There are a number of tests that are used to find out whether a person is infected with HIV, the virus that causes AIDS. These include the HIV antibody test, P24 antigen test and PCR test. There are other types of HIV testing, which are used once a person has been diagnosed with the virus. These include the CD4 test and the viral load test.
In addition to an introduction to the different types of test, we also look at the reasons to get tested for HIV and what an HIV test involves.
The different types of HIV test
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%.
Rapid HIV tests
These tests are based on the same technology as ELISA tests, but instead of sending the sample to a laboratory to be analysed, the rapid test can produce results within 20 minutes.
Rapid tests can use either a blood sample or oral fluids. They are easy to use and do not require laboratory facilities or highly trained staff.
All positive results from a rapid test must be followed up with a confirmatory test, the results of which can take from a few days to a few weeks.
Antigen test (P24 test)
Antigens are the substances found on a foreign body or germ that trigger the production of antibodies in the body. The antigen on HIV that most commonly provokes an antibody response is the protein P24. Early in HIV infection, P24 is produced in excess and can be detected in the blood serum (although as HIV becomes fully established in the body it will fade to undetectable levels).
P24 antigen tests are not usually used for general HIV diagnostic purposes, as they have a very low sensitivity and they only work before antibodies are produced in the period immediately after HIV infection. They are now most often used as a component of ‘fourth generation’ tests.
Fourth generation tests
Some of the most modern HIV tests combine P24 antigen tests with standard antibody tests to reduce the ‘diagnostic window’. Testing for antibodies and P24 antigen simultaneously has the advantage of enabling earlier and more accurate HIV detection.
In the UK, fourth generation tests are the primary recommendation for HIV testing among individuals, but are not offered by all testing sites.1 During June 2010, the FDA approved the first fourth generation test in the United States.2
A PCR test (Polymerase Chain Reaction test) can detect the genetic material of HIV rather than the antibodies to the virus, and so can identify HIV in the blood within two or three weeks of infection. The test is also known as a viral load test and HIV NAAT (nucleic acid amplification testing).
Babies born to HIV positive mothers are usually tested using a PCR test because they retain their mother’s antibodies for several months, making an antibody test inaccurate. Blood supplies in most developed countries are screened for HIV using PCR tests. However, they are not often used to test for HIV in individuals, as they are very expensive and more complicated to administer and interpret than a standard antibody test.
HIV home sampling and HIV home testing
It is generally recommended that an HIV test is carried out in a healthcare setting. However, in some countries home sampling and home testing kits are available.
With a home sampling kit, a person can take a sample (usually a blood sample) and send it to a laboratory for testing. They can phone up for the results a few days later. If the result is positive then a professional counsellor will provide emotional support and referrals. The main advantages of home sampling are convenience, speed, privacy and anonymity.
There is also a company in the UK that offers home sampling services using oral fluid instead of blood. If a person’s test result is positive they will need a follow up blood-test at a clinic.
A home self-test involves a person conducting a rapid antibody HIV test in their home. The person takes either a blood or saliva sample and can interpret the result within minutes. A positive result will require a further confirmatory blood-test in a clinic.
In many countries it is illegal to sell HIV test kits to the public. If a test is purchased over the internet, there is no guarantee that the test kit is genuine or will provide accurate results.
There is currently a debate about allowing the kits to be sold in America and the UK. AVERT opposes the legalisation of the sale of home testing kits in the UK because of the lack of post-test counselling.
Comparison of HIV home sampling and home testing services
|Home sampling (UK)||Home sampling (USA)||Home testing|
|Method||Sampling device purchased; oral sample taken at home and sent to lab for testing||Sampling device purchased; blood sample taken at home and sent to lab for testing||Kit purchased for taking a sample and testing it at home|
|Notification||Reactive results given by phone; negative results given by email||All results given by phone||Results produced at home|
|Availability||Legal in the UK; available online for £34||Legal in the USA; sold in shops, online, by phone and mail order for $44||Illegal in UK; not approved for sale in USA|
|Potential for mistakes||Oral sample may be taken incorrectly, possibly leading to a false negative result||Low potential for mistakes as blood sample is clearly visible on card||Test may be performed or interpreted incorrectly, possibly leading to a false result|
|Reliability||Negative results are definitive; reactive results are preliminary and must be confirmed by further tests at a clinic||All results are definitive; as reliable as conventional testing||Negative results are definitive; reactive results are preliminary and must be confirmed by further tests at a clinic|
|Pre-test counselling||Optional at extra cost, by phone||Optional, by phone||Unlikely to be provided|
|Post-test counselling||Always provided for reactive results, by phone||Always provided for positive results, by phone||Optional, by phone|
What does an HIV test involve?
In most countries, HIV testing is provided in a number of places, such as health clinics, doctor’s surgeries and specialist HIV/AIDS voluntary counselling and testing (VCT) sites. When someone attends a testing site they will usually see a doctor, trained counsellor, nurse or other health professional in private. He or she will explain what the test involves and what the result means.
Either a blood sample (taken from the arm) or an oral fluid sample will be taken, depending on the type of test used at the site. The test is always strictly confidential and only goes ahead if the person agrees to it. Personal doctors are not told about the test without the person’s permission. Depending on the test used, it can take anything from minutes, to days, to weeks, for the results.
When should someone get an HIV test?
A standard antibody test (see above) looks for HIV antibodies in a person’s blood. These antibodies can take up to three months from the time of infection to appear, and so for an accurate HIV test, a person should wait three months since the time of suspected infection. Some test centres may recommend testing again at six months, as in very rare cases it can take this long to develop antibodies. However, in most cases it is not necessary.
It is important to note that although antibodies may take a while to appear, as soon as a person becomes infected with HIV they can transmit the virus on to others.