When an HIV infection is suspected, it's most commonly diagnosed by tests that can detect the presence of HIV antibodies in either the blood or saliva. An antibody is a type of protein secreted by certain white blood cells that can detect and destroy foreign bodies like viruses or bacteria.
The two tests used to confirm HIV infection—the HIV ELISA and Western Blot—are designed to react in the presence of HIV antibodies. A "positive" result means that HIV antibodies have been detected and that an infection has occurred. A "negative" result means that no antibodies have been detected and that HIV infection has not occurred.
Conversely, a "false positive" is when a test incorrectly indicates an HIV infection, while a "false negative" is when the test incorrectly shows that no infection has occurred. Both of these are relatively rare are using current testing assays.
People will usually develop measurable levels of HIV antibodies within 30 days of infection, although some may take longer—up to three months in some cases.
Before this happens, there is a period when antibody levels are too low to reliably be detected. This is called the "window period." It is during this time that an infected person can pass HIV to others, but still have a negative result if given an antibody test.
If you believe you have been infected and suspect when you might have been accidentally exposed to HIV, it's important that you share this with your doctor or testing center. They can then help determine if the HIV test falls within the window period or if it can be performed with reasonable level of reliability.
HIV ELISA and Western Blot Tests
The HIV ELISA ("enzyme-linked immunosorbent assay") is a highly sensitive test that changes color in the presence of HIV antibodies. Results are assigned a numeric value, with values below 1.0 indicating a negative result and values above 1.0 indicating a positive (or reactive) result. Although the ELISA offers a high probability for HIV infection, it is not used alone as in the detection of HIV.
When an ELISA test produces a positive result, a second antibody test, called the Western Blot, is used to confirm the results. Together, the ELISA and Western Blot are shown to have an accuracy of 99.9% in the general U.S. population, with a false-positive rate of about one out of every 250,000 tests performed.
Occasionally, the tests can return with an indeterminate or inconclusive result. This can happen for a number of reasons:
- The infection may have been too recent to reliably detect.
- There may have been problems with the test sample or the testing procedure itself.
- Acute infections like syphilis or Lyme's disease can sometimes elicit a false positive result, as can autoimmune conditions like lupus or diabetes.
If a person has an inconclusive result, the tests should be repeated.
Rapid HIV Testing
Rapid HIV tests (also known as point-of-care tests) are ELISA screening assays that can deliver results in as a little as 20 minutes, as opposed to standard ELISA/Western Blot assays that can take several days.
Rapid tests are performed using saliva or blood samples, and are shown to be as sensitive as a standard ELISA. Results from these tests are either "negative" or "preliminary positive." Preliminary positives must be confirmed with a Western Blot performed in a lab.
Rapid tests are inexpensive and generally well received by patients and providers alike. While errors can occur that may affect results—such as improper storage, temperature or handling—these concerns have been largely minimized by FDA regulations limiting sales to authorized providers with approved quality assurance programs.
However, the picture changed completely in 2012 when the FDA approved the first in-home rapid HIV test for direct sale to consumers. While the products are relatively easy to use and offer linkage to care for those who do test positive (via a 24-hour consumer hotline), some are concerned that the lack of face-to-face counseling and potential breaches in informed consent may undermine prevention efforts.
Furthermore, while the in-home tests offer the same sensitivity and specificity as a point-of-care tests, clinical research has demonstrated a false-negative rate of around 7%—or roughly one out of 12 tests.
Another study published by the University of California, San Francisco has suggested that fourth generation rapid tests have a "real life" accuracy of about 86% in correctly identifying HIV-positive cases, but only 54% accuracy in confirming serostatus during the early, acute stage of HIV infection.
With mounting evidence suggesting that early diagnosis and treatment may prevent a number of long-term complications—including a possible reduction in the viral reservoirs where HIV can persist for decades—the need for accurate identification during acute HIV infection has become less a consideration and more of a "must" among public healthcare officials.
Chou, R.; Huffman, L.; Fu, R.; et al. "Screening for HIV: a review of the evidence for the U.S. Preventive Services Task Force." Annual Internal Medicine. July 2005; 143(1):55-73.
U.S. Food and Drug Administration (FDA). "First Rapid Home-Use HIV Kit Approved for Self-Testing." FDA Consumer Health Information. Silver Spring, MD; July 2012. Document: UCM311690.
Greenwald, J.; Burstein, G.; Pincus, J.; et al. "A Rapid Review of Rapid HIV Antibody Tests." Current Infectious Disease Reports. March 2006; 8(2):125-131.
Pilcher, C.; Louie, B.; Facente, S.; et al. "Performance of Rapid Point-of-Care and Laboratory Tests for Acute and Established HIV Infection in San Francisco." PLOS | One. December 12, 2013; DOI: 10.1371/journal.pone.0080629.