Question: How often do false positive and false negative HIV test results Occur?
Answer: Although the rate of false positive and false negative HIV test results are low, they do sometimes occur. Their incidence is largely influenced by a number of factors, including the type of test used, the limitations of current testing technologies, and even the time at which a person is given a test.
Today, the false negative rate in the general U.S. population is around 0.003%, or three times out of every 100,000 tests. False positive rates are far lower—between 0.0004% and 0.0007%—due to the practice by which the initial positive result (using an HIV ELISA) is confirmed with a secondary test (called the Western blot).
What Is a False Positive and a False Negative?
By its broadest definition, a false positive result is one that incorrectly identifies a person who is not infected with HIV as being HIV-infected. Conversely, a false negative is one that incorrectly identifies an HIV-infected person as not being infected.
An HIV test works by detecting HIV antibodies in either a blood or saliva sample. (Antibodies are Y-shaped proteins produced by the immune system to help defend the body against foreign agents, such as HIV.)
Therefore, in a false-negative result, the test either fails or is unable to detect HIV antibodies. This most often occurs not because of the test itself, but because the HIV-infected person is tested prematurely during the so-called "window period." It is during this period that the immune system has not produced enough HIV antibodies to register a detectable response by current testing assays.
By comparison, a false positive occurs when an HIV test incorrectly identifies a non-HIV-antibody as being an HIV antibody. On the rare occasion when this does occur, it is usually because antigens similar to HIV are detected. Some autoimmune diseases, like lupus, have been known to elicit such a response.
How Accurate Are the Tests?
Within a general population, the rate of false negatives and false positives is largely determined by the sensitivity and specificity of the test used. In the framework of an HIV test:
- Sensitivity is defined as the percentage of true positives who are actually identified as such. Lower rates of sensitivity equate to more false negative results.
- Specificity is defined as the percentage of true negatives who are actually identified as such. Lower rates of specificity equate to more false positive results.
Current generation HIV tests are considered extremely accurate. The blood-based HIV ELISA has a demonstrated sensitivity of between 99.3% to 99.7%, with a specificity of between 99.91% and 99.97%. When combined with a Western blot, this translates to approximately one false positive out of every 250,000 tests in the general U.S. population.
Comparing Blood-Based vs Rapid Oral HIV Tests
The rapid oral HIV test has gained popularity in health clinics and, most recently, as an over-the-counter, in-home test. While the ELISA-based assay has similar specificity to blood-based ELISA, it has a 2% lower sensitivity.
While the accuracy of these tests is still considered high, the U.S. Food and Drug Administration (FDA) has reported that as many as one out of our 12 in-home tests will produce a false negative result. Independent studies have shown that product misuse (e.g. insufficient swabbing) and premature testing are largely to blame, particularly since the amount of antibodies found in saliva are especially low after recent infection.
Despite these shortcomings, the U.S. Centers for Disease Control and Prevention (CDC) support the use of in-home tests in the U.S. due to their convenience, ease of use, and affordability. They are also seen as a mean by which to increase acceptability of HIV testing in marginalized communities where as many 21% of infections so undiagnosed.
If taking an in-home HIV test, you can reduce the risk of a false negative result by:
- Following manufacturer's product instructions and contacting the consumer hotline should the results be unclear.
- Ensuring that you don't take the test too early. People usually develop measurable levels of HIV antibodies within 30 days of infection, although it can sometimes take longer—up to three months in some cases.
- Treating a negative oral result with caution if you believe yourself to be at higher risk of infection.
If in doubt about your in-home test results or are uncertain when an exposure may have taken place, consider retesting in several weeks or visiting an approved HIV testing center near you. A comprehensive list of HIV testing sites is available from AIDS.gov, the HIV/AIDS website operated by the U.S. Department of Health and Human Services (DHHS).
Finally, in the event you've been exposed to HIV within the space of 72 hours, do not wait to get tested. Go immediately to your nearest clinic, doctor or emergency room and get a course of medication called post-exposure prophylaxis (PEP), which can potentially avert infection if started soon after exposure.
Malm, K.; von Sydow, M.; and Andersson, S. "Performance of three automated fourth-generation combined HIV antigen/antibody assays in large-scale screening of blood donors and clinical samples." Transfusion Medicine. 2009: 19(2):78-88.
U.S. Food and Drug Administration (FDA). "First Rapid Home-Use HIV Kit Approved for Self-Testing." Silver Springs, Maryland. Media Release: April 13, 2013.
Pant Pai, N.; Balram, B.; Shivkumar, S.; et al. "Head-to-head comparison of accuracy of a rapid point-of-care HIV test with oral versus whole-blood specimens: a systematic review and meta-analysis." The Lancet Infectious Diseases. January 24, 2012; 12(5):373-380.