Q and A

Question

What different tests are used to test for HIV?

Answer

There are three types of tests that can be used to diagnose HIV.

i) Antibody only tests – window period: from 6 weeks after exposure (i.e. rapid, Tri-Dot and Centaur tests). Self-testing tests are antibody only tests.

The most common test for HIV is an antibody test. This tests for a persons immune response to having come into contact with HIV. An antibody is a protein produced by your body when it recognises an infection.

The main antibody test is called ELISA (Enzyme-Linked Immunosorbent Assay). If blood tested is from a finger prick, then ‘rapid’ tests can give the results in 10-30 minutes. If the blood is taken with a syringe, it is usually then sent to a lab, and results can take anything from a few hours to a few weeks, depending on the urgency of the sample, and procedures of that clinic.

If this result is negative or non-reactive, then you are HIV negative (allowing for the window period – see below).

If the result is positive, this does not mean that you are HIV positive. It means you need a second confirmatory test. A small percentage of people can test positive with ELISA who are not HIV-positive (called a ‘false-positive’ result).

All positive results need to be confirmed by a second, more sensitive antibody test called western blot. The western blot test takes longer (usually a week or so) and is the most accurate at identifying genuine positive results. When it is used to confirm a positive result the western blot test is 100% accurate.

HIV antibody tests do not work as soon as you are infected. This is because it usually takes several weeks to generate antibodies to HIV. This is called the ‘window period’. Most people generate this response within 4–6 weeks, but approximately 5% of people take up to 3 months. Very rarely it can take longer.

The result of an antibody tests therefore only tells your your HIV status three months prior to the test. This is why people are advised to re-test three months after the exposure if you test at 4-6 weeks, or to wait three months before taking an HIV test.

Taking an antibody test less than 4 weeks after exposure will not tell you very much. You need to wait until at least 4–6 weeks after the exposure.

You then need to confirm a negative result with a second test three months after the exposure.

This confirmatory test is to cover the small chance (less than 5%) that you may not have developed an antibody response this early.

A modified ELISA tests called RITA (Recent Infection Testing Algorithm) can indicate whether the infection is likely to have been in the previous six months. Detuned means that the test has been modified, so that it is less sensitive.

It is provided to every NHS clinic free by the public health laboratory based in Colindale. In the UK this is recommended with every positive HIV test. The results given a good indication that the infection is recent but are not a guarantee. This is now referred to as RITA .

RITA used to be called STARHS (Serological Testing Algorithm for Recent HIV Seroconversion). A technical review of STARHS is at this link.

ii) p24 antigen tests and combined antibody/antigen tests (ie Duo tests) – window period: 3–4 weeks after exposure (UK clinics generally say 28 days).

Some HIV tests combine an antibody (Ab) test with an antigen (Ag) test.

An antigen is a name for genetic material caused by a virus or other infection. In these tests the antigen being tested is p24 (protein 24), a major protein that is part of HIV.

p24 is detected 2–3 weeks after infection – before antibodies are produced, but not really afterwards – and p24 levels only stay high for the next 1–2 months.

p24 antigen tests can be used 3–4 weeks after exposure – so can give an earlier result than an antibody test. The are combined with antigen tests because the window period that p24 is detected is very short.

As with antibody tests, a small percentage of people may have a delayed response to HIV so people using this test four weeks after any potential exposure are routinely recommended to confirm a negative result three months later.

iii) Viral load tests (PCR RNA and DNA) – window period: 1–4 weeks after exposure

Viral load tests are also called PCR (Polymerase Chain Reaction) tests. They either test for HIV RNA or HIV DNA.

Rather than look for an immune response, they look directly for HIV, usually in a blood sample. In a new infection, viral load can reach very high levels within a few weeks of infection. More rarely this can be within if few days.

Viral load tests are not approved for diagnosing HIV testing.

However, viral load tests are sometimes used in specific circumstances. This is usually after a very high risk or traumatic risk,  especially if several symptoms suggest seroconversion. In the presence of symptoms,  a negative viral load test is useful. It can rule out HIV because symptoms caused by HIV seroconversion would by definition need viral load to be both high and detectable. A negative viral load test can reduce initial anxiety and worry until an HIV antigen test can confirm there is no infection.

Viral load test are neither approved or recommended for routine HIV testing by people who just want to test more quickly than the four weeks recommended with an antigen/antibody test. They are less accurate (actual infections can easily be missed), more expensive and require more complicated laboratory technology.

After infection, viral load is usually very high within the first 1–2 weeks, and so this test can be usbloed to confirm a suspected early infection.

In adults, viral load tests look for RNA and are only usually done when there is both:

  1. A history of recent high risk exposure (ie condom break an HIV positive partner who is not on treatment); and
  2. Symptoms consistent with HIV seroconversion (fever, extreme tiredness, heavy ‘flu-like illness etc).

Viral load tests are also used in babies born to HIV positive mothers, and test for DNA.

This is important to know to decide on whether the baby needs early treatment. Because a baby carries his or her mothers anitbodies for the first two years, antibody testing is not useful until a child is around two years old.

Note: This answer was updated in January 2018 and October 2011 from an original post in January 2008.

131 comments

  1. Simon Collins

    Please see this new guide to HIV testing and the risk of sexual transmission:
    http://i-base.info/home/new-i-base-guide-hiv-testing-and-risks-of-sexual-transmission/

    This resource has been developed in response to the many questions i-Base get on testing and transmission.

    It explains whether you need to test and how to interpret test results.

    This discussion thread is now closed.

    New questions about HIV treatment can still be asked at this link.