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Screening and window period
      09/13/01 12:37 PM

Accuracy of Screening Tests

The initial screening test to detect antibodies to HIV is the enzyme immunoassay (EIA). Commercially available EIAs use antigens from whole disrupted virus (first generation), recombinant viral proteins (second generation), or chemically synthesized peptides (third generation).22 EIA results are considered "reactive" only when a positive result has been confirmed in a second test of the original sample. In subjects with clinical AIDS, the sensitivity of EIA is close to 100%, but newly infected individuals may not develop detectable antibodies for periods of weeks to months after infection.23 The median interval between infection and seropositivity has been estimated at 3 months with earlier EIAs, with 95% seroconverting within 6 months.24 Estimates based on more sensitive third-generation EIAs, which may be positive within 1 week of peak antigen levels, suggest that the "window" period with current tests is substantially shorter (3-4 weeks).25,25a Specificity of EIA is above 99.5% with most tests. Third-generation EIAs had specificities of 99.7-99.9% when tested against uninfected controls.25-27 False-positive results can be caused by nonspecific reactions in persons with immunologic disturbances (e.g., systemic lupus erythematosus or rheumatoid arthritis), multiple transfusions, or recent influenza or rabies vaccination.28,29 Food and Drug Administration (FDA) approval has been granted to rapid tests (<15 minutes) using colorimetric assays,30 and to an EIA-based test using oral fluid samples collected with a cotton pad.31,32 Although these tests are sensitive and specific (>99%), neither method is recommended for the definitive diagnosis of HIV infection.33

To prevent the serious consequences of a false-positive diagnosis of HIV infection, confirmation of positive EIA results is necessary, using an independent test with high specificity. The Western blot (WB) is the most commonly used confirmatory test in the U.S. Indirect immunofluorescence assay (IFA), which is less expensive and less time-consuming, is also approved for confirmatory testing.34 The specificity of WB is close to 100% under controlled settings,35 but it is dependent on the skill and experience of the laboratory and on criteria used to determine positive WB results.33,35 The sensitivity of WB was 98.5% and specificity 92.5% in a 1989 CDC quality control survey of 140 laboratories;26 most errors involved misclassification of positive or negative samples as "indeterminate." Criteria for interpreting WB have been developed to improve accuracy in clinical testing.35 Many laboratories doing a large volume of HIV testing achieved sensitivities and specificities of 100%.36

The precise false-positive rate of current HIV testing is not known. A false-positive rate of less than 1 in 100,000 was reported in experienced, centralized laboratories using careful quality control procedures.37,38 In practice, false-positive diagnoses can result from contaminated or mislabeled specimens,39 cross-reacting antibodies,33 failure to perform confirmatory tests,40 misinterpretation of WB patterns, or misunderstanding of reported results by clinicians or patients.41 In one study, 8 of 900 women referred for treatment of HIV were not infected on repeat testing.40 To prevent errors due to mistakes in specimen handling or WB interpretation, a consensus laboratory panel recommended confirming all new diagnoses of HIV with tests on a freshly obtained specimen.42

Indeterminate Western Blot Results

Indeterminate WB results, due to antibody patterns that do not meet full criteria for positive test, occur in 3-8% of EIA-positive specimens.33,43 In a survey of over 1 million newborn specimens for maternal HIV antibody, <1 in 4,000 screened samples produced an indeterminate WB result.43a An indeterminate WB result may indicate evolving antibody response in recently infected subjects, but in low-risk persons it usually represents the presence of nonspecific antibodies. Follow-up of nearly 700 EIA positive/WB indeterminate blood donors documented seroconversion in only eight subjects, all but one of whom reported a high-risk behavior.43-45 An indeterminate WB is more significant in high-risk subjects, but the risk of seroconversion is variable (13-28%).44,46 Antibodies to p24 antigen were present in 29 of 30 subjects who subsequently converted.43,44 A new indeterminate WB in subjects who were previously seronegative is more likely to represent recent infection.47 Seroconversion usually occurs within 2-3 months (range 2-16 weeks in one study).44 A stable indeterminate WB after 6 months can be assumed to be due to nonspecific antibody reaction rather than HIV infection.33,48

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* Screening and window period Anonymous 09/13/01 12:37 PM
. * Re: Screening and window period Anonymous   09/13/01 07:11 PM
. * Re: Screening and window period Anonymous   09/13/01 12:47 PM

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