Health care providers generally diagnose human immunodeficiency virus (HIV) using one or more blood tests. If your first round of tests is inconclusive, your doctor may order follow-up tests. For tests to be effective and conclusive, you’ll need to wait a certain period of time for the virus to replicate (multiply) and reach a detectable level. The earliest reliable test for HIV is effective within one to three weeks of a person being exposed to HIV. Nearly all tests are reliable about two months after infection.

The time between exposure and eligibility for testing is called the window period. During the window period, HIV is multiplying within the body, creating molecules that diagnostic tests can identify. Testing too soon may provide false negatives — that is, the test may not find the virus, despite the person being infected — or it may provide inconclusive results. During the window period, if you’ve been infected, you’re very contagious.

If you know or suspect you’ve been exposed to HIV, you should get tested as soon as possible. In the meantime, avoid behavior that can increase their risk of HIV transmission to others. If you’re concerned you may have recently been exposed to HIV, talk to your doctor about post-exposure prophylaxis (PEP), a treatment regimen that can help prevent you from becoming infected.

Who Should Be Tested for HIV?

Starting HIV treatment early gives people with HIV the best chance for long, healthy lives and also reduces the risk of spreading the virus. For these reasons, the Centers for Disease Control and Prevention (CDC) recommends a routine HIV test at least once for everyone between the ages of 13 and 64.

Additionally, the CDC recommends that people who are pregnant undergo screening for HIV, even if they’ve been screened during previous pregnancies.

The CDC recommends annual testing for people who:

• Are men who’ve ever had sex with another man, particularly those with partners who are either HIV-positive or have an unknown serostatus
• Have had anal or vaginal sex with an HIV-positive person
• Have had more than one sexual partner since their last HIV test
• Inject drugs and share needles, syringes, or other equipment with someone else who does drugs
• Have sold sex for money or drugs
• Have ever had a sexually transmitted infection (STI)
• Have ever been diagnosed with hepatitis or tuberculosis
• Have had sex with anyone who has done or may have done any of the above

For gay or bisexual men who are sexually active, the CDC recommends testing for HIV every three to six months.

How Is HIV Diagnosed?

A doctor can diagnose HIV by analyzing molecules of blood or saliva samples for molecules related to HIV. Some research suggests that saliva tests appear to be less sensitive than blood tests.

Types of Tests for HIV

Different types of HIV tests look for different molecules that can provide evidence of infection. Some HIV tests are not conclusive for weeks or months after a person is infected, while others can be used sooner. Remember that a false-negative result can occur if a test is taken too early.

Viral Load Tests

The earliest tests that are effective for diagnosing HIV are viral load tests, which can be used between 10 to 33 days after exposure. One type of viral load test is the RNA or nucleic acid test (NAT). Viral load tests, which require a blood sample, can measure how much HIV genetic material is in a person’s blood.

The polymerase chain reaction (PCR) test is another early test that can diagnose HIV and determine viral load. PCR tests can be effective within two to three weeks of exposure. This method is not generally better than serologic testing — the regular HIV blood test — for most situations. However, the PCR test is particularly useful in a few specific cases:

• When diagnosing newborn babies for HIV infection
• When a regular blood test does not give a clear result
• When evaluating cases where someone might be newly infected and is in the window period where the infection might not show up in some tests
• When screening blood donors to ensure the blood donated is safe

Rapid Antibody/Antigen Combination Tests

Also known as a fourth-generation HIV test, rapid antibody/antigen tests look for fragments of the virus known as p24 antigen as well as HIV antibodies, immunoglobulin M (IgM) and immunoglobulin G (IgG). IgM and IgG are types of proteins the body produces to try to fight HIV. Rapid antibody/antigen tests are considered to become effective between 12 and 45 days after HIV exposure.

Researchers have found that these types of rapid tests are nearly 100 percent effective at identifying HIV in the chronic/latent stage — a few months after a person is infected — and about 80 percent effective at diagnosing early infections, within the first two to three months. Some rapid HIV tests can provide results within 20 to 30 minutes.

Learn more about HIV stages and symptoms.

ELISA Test

The enzyme-linked immunosorbent assay (ELISA) test checks only for IgM and IgG antibodies. This antibody test is nearly 100 percent effective at diagnosing HIV in the chronic/latent stage but may miss HIV infections in the acute or early stage. If your ELISA test result is negative but you strongly suspect you’ve been exposed to HIV, ask your doctor about getting another test after waiting one to three months.

If an ELISA test yields a positive result for HIV, the clinician may perform an HIV-1/HIV-2 differentiation immunoassay or a Western blot test to confirm the infection.

HIV-1/HIV-2 Differentiation Immunoassay

Doctors most often perform HIV-1/HIV-2 differentiation immunoassays to confirm a positive ELISA test result and identify different HIV strains — HIV-1 or HIV-2. These two viruses share less than half of their genes and can work differently. HIV-1 and HIV-2 affect various groups of people, are diagnosed using different tests, and may require different treatments. Most people with HIV have HIV-1 infection.

The blood test provides very accurate results in less than 20 minutes.

Western Blot Test

Western blot tests separate blood proteins and look for HIV antibodies. Once the gold standard for diagnosing HIV-1, these tests are performed less often for a couple of reasons. First, they sometimes miss a subtype of HIV-1 called subtype O. Second, special types of Western blot tests are necessary for detecting an HIV-2 infection.

Western blots become effective about two months after HIV exposure. They’re mainly performed to confirm positive ELISA test results, and the test combination has near-100-percent accuracy.

Home HIV testing

The U.S. Food and Drug Administration (FDA) has approved two HIV test kits.

The OraQuick In-Home HIV Test is performed at home using oral fluids (fluid that comes from around your gums, similar to saliva). Results are available within 20 to 40 minutes. The self-test is designed to be used at least three months after exposure to HIV. About 1 in 12 people who take an OraQuick In-Home HIV Test receive a false-negative result.

The Home Access HIV-1 Test System allows you to collect a blood sample at home using a finger prick and mail it to a lab for anonymous testing. The test is effective three to six months after exposure to HIV. Results are available by phone within seven business days of mailing in the blood sample. Clinical trials show that the test is extremely reliable.

How Is AIDS Diagnosed?

Acquired immune deficiency syndrome (AIDS) is the third and final stage of HIV. AIDS is most commonly diagnosed with blood tests, including tests that look at viral load and tests that measure a person’s levels of CD4 lymphocytes (also called T-helper cells). Healthy CD4 cells organize other immune system cells to respond to infection. HIV specifically attacks CD4 cells, infecting them to help itself replicate and also destroying them.

A doctor will render an AIDS diagnosis under any of the following conditions:

• CD4 cell count has fallen below 200 cells per cubic millimeter
• CD4 cells make up less than 14 percent of a person’s lymphocytes
• They find one or more opportunistic infections, such as candidiasis (yeast infection) in the throat or lungs, toxoplasmosis of the brain, Kaposi’s sarcoma, invasive cervical cancer, or many others

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