HIV is a virus that attacks the immune system. If HIV infection isn’t treated, a person can develop AIDS, which is a prolonged and often fatal condition. HIV is spread through vaginal, oral, or anal sexual contact. It’s also spread through blood, blood factor products, injection drug use, and breast milk.
To test for HIV, a series of blood screenings may be done, including one called the ELISA test. Read on to learn how these tests are done, what to expect during the tests, and what the results can mean.
The enzyme-linked immunosorbent assay (ELISA), also known as an enzyme immunoassay (EIA), detects HIV antibodies and antigens in the blood.
Antibodies are proteins produced by the immune system, which helps your body fight disease. The immune system produces the antibodies in response to the presence of foreign substances, such as viruses. By contrast, antigens are any foreign substance in the body that causes the immune system to respond.
The ELISA test is typically the first test ordered by a healthcare provider. In case of a positive result from this test, the ELISA test was previously followed by a test called a Western blot to confirm the diagnosis. However, the Western blot is no longer used, and today the ELISA test is followed by an HIV differentiation assay to confirm HIV infection. The provider may also order an HIV genetic material detection test.
The ELISA test is recommended if a person has been exposed to HIV or is at risk for contracting HIV. Those at risk for contracting HIV include:
- people who use intravenous (IV) drugs
- people who have sex without a condom, especially with someone who has HIV or an unknown HIV status
- people who have had sexually transmitted diseases (STDs)
- people who had blood transfusions or blood clotting factor injections before 1985
People may opt to have the test done if they’re uncertain about their HIV status, even if they’re not in a high-risk group. For people who participate in high-risk behaviors, such as IV drug use or sex without a condom, it’s a good idea to get tested on a regular basis. And the Centers for Disease Control and Prevention (CDC) recommends that all adults get tested at least once for HIV.
There’s no need to prepare for an ELISA test or a differentiation assay. These tests are done using a blood sample, and it takes very little time to give a blood sample. However, to get the test results, it may take several days, and in some cases weeks.
People with a fear of needles or who faint at the sight of blood should be sure to tell the healthcare provider as well as the laboratory technician. These clinicians can take precautions to help ensure safety in case the person faints.
Before the test, a healthcare provider will explain the procedure. The person having the test will probably need to sign a consent form.
To help prevent any problems during the test, the person should be sure to tell the healthcare provider if:
- they’ve had trouble giving blood in the past
- they bruise easily
- they have a bleeding disorder, such as hemophilia
- they’re taking anticoagulant medications (blood thinners)
During the test
The procedure for getting a sample of blood is the same for both tests. A medical professional will:
- clean the skin site where they plan to draw blood
- apply a tourniquet, or elastic band, around the arm to make the veins swell with blood
- place a needle into one of the veins and draw a small sample of blood into a tube
- remove the needle and apply a bandage
To decrease further bleeding, after the test the person may be asked to elevate or flex their arm to reduce blood flow.
Giving a blood sample isn’t painful, though the person may feel a sting or a pricking sensation as the needle goes into their vein. Their arm may throb slightly after the procedure.
Testing the blood
For the ELISA test, the blood sample will be sent to a laboratory for analysis. A lab technician will add the sample to a device that contains HIV antigen and anti-HIV antibodies.
An automated process will add an enzyme to the device. The enzyme helps speed up chemical reactions. Afterward, the reaction of the blood and the antigen will be monitored. If the blood contains antibodies to HIV or antigens of HIV, it will bind with the antigen or antibody in the device. If this binding is detected, the person may have HIV.
The differentiation assay is very similar, but instead of an automated machine, the device can be handled by a lab technician. The specific antibodies and antigens in the blood are separated and identified in a different immunoassay device.
These tests are very safe, but rare complications can occur. For example, the person may:
- feel lightheaded or faint, especially if they have a fear of needles or blood
- get an infection at the site of the needle insertion
- develop a bruise at the puncture site
- have trouble stopping the bleeding
The person should contact their healthcare provider right away if they experience any of these complications.
If a person tests positive for HIV on the ELISA test, they might have HIV. However, there can be false positives with the ELISA test. This means that test results indicate that the person has HIV when they actually do not. For example, having certain conditions such as Lyme disease, syphilis, or lupus may produce a false positive for HIV in an ELISA test.
For this reason, after a positive ELISA test, more sophisticated tests are done to confirm whether the person has HIV. These tests include the differentiation assay and a test called the nucleic acid test (NAT). If the person tests positive for HIV with either one of these tests, they probably have HIV.
Sometimes, HIV doesn’t show up on the ELISA test even though a person is infected. This can happen if someone is in the early stages of the infection and their body hasn’t produced enough antibodies (in response to the virus) for the tests to detect. This early stage of HIV infection, in which a person has HIV but tests negative for it, is known as the “window period.”
According to the CDC, a person’s window period is usually between three and 12 weeks. However, in rare cases, some people can take as long as six months to develop antibodies.
Though both the ELISA test and the differentiation test are simple and straightforward, waiting for the results can create anxiety. In many cases, a person will have to talk to someone either in person or over the phone to receive their results, regardless of whether they’re positive or negative. A positive test result can trigger strong emotions. If needed, the person’s healthcare provider can refer them to counseling or HIV support groups.
Although HIV is very serious, it’s important to keep in mind that today there is medication available that can help prevent HIV infection from developing into AIDS. It’s possible for someone with HIV to live a long and full life. And the earlier a person learns their HIV status, the earlier they can begin treatment to prevent health complications or transmission of the infection to others.