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 tests – window period: 6–12 weeks after exposure (i.e. rapid, Tri-Dot and Centaur 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.

If the result is positive though, this does not mean that you are HIV-positive. 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 identifing genuine positive results.

HIV antibody tests do not work as soon as you are infected because it usually takes several weeks to generate antibodies to HIV (the ‘window period’). Most people generate this response within 4–6 weeks, but occasionally it can take longer.

The result only tells your your HIV status three months prior to the test. This is why people are advised to wait three months to take an HIV test, or to re-test three months after a negative result.

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 and to confirm a negative result with a second test three months later. This confirmatory test is to cover the small chance (less than 5%) that you may not have developed an antibody response this early.

A ‘detuned’ ELISA called STARHS is sometimes used in research to see if someone was infected in the previous six months. Detuned means that the test has been modified, so that it is less sensitive.

STARHS stands for Serological Testing Algorithm for Recent HIV Seroconversion, and is provided to your clinic free by the public health laboratory based in Colindale. In the UK anyone can ask for this test if they think they have a recent infection. This is sometimes referred to as RITA (Recent Infection Testing Algorithm).

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) – window period: 1–4 weeks after exposure

Viral load tests are also called PCR (Polymerase Chain Reaction) tests. They either test for RNA of 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 recommended for HIV testing except in specific circumstances because they are less accurate, more expensive and take longer to get a result.

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

In adults, viral load tests look for RNA and are only usually done when there is both
i) a history of recent high risk exposure (ie condom break with a known HIV-positive partner), and
ii) symptoms of HIV infection (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.

Answers by: Simon Collins