Diagnosis of HIV Infection

Anyone who has unprotected sex with or shares a needle or syringe with a person who is HIV-positive, even once, is at risk for HIV infection. Making the decision to be tested for HIV can be difficult. If you have questions or concerns, talk to a trained counselor at an HIV/AIDS testing center.

People who want to be tested should look for a center that offers free, anonymous, confidential testing, and HIV counseling. People under the age 18 should find out if parental permission is required in their state.

For information on where to go for testing and HIV counseling, look under "Drug Abuse" in the Yellow Pages of the phone book or call the Centers for Disease Control and Prevention (CDC) National HIV/AIDS Hotline:

  • 1.800.342.2437 (English)
  • 1.800.344.7432 (Spanish)
  • 1.800.243.7889 (TTY, for hearing-impaired)

If a person is infected with HIV, the sooner he or she knows the better. Patients can stay healthier longer by seeking early treatment and can protect others by preventing transmission. Being tested too soon, however, (i.e., within 3 to 6 weeks of exposure) can result in unreliable test results.

This is because most tests that are used do not detect the presence of the virus itself. They are used to detect proteins that the immune system produces to fight the virus. These proteins, which are known as antibodies, usually cannot be detected until about 3 to 6 weeks after the initial infection.

The ELISA (enzyme-linked immunosorbent) and Western blot assay are the two most common ways to determine if a person is infected with HIV. In certain circumstances, polymerase chain reaction (PCR) may be used. All three of these tests are performed on a small blood sample, usually drawn from the arm.

Another test, called the OraQuick test, uses saliva samples to diagnose HIV infection. In some cases, results from this test, which are more than 99 percent accurate, are available in less than 20 minutes. A recent study has shown that the OraQuick test may be useful to diagnose HIV in women who are in labor. Using information from this test, physicians can administer anti-retroviral drugs immediately to newborns who have been exposed to the virus—which may reduce the risk for infection.

Enzyme-Linked Immunosorbent Assay (ELISA)

Enzyme-linked immunosorbent assay is a quick and easy way to test for HIV antibodies. The sensitivity (percentage of positive results that are truly positive) and specificity (percentage of negative results that are truly negative) of ELISA approach 100 percent, but false-positive and false-negative results do occur. A false-negative result is slightly more common, especially in women who have had multiple pregnancies or people who have received multiple blood transfusions.

If the result is positive or indeterminate, the test is repeated. If the second test is positive or indeterminate, a different test, usually the Western blot, is done to confirm the results.

What exactly is an enzyme-linked immunosorbent assay? In this test, the blood sample is added to a slide that is coated with HIV antigens (HIV molecules that trigger an immune system response) and a dye is then added that appears only if HIV antibodies (proteins that the body produces in response to HIV infection) bind with the antigens.

Western Blot

The Western blot is generally used to confirm positive ELISA results, not to screen for infection. It produces more false positive results (i.e., positive tests results that are actually negative) than ELISA.

How does the Western blot work? This test involves putting the blood sample on a strip of paper (made out of nitrocellulose), which is embedded with HIV antigens (HIV molecules that trigger an immune system response). The blood migrates along the paper, and a visible band shows up in places where HIV antibodies (proteins that the body produces in response to HIV infection) bind with the antigens.

Polymerase Chain Reaction (PCR)

Whereas the ELISA and the Western blot detect the presence of antibodies to the virus, PCR detects the human immunodeficiency virus (HIV). This test can detect HIV even in people who are not currently producing antibodies to the virus.

Specifically, PCR detects the presence of what is known as "proviral DNA." HIV is made up of a genetic material known as RNA, which is a slightly different type of material than DNA. Proviral DNA is a DNA copy of the RNA of the virus.

PCR is used:

  • to confirm the presence of HIV when the ELISA and Western blot are negative;
  • in the first few weeks following infection, before antibodies are detectable;
  • if the Western blot is indeterminate;
  • in newborns for whom the presence of their mothers' antibodies complicates the other tests.

Test to Differentiate Between HIV Types

In July 2015, the U.S. Food and Drug Administration (FDA) approved the first diagnostic test to differentiate HIV-1 antibodies, HIV-2 antibodies, and HIV-1 p24 antigens. (These antigens can be detected before measurable levels of HIV antibodies have developed.) The test is called the Bio-Rad BioPlex 2200 HIV Ag-Ab assay. It's approved for use in adults (including women who are pregnant) and children over the age of 2.

The Bio-Rad BioPlex 2200 HIV Ag-Ab assay allows antibody and antigen results to be reported separately—this distinction can help differentiate between established and acute HIV infections. It can be used to screen organ donors while the donor's heart is still beating, but it is not approved for use in screening blood/plasma donors—except in urgent situations when traditional screening tests cannot be used.

Updated by Remedy Health Media

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 30 Nov 2000

Last Modified: 12 Aug 2015