There are three main types of HIV test.
The first type of test is the HIV antibody test. This test shows whether a person has been infected with HIV, the virus that causes AIDS. Information on this page concentrates mainly on HIV antibody testing. Antibody tests are also known as ELISA (Enzyme-Linked Immunosorbent Assay) tests.
The second type of test is an antigen 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 the infection, P24 is produced in excess and can be detected in the blood serum by a commercial test (although as HIV becomes fully established in the body it will fade to undetectable levels). P24 antigen tests are sometimes used to screen donated blood, but they can also be used for testing for HIV in individuals, as they can detect HIV earlier than standard antibody tests. Some of the most modern HIV tests combine P24 and other antigen tests with standard antibody identification methods to enable earlier and more accurate HIV detection.
Blood supplies in most developed countries are screened for HIV using an RNA PCR test, which can produce positive results several days before a DNA test.
The third type of test is a PCR test (Polymerase Chain Reaction test). The whole process of extracting genetic material and testing it with a PCR test is referred to as Nucleic Acid-amplification Testing or 'NAT'. PCR tests detect the genetic material of HIV itself, and can identify HIV in the blood within two or three weeks of infection.
PCR tests come in two forms: DNA PCR and RNA PCR. Babies born to HIV positive mothers are usually tested using a DNA PCR 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 an RNA PCR test, which can produce positive results several days before a DNA test. When a person already knows that she or he is infected with HIV, they may also have a viral load test to detect HIV genetic material and estimate the level of virus in the blood. This can be performed using either an RNA or DNA PCR test. PCR tests are not often used to test for HIV in adults, as they are very expensive and more complicated to administer than a standard antibody or P24 test. However they may be offered in special circumstances, or by private clinics where patients are willing to pay.
HIV testing
The standard HIV test looks for antibodies in a person's blood. When HIV (which is a virus) enters a person's body, special proteins are produced. These are called antibodies. Antibodies are the body's response to an infection. So if a person has antibodies to HIV in their blood, it means they have been infected with HIV. There are only two exceptions to this rule. Firstly, babies born to positive 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. This is why babies born to positive mothers may receive a PCR test after birth. Secondly, some people who have taken part in HIV vaccine trials may have HIV antibodies even if they are not infected with the virus.
Some test centres may recommend testing again at 6 months if you're deemed to be at particularly high risk of infection.
Most people develop detectable HIV antibodies within 6 to 12 weeks of infection. In very rare cases, it can take up to 6 months. It is exceedingly unlikely that someone would take longer than 6 months to develop antibodies.
Getting tested earlier than 3 months may result in an unclear test result, as an infected person may not yet have developed antibodies to HIV. The time between infection and the development of antibodies is called the window period. During the window period people infected with HIV will not yet have antibodies in their blood that can be detected by an HIV test. However, the person may already have high levels of HIV in their blood, sexual fluids or breast milk. Someone can transmit HIV to another person during the window period even though they do not test positive on an antibody test. So it is best to wait for at least 3 months after the last time you were at risk before taking the test, and abstain from unprotected sex or drug use with shared needles in the meantime. Some test centres may recommend testing again at 6 months if you're deemed to be at particularly high risk of infection.
It is also important that you are not exposed to further risk of getting infected with HIV during the window period. The test is only accurate if there are no other exposures between the time of possible exposure to HIV and testing.
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 had unprotected sex or shared needles again in the meantime, it means that they do not have HIV, and will not therefore go on to develop AIDS.
The only way to know for sure whether you are infected with HIV is to have an HIV antibody test. It is not possible to tell from any symptoms.
What are the reasons to have an HIV test?
Many people who have an HIV test have been worrying unnecessarily. Getting a negative result (which means you are not infected with HIV) can put your mind at rest. If your test result is positive, many things can be done to help you to cope with the HIV positive result and look after your health. If your test is positive, then:
- A doctor can keep an eye on your health. Many people who test positive stay healthy for several years. But if you fall ill, there are many drugs called antiretrovirals that can help to slow down the virus and maintain your immune system. You can also have medicines to prevent and treat some of the illnesses that people with HIV get. You may also have access to trials of new HIV drugs and treatments.
