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HIV Testing

Why get tested for HIV?

Sign promoting HIV testing in Livingstone, Zambia

For people who think they may have been exposed to HIV, having a test and receiving a negative result (which means they are not infected with HIV), can put their mind at rest.

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 the treatment will work. Doctors can monitor a 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 prevent HIV transmission to other people. They can take treatment, practice safe sex, and inform previous sexual partners that they may have been at risk of infection.

Those who test positive who were thinking of starting a family can learn about ways to prevent their child from becoming infected with HIV through mother-to-child transmission.

Where can you be tested for HIV?

Globally, HIV testing is provided at a number of locations including:

  • hospitals
  • health clinics
  • doctors surgeries
  • specialist HIV/AIDS voluntary counselling and testing (VCT) sites

In many countries, HIV testing services are delivered in more innovative and creative ways such as:

  • in the home
  • through mail order or online
  • mobile and outreach testing in the community
  • workplaces
  • schools
  • religious facilities
  • transport hubs. 1

What does a HIV test involve?

When someone attends a testing site they will usually see a doctor, trained counsellor, nurse or other healthcare professional in private. They will have a pre-test consultation during which they can ask any questions about the test.

The healthcare professional takes a blood sample (from the arm or finger) or an oral fluid sample, 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 the results arrive, the healthcare professional will explain what the results means, and if they are positive, the next course of action regarding treatment.

When should you have a HIV test?

The 'window period' is the term used to describe the period of time between HIV infection and the indicators of HIV infection showing up on tests, such as antibodies and antigens. During this time, a test may give a ‘false negative’ result, which means the test will give a negative result even though a person is infected with HIV. A test must be taken after the window period for an accurate result.

As described below, different types of test have different window periods. A person should always discuss the limitations of the test they are using with a healthcare professional before being tested.

It is very important to note that if a person is infected with HIV, HIV transmission is possible to others during the window period. 2

Types of HIV tests

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 rapid tests. These tests are sometimes used in combination.

There are other types of HIV testing which are used once a person has been diagnosed with the virus, such as the CD4 test and the viral load test.

Table summarising the different types of HIV tests

HIV testsWhat do they test for?What is the window period?How long for the results?ReliabilityCommonly used in
Antibody testsHIV antibodiesThree monthsBetween a few days and a few weeksHighEurope
Antigen (p24) testp24 viral proteins11 days to one monthA few days to a weekHigh for primary infection, increasingly unreliable afterwardsUK – standard screening assay
Fourth generation testsHIV antibodies & p24 viral proteins11 days to one month2-14 daysHighUK – recommended as first-line assay
United States
PCR / NAT testGenetic material belonging to HIV12 daysUp to a weekHighMost developed countries
Rapid testHIV antibodiesThree monthswithin 20 minutesSatisfactory for uncomplicated HIV infectionResource-poor settings

Antibody tests

HIV antibody tests are specifically designed for routine diagnosis of HIV in adults. They are both inexpensive and very accurate and are sometimes called 'third generation' tests. The ELISA antibody test (enzyme-linked immunoabsorbent) also known as EIA (enzyme immunoassay) was the first HIV antibody test to be widely used. 3

  • How do antibody tests work?

When a person is infected with HIV, their body responds by producing special proteins to fight the infection called antibodies. A 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 a HIV antibody test, even if they are actually HIV negative. 4
  • Some people who have taken part in HIV vaccine trials may have HIV antibodies even if they are not infected with the virus. 5

Most people develop detectable HIV antibodies within 6 to 12 weeks of infection. In very rare cases, it can take up to 6 months, though there are very particular reasons why this may happen, such as auto-immune disorders. 6

  • What is the window period for antibody tests?

The window period for a HIV antibody test is estimated to be three months after 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 during that period, they are not infected with HIV. 7

  • How accurate are antibody tests?

Antibody tests are extremely accurate in detecting the presence of HIV antibodies. Antibody tests are very sensitive and so will detect very small levels of HIV antibodies. This high level of sensitivity however, means that their specificity (ability to distinguish HIV antibodies from other antibodies) is slightly lowered. As a result, there is 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 comes 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 antibody tests. It is complex to administer and may produce inaccurate results if a person is also infected with a different, transient virus. A transient virus is a virus that a persons immune system has removed all viral DNA from the body.
  • 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 equally accurate as the Western blot.
  • A second antibody test – in resource-poor settings with a high HIV 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 an inaccurate result is less than 0.1 percent. 8

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. However, as HIV becomes fully established in the body levels of p24 fade to undetectable levels. Because of this, the test is no longer routinely used in the United States or the European Union to screen blood donations. 9 10

Because of their very low sensitivity and the fact that they only work before antibodies are produced in the period immediately after HIV infection, P24 antigen tests are not usually used for general HIV diagnostic purposes. They are now most often used as a component of 'fourth generation' tests.

  • What is the window period for a p24 antigen test?

The window period for an antigen test is estimated to be from 11 days to one month after exposure to HIV infection.

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 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. 11 In June 2010, the FDA approved the first fourth generation test in the United States. 12

  • What is the window period for a fourth generation test?

