From a terminology standpoint, when discussing the diagnostic tests associated with HIV, we often take that to mean the so-called HIV test—or the test that confirms whether we have been infected or uninfected by the human immunodeficiency virus (HIV).
In fact, there are a number of different tests that are used in the detection, treatment and management of HIV, which can tell us:
- if we are HIV-negative or HIV-positive
- when to start antiretroviral therapy
- what drugs are best suited for us
- how effectively we are responding to therapy
- if any drug side effects are developing, and
- the levels of drug resistance that may be developing in our virus.
Testing and Confirmation of HIV Infection
When an infection is suspected, it's most commonly diagnosed by a test 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 exposed, it's important 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 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 testing results—such as improper storage, temperature, handling, etc.—these concerns have largely been minimized by FDA regulations limiting sales to authorized providers with approved quality assurance programs.
However, with the FDA's approval of in-home rapid HIV tests in 2012, consumers can now purchase the tests directly themselves. While the products offer linkage to care for those who 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.
That said, the in-home tests offer the same sensitivity as the standard rapid test, although a clinical study of untrained consumers has shown a false-negative rate of around 7%—or about one out of 12 tests.
(Finally, it's important to note that "rapid" only applies to the speed of the tests. They neither foreshorten nor "speed up" the window period. Standard testing guidelines still apply.)
NEXT PAGE: CD4 Count and Viral Load