How is it used?
HIV testing is used to determine whether or not a person is infected with HIV. Early treatment of HIV infection and immune system monitoring can greatly improve long-term health. Also, knowing your HIV status may help you change behaviours so that you and others are not put at risk.
Antibodies to the HIV virus can be detected by a screening test called an ELISA (enzyme linked immunosorbent assay) or EIA (enzyme immunoassay). These methods are very sensitive but require another test called, a Western Blot, to confirm the results because can occur.
How do the tests work?
The ELISA and EIA methods start with wells that are pre-coated with inactivated HIV-1 and HIV-2 proteins (). The patient blood sample is added and if antibodies to HIV are present they will bind to the antigens on the well. Antibodies that do not bind are washed away. Then a special enzyme linked antibody is added and this binds to any human antibodies in the well. The enzyme causes a colour change in the well. If such a colour change occurs the test is considered positive.
The Western blot is used to confirm a positive result by ELISA or EIA tests. Western blot is performed by combinng the blood sample to be tested with a strip which contains HIV antigens. The antibodies present in the blood sample bind to the antigens on the strip. These are then detected with a labelled antibody to human antibodies. A positive reaction appears as a band on the strip. Several HIV antigens are included in the test. The test is interpreted as positive if antibody to at least two of three antigens is detected (p24, gp41 or gp120/160). When no bands are found the test is negative. When some bands appear but do not meet the criteria for a positive result the result is considered indeterminate.
When is it requested?
testing for HIV should be requested if you think you may have been exposed to HIV. Testing is recommended if:
- You have had multiple sexual partners or sex with a person known or suspected to have a HIV infection.
- You received a blood transfusion before 1985, or a sexual partner received a transfusion and later tested positive for HIV. (Since 1985, all blood donated in Australia, the UK and the US is tested for HIV, and infection by transfusion is highly unusual.)
- You are a male who has had sex with another male.
- You have used street drugs by injection, especially when sharing needles and/or other equipment.
- You have a sexually transmitted disease (STD).
- You are a health care worker with direct exposure to blood at work.
- You are pregnant or planning a pregnancy. (There are now treatments that can greatly reduce the risk that a pregnant woman who has HIV will pass on the virus to her baby)
In Australia the HIV test is free and anonymous. You can be tested by your doctor, at any clinic dealing with STDs or at special clinics for HIV screening. Your doctor is not informed of the clinic result without your permission. Your doctor or clinic staff will give advice about informing sexual partners if your test is positive. You must have a test if you want to become a blood donor, use your sperm for artificial insemination, have an egg implantation or adopt a child.
What does the test result mean?
A healthy individual has no antibodies to HIV, however, persons with very early HIV infection have not yet developed antibodies and will also test negative. If you test positive for HIV antibodies on both the ELISA and the Western Blot tests, you are considered to be infected with HIV.
It is not unusual for the screening ELISA/EIA test to give a false positive result. There are several reasons this may occur but usually it is because of interfering antibodies due to conditions other than HIV. The Western Blot is far less likely to give a false positive. If the Western Blot is indeterminate then the test should be repeated in a few weeks. Western Blot results are only specific for HIV-1. If you may be infected with HIV-2 a specific test for this infection should be requested. Testing for HIV genetic material (RNA) by nucleic acid amplification (NAAT or PCR) is a useful way to determine if HIV-1 is present when antibody tests are conflicting or indeterminate.
Is there anything else I should know?
Antibody testing will not detect HIV immediately after exposure, during the window before the development of antibodies. If you are tested too soon, your result may be negative despite the fact that you are infected (). Because of this, repeat testing is important. You should have another HIV antibody test in 3 - 6 months from the time of a possible exposure to the .