False positive HIV test?

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 if you agree with my doctor? 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,

Getting a false positive human immunodeficiency virus (HIV) test result may be stressful, especially when you’re pregnant. At the time your test was administered, the western blot was used as confirmation for a positive test and was considered definitive. Based on your question, your health care provider’s guidance was in alignment with the recommendations of the time. However, since then, the Centers for Disease Control and Prevention (CDC) have issued additional guidelines and recommendations for HIV detection tests. Since you didn't give specifics on your first HIV test, it could be helpful to run through the common screening methods for HIV for you to better understand what might have occurred with the positive result from your first screening. It may also be helpful to know what current guidelines are and what expectations are regarding those tests.

Historically, some of the tests used to test for HIV were:

  • Enzyme-linked immunosorbent assay (ELISA/EIA) test: It’s possible you were given an ELISA/EIA test, which looked for antibodies in the blood that might be fighting an HIV infection. This was one of the tests used in routine HIV screenings and may detect HIV antibodies up to three weeks following exposure to the virus. These tests were highly sensitive in order to ensure that those who had HIV screen positive. However, this resulted false positives, as the test was sensitive to antibodies that respond to other conditions besides HIV including: autoimmune diseases, multiple pregnancies, blood transfusions, liver diseases, parental substance abuse, hemodialysis, vaccinations for hepatitis B, rabies, or influenza.
  • Rapid HIV test: Rapid HIV tests were similar to ELISA/EIA, but instead of sending a blood sample to the lab and waiting two weeks, they were done on the spot from a finger prick or a saliva sample. They were also commonly used for routine screenings. The results of a rapid HIV test were usually ready in about 20 minutes. Along with the ELISA/EIA test, any positive results found by the rapid test were confirmed by a western blot.
  • Western blot test: The western blot test also looked for HIV antibodies but was better able to recognize HIV antibodies than ELISA/EIA tests. However, it wasn't usually the first test performed, as it occasionally came up as inconclusive and was a difficult test to administer. Given this, it’s no longer recommended by the CDC as a diagnostic test.
  • Indirect fluorescent antibody (IFA) test: The IFA test was relatively uncommon, due to how expensive it was to perform. Similar to the western blot, it confirmed the results of the ELISA/EIA tests through measuring antibodies. Due to the frequency of false negatives and inconclusive results, the CDC no longer recommends its use.

In your situation, it’s possible that the test you took was more sensitive and detected antibodies that weren’t related to HIV. The Western Blot was then used to confirm the results of that test. However, the CDC has some different guidelines now around diagnostic testing for HIV, so if you were to be tested again now, your experience would likely be different. The tests that are currently approved include:

  • Antigen/antibody tests: These tests look for both HIV antibodies and antigens, which are the foreign substances produced in the body by a virus. They usually show an accurate result within 18 and 45 days of exposure to the HIV virus.
  • Antibody tests: These tests measure the antibodies that develop in response to the presence of the virus. It doesn’t actually measure the virus. Of these tests, antibody tests can take the longest to detect HIV, with it taking 23 to 90 days for the virus to be detectable.
  • Nucleic acid test (NAT): Also called viral load testing, this is a method to measure the actual genetic material of the virus. This is an expensive test, and it isn’t routinely administered for everyone. It’s generally done for those who may be at high risk for exposure or who believe they may have been exposed. If someone is taking pre-exposure prophylaxis or post-exposure prophylaxis, the results of this test may be less accurate. These tests can detect HIV as early as between 10 to 33 days after exposure to HIV.

Both the antigen/antibody and antibody tests can be done as rapid tests. As a whole, these tests are able to detect HIV sooner than earlier tests and with higher accuracy. It’s great to be seeking more information during pregnancy, especially when information from your health care provider seems confusing.

Last updated Dec 20, 2019
Originally published Dec 01, 2000

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