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AIDS/HIV
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False-positive HIV tests
Originally Published: December 01, 2000
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Dear Alice, I recently had a routine HIV screening done for prenatal labs. It was positive. Thank GOD, western blot came out to be negative. My OB is reassuring me not to worry, that false-positives sometimes come out in pregnancy, and as long as the western blot is negative — I'm negative. Could you explain a little more in detail as to why and do you agree with my Dr.? I'm at very little risk otherwise and had a negative HIV test about five years ago and am with the same person (my husband).
Dear Reader, The Western Blot test is a confirmatory test that is definitive, provided you were not infected during the three-month window period before your exam. For more info on the window period, see How long does an HIV test take? and HIV Transmission: When does it show up on a blood test? in Alice's Sexual Health archive. If the Western Blot is negative, you do not have any HIV antibodies in your blood. If this still concerns you, you can always have another HIV test. When your blood is tested for HIV, the lab first performs what is known as an EIA or ELISA (enzyme-linked immunosorbent assay). A negative EIA means that no antibodies were found in the blood and that the person is HIV-negative. A positive result is confirmed with a second EIA. If the second EIA is positive, a Western Blot must be done to ensure that the antibodies detected in the EIA test are really HIV antibodies. The Western Blot test can come back positive, negative, or inconclusive. It's uncertain why you were given your EIA result when your Western Blot was negative. The negative result is definitive, and you needn't worry. About 0.2 percent of EIA tests give positive results that are proven false by the Western Blot test. This percentage does not vary between the general population and pregnant women. As noted in an article published in the August 14-28, 2000 Archives of Internal Medicine, an EIA can be falsely positive for several reasons, including a patient's autoimmune disease, multiple pregnancies, blood transfusions, liver diseases, parenteral substance abuse, hemodialysis, or vaccinations for Hepatitis B, rabies, or influenza. There is a very, very small chance that a Western Blot result is incorrect. False negatives usually happen when people test during the window period. In low-risk groups, a false negative Western Blot result occurs 0.001 percent of the time. False positives occur even less frequently — 0.00001 percent. False positives most often result from volunteers in HIV vaccine studies, clerical error, contaminated specimens, or misinterpretation of the results.
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