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HIV Screening of Pregnant Women and Newborns 3 SCREENING FOR HIV INFECTION The widening epidemic of HIV infection and AIDS among women and children has prompted consideration of various HIV screening proposals for pregnant women and newborns. In any discussion of such screening, it is important to distinguish between what is meant by screening and what is meant by testing. In the public health lexicon, screening generally refers to the application of a test or measurement (in this case an HIV-antibody test) to all individuals in a defined population . Most often, screening is instituted for the purpose of identifying a previously unknown or unrecognized condition in apparently healthy or asymptomatic persons and offering presymptomatic treatment to those so identified. Historically, screening has also been implemented for two other purposes: (1) to offer counseling to individuals at reproductive risk for producing affected offspring and (2) to conduct research, including enumeration (i.e., surveillance), natural history studies of a disorder, or recruitment of potential subjects for experimental treatment protocols.1 Additionally, screening for infectious conditions can provide an opportunity to initiate prevention activities that may limit or reduce the spread of infection. Testing is the application of a test or measurement to selected individuals. With regard to HIV, screening means that the HIV-antibody test is offered to all individuals within a defined population. In the case of testing, individuals with clinical findings suggestive of HIV infection or behavioral risk factors for HIV exposure are offered the HIV test. 2 Before a screening program is undertaken, its goals must be dearly specified and shown to be achievable. For example; if the purpose of 1 More detailed discussions of the specific goals of screening newborns and pregnant women for HIV infection appear in Chapters 4 and 5, respectively. 2 The committee recognizes that the concepts of ''screening'' and "testing" can be somewhat ambiguous. Screening, for example, is ultimately conducted among individuals, and testing, if widely practiced by health care providers, can be indistinguishable from screening.
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HIV Screening of Pregnant Women and Newborns screening is early diagnosis and treatment of a particular condition, effective therapy must be available for asymptomatic individuals. In addition, there must be a test or measurement that distinguishes those individuals who are likely to have the condition from those who are unlikely to have it.3 Also to be considered is the availability of resources and facilities to collect and process test specimens, as well as to provide the range of follow-up services for affected individuals identified through screening. Provisions for evaluation must be incorporated into the initial design and development of the program. (See the discussion in Chapter 7 of the elements of a screening program.) Finally, the benefits of identifying and treating individuals early in the course of disease must be carefully weighed against the costs—both personal (e.g., clinical, psychological, and social ramifications for the individual) and societal (e.g., the actual costs of screening and subsequent medical evaluation and treatment, potential consumption or diversion of resources from other public health or social programs). Technical Characteristics of a Screening Test and the HIV Testing Algorithm Among the characteristics that must be considered in assessing the performance and utility of a test, particularly for screening purposes, are its sensitivity, specificity, and predictive value. Sensitivity is the ability of a test to detect the condition of interest in those individuals who truly have that condition. Specificity, on the other hand, reflects the ability of a test to exclude those who do not the condition. In the case of HIV infection, sensitivity refers to the probability that the test will be positive if infection is present; specificity indicates the probability that the test will be negative if infection is absent The predictive value of a test reflects false-positive and false-negative results. The predictive value of a positive test is the probability that an individual is infected, given that the test result is positive; the predictive value of a negative test is the probability that an individual is not infected, given that the test is negative. The predictive value of any test will vary according to the prevalence of the condition in the population being studied (Tables 3-1a and 3-1b). For example, the predictive value of a positive test diminishes with declining 3 A screening tool identifies individuals who are at increased risk of having a particular condition. A positive screening test result, therefore, does not necessarily signify the presence of the condition. Rather, it indicates the need for further evaluation of the individual through available diagnostic techniques.
