for this calculation. First, the marginal cost of including an HIV test in the standard prenatal panel is $3 to $5. Costs of testing vary markedly according to the circumstances in which the