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Predictive Value of Laboratory Tests

Predictive value (PV) theory deals with the usefulness of tests as defined by their clinical sensitivity (ability to detect a disease) and specificity (ability to define the absence of a disease).

The problems addressed by PV theory are false-negative and false-positive test results. Both are major considerations in interpreting the results of screening tests in general and neonatal screening tests in particular.

Testing for HIV seroreactivity illustrates some of these considerations. If it is assumed that approximately 1,100,000 of 284,000,000 residents of the USA are infected with HIV (prevalence = 0.39%) and that 90% of those infected demonstrate antibodies to HIV, then we can consider the usefulness of a simple test with 99% sensitivity and 99.5% specificity. If the entire population of the USA were screened, it would be possible to identify most of those infected with HIV.

However, there will be 119,900 false-negative test results. Even with 99.5% specificity, the number of false-positive test results would be larger than the number of true-positive results:

In addition, there will be 281,480,000 true-negative results.

Given the high cost associated with follow-up and the anguish produced by a false-positive result, it is easy to see why universal screening for HIV seropositivity received a low priority immediately after the introduction of testing for HIV infection.

By contrast, we can consider the screening of 100,000 individuals from groups at increased risk for HIV in whom the overall prevalence of disease is 10%, with all other considerations being unchanged.

These 2 hypothetical testing strategies show that the diagnostic efficiency of testing depends heavily on the prevalence of the disease being tested for, even with a superior test, such as the test for HIV antibodies. Because the treatment of pregnant women infected with HIV is effective in preventing vertical transmission of the infection, screening has now been expanded to all pregnant women. The proven effectiveness of current therapy in preventing neonatal infection has intensified screening for HIV early in pregnancy.

However, because of the long time needed to test for HIV antibodies, it was difficult to screen women during labor and provide the necessary therapy. Recently, rapid HIV antibody testing procedures using a fingerstick or venipuncture to obtain whole blood, plasma, or serum, and tests using oral fluid were approved (Table 707-2). The HIV test results are usually obtained in <20 min. The collection of oral fluid samples provides an alternative for individuals who avoid HIV testing because of their dislike of needlesticks. HIV testing using whole blood or oral fluid is classified as a waived test under the Clinical Laboratory Improvement Amendments of 1988 (CLIA), and these tests are allowed in a point-of-care setting. Waived tests are simple laboratory procedures that use methodologies that are so simple and accurate as to render the likelihood of an erroneous result by the user negligible. A positive rapid HIV test result is then confirmed by Western blot analysis or immunofluorescence assay.

Table 707-2   — RAPID HIV ANTIBODY TESTS AND STATUS UNDER THE CLINICAL LABORATORY IMPROVEMENT AMENDMENTS OF 1988 (CLIA)

RAPID HIV TEST SPECIMEN TYPE CLIA CATEGORY TIME FOR PERFORMING ASSAY WAIT TIME TO READ RESULTS MANUFACTURER
OraQuick ADVANCE Rapid HIV-1/2 Antibody Test Oral fluid Whole blood (fingerstick or venipuncture) Waived Waived <5 min 20-40 min OraSure Technologies, Inc. www.orasure.com
  Plasma Moderate complexity      
Uni-Gold Recombigen HIV-1 Whole blood (fingerstick or venipuncture) Waived <5 min 10-12 min Trinity Biotech www.unigoldhiv.com
  Serum and plasma Moderate complexity      
Reveal G-2 Rapid HIV-1 Antibody Test Serum and plasma Moderate complexity <5 min Read result immediately MedMira, Inc. www.medmira.com
MultiSpot HIV-1/HIV-2 Rapid Test Serum and plasma Moderate complexity 10-15 min Result can be read immediately or up to 4 hr later BioRad Laboratories www.bio-rad.com

According to the U.S. Centers for Disease Control and Prevention, in the USA, between 280 and 370 infants were born with HIV in 2000. Rapid HIV testing during labor allows for implementation of antiretroviral therapy for HIV-infected women who have not been tested or are unaware of their HIV status. The initiation of therapy at the time of labor or within the 1st 12 hr of an infant’s birth significantly reduces the risk of mother-to-child transmission. In the mother-infant rapid intervention at delivery study, it was shown that the sensitivity and specificity of a rapid whole blood test for HIV during labor were 100% and 99.9%, respectively, with a positive PV of 90%. The median turnaround time for obtaining results from blood collection to patient notification was only 66 min. The performance of the rapid blood test was better than that of the standard HIV enzyme immunoassay, which had sensitivity and specificity of 100% and 99.8%, respectively, with a positive PV of 76%. In addition, the median turnaround time from blood collection to patient notification was 28 hr. As a result, rapid whole blood HIV testing is now the standard of care for women in labor with undocumented HIV status.

Rapid HIV testing can also be used in developing countries. In resource-poor settings, because of the lack of properly equipped laboratories, skilled technologists, and basic resources, such as electricity and water, these self-contained, point-of-care HIV tests are very attractive. In areas of Asia and Africa in which HIV is epidemic, screening pregnant women with rapid HIV tests and offering antiretroviral therapy can significantly reduce the transmission of HIV to hundreds of thousands of infants.

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