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HIV Screening of Pregnant Women and Newborns TABLE 3-1a Predictive Value of a Repeatedly Reactive Screening Test for Asymptomatic HIV Infection with a Prevalence of Infection in the Population of 0.2 Percent Actual Condition Totals Predictive Value Test Result Infected Not Infected Number Percent (percent) Reactive 196 998 1,194 1.2 16.4 Nonreactive 4 98,802 98,806 98.8 99.99 Totals 200 99,800 100,000 100.0 Note: Assumptions-test sensitivity, 98.0 percent; test specificity, 99.0 percent. SOURCE: Allen (1988:425). TABLE 3-1b Predictive Value of a Repeatedly Reactive Screening Test for Asymptomatic HIV Infection with a Prevalence of Infection in the Population of 20 Percent Actual Condition Totals Predictive Value Test Result Infected Not Infected Number Percent (percent) Reactive 19,600 800 20,400 20.4 96.1 Nonreactive 400 79,200 79,600 79.6 99.5 Totals 20,000 80,000 100,000 100.0 Note: Assumptions-test sensitivity, 98.0 percent; test specificity, 99.0 percent. SOURCE: Allen (1988:425). prevalence; hence, the proportion of positive test results that are liable to be falsely positive is likely to increase. The serological tests most commonly used to diagnose HIV infection are the Enzyme-linked Immunosorbent Assay (ELISA) in combination with the Western Blot. The ELISA is reliable, relatively inexpensive, and easy to perform—it is therefore appropriate for screening purposes. The Western Blot is used to confirm the results of the ELISA. The ELISA, which detects antibodies to virus antigens (i.e., HIV vital proteins), is first performed on the patient's serum. If the test is positive (i.e., "reactive"), it is repeated on the same blood sample. If positive again, the specimen is referred to as "repeatedly reactive" on the ELISA. Before the test can be considered truly positive, however, the specimen must be subjected to a more specific confirmatory test (usually the Western Blot), which is used to validate the ELISA test result—to determine whether the ELISA reactive
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HIV Screening of Pregnant Women and Newborns specimen is a true or false-positive result. A repeatedly reactive ELISA test that is confirmed positive by a Western Blot is generally diagnostic of HIV infection and can be reported as positive. Format of a Screening Program Screening programs can take various forms: mandatory, voluntary with fight of refusal, or voluntary with specific informed consent. Mandatory screening means that all individuals within a defined population are tested without an opportunity for refusal. Voluntary screening with right of refusal means that each individual within a defined population is informed that the test will be performed unless he or she explicitly refuses. Voluntary screening with specific informed consent means that each individual within a defined population is informed that the rest is available but that it will be performed only with a person's specific informed consent. Mandatory HIV testing and screening have not been generally implemented in this country for civilian, noninstitutionalized populations or populations at risk. Such programs have been rejected largely because of the powerful psychological and social impacts (including the threat of discrimination in employment, housing, access to health care, and insurance, as well as stigmatization and ostracism by friends, family, and others) that an antibody-positive test result may produce for an individual.4 In light of these potentially adverse social consequences, the HIV test (unlike, for example, a complete blood count [CBC]) does not qualify as a benign, routine medical test that may be performed under the conditions of general or presumed consent, which govern many, but not all, tests routinely conducted in medical practice (Livine and Bayer, 1989).5 Thus, the committee concludes that individuals (or their legally recognized representatives) should have the right to consent to or refuse HIV testing (except when such testing is conducted anonymously for epidemiological 4 Screening in populations with a low prevalence of infection is also likely to yield an increased proportion of false-positive results (see Tables 3-1a and 3-1b). 5 Presumed consent generally means that when a patient supplies urine or allows blood to be drawn in the medical setting, he or she agrees to the routine testing of these materials (Levine and Bayer, 1989).
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HIV Screening of Pregnant Women and Newborns purposes).6 The committee found no compelling evidence to suggest that women and children should constitute an exception to this principle. History has revealed that mandatory screening programs are frequently inflexible, often because they are legislated, and that program modification over time proves difficult. HIV testing and screening policies must be responsive to advances in diagnostic technology, scientific understanding of the disease, and medical therapy. Voluntary HIV screening (with specific informed consent) permits greater flexibility than mandatory screening in accommodating change. The committee opposes any mandatory newborn or prenatal HIV screening program (other than anonymous screening for surveillance purposes ). The following discussions, therefore, focus primarily on voluntary HIV screening—in particular, whether at this time such screening is warranted for pregnant women or newborns. 6 There may be rare circumstances in which this right might be overridden, such as in court-ordered testing, for a compelling state interest.
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