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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