HARDBACK
\$39.95

• #### Index 377-397

standard panel of prenatal blood tests with no additional blood samples, and there is no cost for office visits or counseling (since the testing is done in the context of prenatal care).

##### Predictive Value of Prenatal HIV Testing

The positive predictive value (PPV) of a test is the probability that an individual with a positive test result is truly infected with HIV. The key assumptions for this calculation are the following. First, a two-stage testing procedure is used, as described by Pins and colleagues (1997). One specimen is subjected to an initial ELISA test, and, if positive, to a second. If repeatedly positive, the same specimen is subjected to a confirmatory Western blot test. Second, the sensitivity of the repeated ELISA test is 100%, and the specificity is 0.999 (Pins et al., 1997). Third, the prevalence of HIV in pregnant women ranges from 1 per 10,000 to 100 per 10,000 (or 1%). This range parallels the range of values found in the 1994 Survey of Childbearing Women.

Table K.1 displays the number of true positives (women truly HIV-positive) and the number of positive ELISA tests that would result for every 10,000 pregnant women tested for a given HIV prevalence rate. The table also shows the PPV for a range of prevalence values. As is generally the case, the positive predictive value of the test is lower where the prevalence of HIV is also low. If the prevalence of HIV in the population tested is above 20 per 10,000 (as is the case in about seven states, the District of Columbia, and Puerto Rico), the PPV exceeds 67%. If the prevalence is as low as 2 per 10,000 (as is the case in Utah or Oklahoma), the PPV is only about 17%. This means that there is less than a one

TABLE K.1 Cost-Effectiveness of HIV Screening Incorporated into Prenatal Care

 Cost of ELISA (dollars) Per 10,000 Women Tested At \$5/Test At \$3/Test Prevalence TRUE Positives Positive ELISA PPV Total \$/True + Total \$/True + 0.0001 1 11 0.091 51,100 51,100 31,100 31,100 0.0002 2 12 0.167 51,200 25,600 31,200 15,600 0.0005 5 15 0.333 51,500 10,300 31,500 6,300 0.001 10 20 0.500 51,999 5,200 31,999 3,200 0.002 20 30 0.667 52,998 2,650 32,998 1,650 0.005 50 60 0.834 55,995 1,120 35,995 720 0.01 100 110 0.910 60,990 610 40,990 410 0.02 200 210 0.953 70,980 355 50,980 255 0.05 500 510 0.981 100,950 202 80,950 162

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