About HIV Test (1)

HIV antibody test

HIV antibody tests are the most appropriate test for routine diagnosis of HIV among adults. Antibody tests are inexpensive and very accurate. The ELISA antibody test (enzyme-linked immunoabsorbent) also known as EIA (enzyme immunoassay) was the first HIV test to be widely used.

How do antibody tests work?
When a person is infected with HIV, their body responds by producing special proteins that fight infection, called antibodies. An HIV antibody test looks for these antibodies in blood, saliva or urine. If antibodies to HIV are detected, it means a person has been infected with HIV. There are only two exceptions to this rule:

Babies born to HIV infected mothers retain their mother's antibodies for up to 18 months, which means they may test positive on an HIV antibody test, even if they are actually HIV negative. Normally babies who are born to HIV positive mothers receive a PCR test (see below) after birth. Some people who have taken part in HIV vaccine trials may have HIV antibodies even if they are not infected with the virus.

Most people develop detectable HIV antibodies within 6 to 12 weeks of infection. In very rare cases, it can take up to 6 months and there are nearly always very particular reasons for antibodies developing so late such as other auto-immune disorders. It is exceedingly unlikely that someone would take longer than 6 months to develop antibodies.

What is a window period?
The ‘window period’ is a term used to describe the period of time between HIV infection and the production of antibodies. During this time, an antibody test may give a ‘false negative’ result, which means the test will be negative, even though a person is infected with HIV. To avoid false negative results, antibody tests are recommended three months after potential exposure to HIV infection.

A negative test at three months will almost always mean a person is not infected with HIV. If an individual’s test is still negative at six months, and they have not been at risk of HIV infection in the meantime, it means they are not infected with HIV.

It is very important to note that if a person is infected with HIV, they can still transmit the virus to others during the window period.

How accurate are antibody tests?
Antibody tests are extremely accurate when it comes to detecting the presence of HIV antibodies. ELISA tests are very sensitive and so will detect very small amounts of HIV antibody. This high level of sensitivity however, means that their specificity (ability to distinguish HIV antibodies from other antibodies) is slightly lowered. There is therefore a very small chance that a result could come back as ‘false positive’.

A false positive result means that although a person may not be infected with HIV, their antibody test may come back positive. All positive test results are followed up with a confirmatory test, such as:

A Western blot assay – One of the oldest but most accurate confirmatory antibody tests. It is complex to administer and may produce indeterminate results if a person has a transitory infection with another virus.
An indirect immunofluorescence assay – Like the Western blot, but it uses a microscope to detect HIV antibodies.
A line immunoassay - Commonly used in Europe. Reduces the chance of sample contamination and is as accurate as the Western Blot.
A second ELISA – In resource-poor settings with relatively high prevalence, a second ELISA test may be used to confirm a diagnosis. The second test will usually be a different commercial brand and will use a different method of detection to the first.
When two tests are combined, the chance of getting an inaccurate result is less than 0.1%.