Screening and diagnosis for HIV
In general, testing for the human immunodeficiency virus (HIV) is a 2-step process that involves a screening test and follow-up tests.
These are tests that check if you've been infected with HIV. The most common tests are described below.
An antibody test (also called immunoassay) checks for antibodies to the HIV virus. Your health care provider may order the test for you to have done at a lab. Or, you may have it done at a testing center or use a home kit. These tests can detect antibodies starting a few weeks after you’re infected with the virus. Antibody tests can be done using:
- Blood -- This test is done by drawing blood from a vein, or by a finger prick. A blood test is the most accurate because blood has a higher level of antibodies than other body fluids.
- Oral fluid -- This test checks for antibodies in the cells of the mouth. It is done by swabbing the gums and inside cheeks. This test is less accurate than the blood test.
- Urine -- This test checks for antibodies in the urine. This test is also less accurate than the blood test.
An antigen test checks your blood for an HIV antigen, called p24. When you're first infected with HIV, and before your body has a chance to make antibodies to the virus, your blood has a high level of p24. The p24 antigen test is accurate 11 days to 1 month after getting infected. This test is usually not used by itself to screen for HIV infection.
An antibody-antigen blood test checks for levels of both HIV antibodies and the p24 antigen. This test can detect the virus as early as 3 weeks after getting infected.
A follow-up test is also called a confirmatory test. It is usually done when the screening test is positive. Several kinds of tests may be used to:
- Detect the virus itself
- Detect antibodies more accurately than screening tests
- Tell the difference between the 2 types of virus, HIV-1 and HIV-2
HIV testing; HIV screening; HIV screening test; HIV confirmatory test
How the Test is Performed
HIV testing can be done by:
How to Prepare for the Test
No preparation is necessary.
How the Test will Feel
When taking a blood sample, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
There is no discomfort with an oral swab test or the urine test.
Why the Test is Performed
Testing for HIV infection is done for many reasons, including for:
- Persons who want to be tested
- Persons in high-risk groups (men who have sex with men, injection drug users and their sexual partners, and commercial sex workers)
- Persons with certain conditions and infections (such as Kaposi sarcoma or Pneumocystis jirovecii pneumonia)
- Pregnant women, to help prevent them from passing the virus to the baby
A negative test result is normal. People with early HIV infection may have a negative test result.
What Abnormal Results Mean
A positive result on a screening test does not confirm that the person has HIV infection. More tests are needed to confirm HIV infection.
A negative test result does not rule out HIV infection. There is a period of time, called the window period, between HIV infection and the appearance of anti-HIV antibodies. During this period, antibodies and antigen may not be measured.
If a person might have acute or primary HIV infection and is in the window period, a negative screening test doesn't rule out HIV infection. Follow-up tests for HIV are needed.
With the blood test, veins and arteries vary in size from one patient to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others. Other risks associated with having blood drawn are slight, but may include:
- Excessive bleeding
- Fainting or feeling lightheaded
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
There are no risks with the oral swab and urine tests.
Ferri, FF. Human immunodeficiency virus. In: Ferri FF, ed. Ferri's Clinical Advisor 2016. Philadelphia: PA: Elsevier Mosby; 2015:639-645.
Simonetti FR, Dewar R, Maldarelli F. Diagnosis of human immunodeficiency virus infection. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 8th ed. Philadelphia, PA: Elsevier Saunders; 2015:chap 122.
U.S. Preventive Services Task Force. Final Update Summary: Human Immunodeficiency Virus (HIV) Infection: Screening. July 2015. www.uspreventiveservicestaskforce.org/Page/Document/UpdateSummaryFinal/human-immunodeficiency-virus-hiv-infection-screening. Accessed August 25, 2015.