- If you do fall ill, the doctor is going to take your symptoms more seriously if they know that you are HIV positive.
- If you know that you are HIV positive, you can take steps to protect other people. For example, by practising safe sex and informing you past sexual partners.
- Knowing that you have HIV may affect some of your future decisions and plans, for example starting a family.
Read more about learning that you are positive.
What does the HIV test involve?
In most countries, there are many places that you can get tested for HIV. It is recommended that you get the HIV test done at a health clinic, at the doctor's surgery, or at a specialist HIV/AIDS voluntary counselling and testing (VCT) site. When you attend to get tested, you will see a doctor, trained counsellor, a nurse or some other health professional in private. He or she will explain what the test involves and what the result means.
Normally a small sample of blood will be taken from your arm, sent to a laboratory and tested. In the USA and a number of places in Africa, the Middle East and Russia, oral tests are also available which do not require the use of needles. The test is always strictly confidential and only goes ahead if you agree. Your personal doctor will not be told about the test without your permission. Depending on the test used, it can take anything from a few days to a week or longer to get the result back.
A rapid HIV test is also an antibody test. The advantage of a rapid test is that you do not have to return to get your test result. The test results from a rapid test are usually available in approximately 30 minutes. Rapid tests are single-use, and do not require laboratory facilities or highly trained staff. This makes rapid tests very suitable for VCT in resource-poor countries.
How accurate are HIV tests?
Standard HIV antibody (ELISA) tests are at least 99.5% accurate when it comes to detecting the presence of HIV antibodies. This high level of sensitivity however means that their specificity (ability to distinguish HIV antibodies from other antibodies) is slightly lowered. Once an individual is out of the window period, it is more likely that they will receive a false positive result than a false negative.
Any HIV positive result given by an ELISA test must therefore be confirmed using a second test. Secondary tests include:
- Western Blot Assays – 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.
- Indirect Immunofluorescence Assay – Like the Western blot, but uses a microscope to detect HIV antibodies.
- Line Immunoassay - Commonly used in Europe. Reduces 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%.
What is HIV home sampling?
It is generally recommended that the HIV test is done in a health care setting. However, in some countries 'home sampling' kits are available. With a home sampling kit, a person can take a sample (usually a blood sample) and then send it to a laboratory for testing. A few days later, the person phones up a special number, gives their individual identification code, and is given the result over the phone. If the result is positive then a professional counsellor will provide emotional support and referrals.
For home sampling, the major advantages are convenience, speed, privacy and anonymity. In countries where HIV tests are not free, home sampling may be a cost-effective way to get tested. But for some people the lack of face-to face counselling before and after the test may be a disadvantage. There is one company in the USA that offers an FDA-approved home sampling kit for HIV.
There is also a company in the UK that offers home sampling services using oral fluid instead of blood. However, this company only conducts a preliminary screening test instead of the full diagnostic procedure, so clients with reactive test results must visit a clinic for further testing. This service is only suitable for people at low risk of being infected.
What about home testing?
Using an HIV test kit at home means that the results are learned on the spot without any counselling. Reactive test results must be confirmed by further testing at a clinic. If purchased over the internet, there is no guarantee that the test kit is genuine or will provide accurate results. In the event of an incorrect result, there may be no legal recourse.
In many countries it is illegal to sell HIV test kits to the public. There is currently some debate about allowing them to be sold in the USA 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 screening, sampling and testing services
| Home screening (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 £25 | Legal in the USA; sold in shops, online, by phone and mail order for $44 ($60 for rapid service) | 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
This page has so far contained some factual information about HIV and different types of tests. But testing is also about the lives people lead, and the personal views they hold, as Jenny explains below.
"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
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