The window period for a fourth generation test is estimated to be from 11 days to one month after exposure to HIV infection.

PCR or NAT test

A PCR test (Polymerase Chain Reaction test) detects the genetic material of HIV in the blood. It can identify HIV in the blood within two to three weeks of infection. The test is also known as a viral load test or NAT (nucleic acid-based test).

Babies born to HIV-positive mothers are typically 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 or NAT tests. In the United States, NAT tests have been used to screen blood donations since 2001 as they shorten the window perod. 13

However, they are rarely used to test for HIV in individuals, as they are very expensive and more complicated to administer and interpret than a standard antibody test.

  • What is the window period for a PCR or NAT test?

The window period for a PCR or NAT test is estimated to be 12 days after exposure to HIV infection.

Rapid HIV tests

A man demonstrates the use of a rapid oral HIV testThe rapid HIV test is an immunoassay that uses blood or oral fluids to produce results within 20 minutes. They are based on the same technology as the tests described above, but are easier to use and do not require laboratory facilities or highly trained technicians. 14

If no antibodies are detected, this does not mean that a person is not infected with HIV. It can take several months for detectable levels of HIV antibodies to occur. All positive results from a rapid test must be followed up with a confirmatory test. Confirmation results can take from a few days to a few weeks to come through. 15

The video below shows how fast and simple the procedure for a rapid test is.

  • What is the window period for a rapid HIV test?

The window period for a rapid test is estimated to be three months after exposure to HIV infection.

HIV home sampling and HIV home testing

It is generally recommended that a HIV test is carried out in a healthcare setting. However, in some countries, HIV home sampling and home testing kits are available.

Home sampling

Home sampling kits can be bought by a person from a chemist or online. It involves taking a sample of blood or saliva and sending 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. The main advantages of home sampling are convenience, speed, privacy and anonymity.

Only one company in the USA offers an FDA-approved home sampling kit for HIV. It can be bought from pharmacies or online. 16 A UK company also offers home sampling services but uses oral fluid instead of blood. If a person’s test result is positive they will need a follow-up blood test at a clinic. 17

Home testing

A home self-test involves a person conducting a rapid HIV antibody or p24 antigen test in their home. The person takes either a blood or saliva sample and can interpret the result within minutes. A positive result must be confirmed by a blood test conducted by a healthcare worker in a clinic. 18

UNAIDS says that self-testing could be a solution in parts of the world where people living with HIV are not reached by existing testing services and do not know their status. 19 In many healthcare settings, people worry about their test results remaining confidential with supervised home-based testing having the potential to overcome some of these issues. 20

Studies in both Kenya and Malawi have shown how home-based testing and counselling can be an effective way of getting people who test positive into treatment and care. 21 22 However, in many countries, it is illegal to sell HIV test kits to the public. In other places, there have been instances of the sale of fake home testing kits. 23

In July 2012, in the United States, the FDA approved a home testing kit for the first time. It requires a saliva sample and gives a result in twenty minutes. 24 Likewise, in the UK, in April 2014, a law was passed allowing the sale of HIV home testing kits over the counter but the tests themselves may not actually be available until early 2015. 25 26

Comparison of HIV home sampling and home testing services in the UK and USA

 Home sampling (UK)Home sampling (USA)Home testing
MethodSampling device purchased; oral sample taken at home and sent to lab for testingSampling device purchased; blood sample taken at home and sent to lab for testingKit purchased for taking a sample and testing it at home
NotificationReactive results given by phone; negative results given by emailAll results given by phoneResults produced at home
AvailabilityLegal in the UK; available onlineLegal in the USA; available in shops, online, by phone and mail orderLegal in the UK but tests themselves not yet available. Approved for sale and use in the USA
Potential for mistakesOral sample may be taken incorrectly, possibly leading to a false negative resultLow potential for mistakes as blood sample is clearly visible on cardTest may be performed or interpreted incorrectly, possibly leading to a false result
ReliabilityNegative results are definitive; reactive results are preliminary and must be confirmed by further tests at a clinicAll results are definitive; as reliable as conventional testingNegative results are definitive; reactive results are preliminary and must be confirmed by further tests at a clinic
Pre-test counsellingOptional at extra cost, by phoneOptional, by phoneUnlikely to be provided
Post-test counsellingAlways provided for reactive results, by phoneAlways provided for positive results, by phoneOptional, by phone

 

HIV testing - personal accounts

"My husband had a test as it is required for his job and it was negative so we thought since he was negative i will be too. After almost one year we went together and i was found positive and my husband negative. It has been 2 years and my cd4 has been stable and viral load undetectable. I’m planning to have a baby soon. You can live a normal life with HIV lets get rid of the stigma." Sue

"I went for a rapid test in the clinic attached to the hospital and never expected the answer to be that i was positive. i fell to the floor, cried like i was in a soap opera, and asked "who's going to raise my children?" The tester was so amazing, so helpful in talking me down from this reaction, telling me he'd known people living with HIV for 25 years +. I latched on to those words like a life vest those first few months." Anonymous

More information

 

References

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