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Why test for HIV?
If the HIV test is positive, there are a number of things that can be done to help a person cope with the result and lead a healthy life. A person who tests positive will at some point need to take antiretroviral treatment to slow down the virus and maintain a healthy immune system. The longer a person remains unaware of their infection, the less likely it is that the treatment will work.
Doctors can monitor an HIV positive person’s health in order to provide the right treatment regimen at the right time. If a person is aware of their HIV status they can take steps to protect other people. They can practice safer sex and inform previous sexual partners that they may have been at risk of infection. If they were thinking of starting a family, they can learn about ways to protect children from becoming infected with HIV through mother-to-child transmission.
When should someone get an HIV test?
If a person believes that they may have become infected, it is very important that they make contact with a doctor or health worker immediately, in order to discuss risk and make plans to test as soon as possible. People who consider themselves as engaging in risky behaviour are advised to get tested at least once a year. In some cases a doctor may advise patients to undergo an HIV test based on their own assessment of an individual's health.
Testing for HIV too soon after potential exposure to the virus may not give an accurate result due to what is known as the window period. A window period refers to the period of time during which HIV is not detected by tests even if the virus has entered the body. During this time, a test may give a ‘false negative’ result. Different types of tests have varying window periods. To be sure of how long a person needs to wait, they should also always discuss the nature of the HIV test with a healthcare worker before getting tested. If a person is not sure of the date or time of their potential exposure to HIV, a healthcare worker will be able to advise appropriately on their HIV testing.
There are a number of tests that are used to find out whether a person is infected with HIV. 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.
What does an HIV test involve?
In most countries HIV testing is provided in a number of places, such as health clinics, doctors 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 finger) 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.
Types of HIV tests
HIV antibody test
HIV antibody tests are the most appropriate test for routine diagnosis of HIV among adults. The ELISA antibody test (enzyme-linked immunoabsorbent) also known as EIA (enzyme immunoassay) was the first HIV test to be widely used. They are sometimes known as 3rd generation tests.
An HIV antibody test looks for HIV fighting proteins called 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.
- 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 auto-immune disorders. It is very unlikely that someone would take longer than 6 months to develop antibodies.
The ‘window period’ for an antibody test is estimated to be three months after exposure to HIV infection.1 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.
Antibody tests are extremely accurate when it comes to detecting the presence of HIV antibodies. Antibody 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 antibody test – In resource-poor settings with relatively high prevalence, a second antibody 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%.
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.
The window period for an antigen test is estimated to be from 11 days to one month after exposure to HIV infection.2 It is therefore safest to assume that the test won’t cover any potential exposure during the previous month.
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. The window period for fourth generation tests is estimated to be about the same as for a p24 antigen test above.
In the UK, fourth generation tests are the primary recommendation for HIV testing among individuals, but are not offered by all testing sites.3 During June 2010, the FDA approved the first fourth generation test in the United States.4
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, PCR tests 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.
Rapid HIV tests
Rapid HIV tests use either a blood sample or oral fluids and can produce results within as little as 20 minutes. They are based on the same technology as the tests described above, but are easy to use and do not require laboratory facilities or highly trained technicians. All positive results from a rapid test must be followed up with a confirmatory test. Cinfirmation results can take from a few days to a few weeks.
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 HIV 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 support and referrals.
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 may require a further confirmatory blood-test in a clinic.
UNAIDS suggest that home testing could be a partial solution in parts of the world where there is a severe number of people living with HIV who don’t know their status.5 In many health-care settings people worry about their test results remaining confidential and supervised home-based testing has the potential to help overcome some of these issues.6 A study in Kenya has also shown that home-based counseling and testing (HBCT) can be the most effective gateway to getting people who test positive into care and treatment.7 However, in many countries it is illegal to sell HIV test kits to the public. One of the worrying aspects of the unregulated use of home tests is fear of fake test kits and the need to ensure accurate results.
The concerns of the debate around allowing the kits to be sold in the United States of America and the UK tend to relate to the potential for a person to test positive without being linked to the appropriate treatment, care and support services. However, in September 2011, the United Kingdom’s House of Lords Select Committee on AIDS recommended repealing laws that prohibit home HIV testing on the basis that this would help to break down the barriers that stand in the way of people coming forward for testing.8
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|
HIV testing - a personal view
"Hi, I am a 30 year old heterosexual woman and I currently have no children. I am not an intravenous drug user or a haemophiliac. However, I have had unprotected sex with a number of heterosexual men. I know this behaviour can produce deadly results and I have had 2 negative HIV tests in the past 10 years with the last one being in 1996. Since my last HIV test I have had unprotected sex 5 times.
I hadn't recently given the subject much thought until I received notice that a local family had contracted HIV. I started thinking about my behaviour and how I have been gambling with my life and also putting the lives of others at risk, since I did not know my current status.
I have been blessed with two prior negative HIV results. For the past two weeks I have been reading articles on HIV/AIDS, reading the stories of women who have contracted the virus and are courageously battling the disease, and also reading the signs and symptoms. I have prayed for guidance and for a repeated chance to begin a new pattern in my life if only my test would come back negative just one more time.
This morning I went for another HIV test and, thank god, it came back negative. I urge everyone black, white, gay, and straight to be tested. I also want to thank the women, men and children who have contributed their stories to this site. I must have read your words a thousand times over. You have made a difference in my life." Signed Jenny in America
If you have any other questions relating to HIV testing visit our HIV testing FAQs page.
- 1. NAT (2011) ‘Types of HIV Test’
- 2. NAT (2011) ‘Types of HIV Test’
- 3. BHIVA/BASHH/British Infection Society (2008) 'UK national guidelines for HIV testing 2008'
- 4. FDA (2010, 18th June) 'Fourth generation HIV diagnostic test approved, permitting earlier detection of infection'
- 5. UNAIDS (2011) 'UNAIDS World AIDS Day Report 2011'
- 6. MacPherson P. et al (2011) ‘Stigmatising Attitudes among People Offered Home-Based HIV Testing and Counselling in Blantyre, Malawi: Construction and Analysis of a Stigma Scale'. PLoS ONE 6 (10)
- 7. Leach-Lemens C. (2012, 10th January) 'Home-based counselling and testing identifies people in need of treatment earlier’ Aidsmap
- 8. House of Lords (2011, 1st September) 'No vaccine, no cure: HIV and AIDS in the United Kingdom’