A blood test is the usual test for HIV. It looks for signs of infection in the blood. These can be ‘antibodies’ (made by the body to try and fight HIV) or a protein the virus makes in the early weeks after infection.
Some tests just look for antibodies, while some look for antibodies and the protein.
If no sign of infection is found the test is ‘negative’, if infection is found it is ‘positive’. Someone who tests ‘positive’ has their blood tested a second time to be absolutely sure the result is accurate. Testing positive doesn’t mean a person has AIDS or will go on to get it, but it does mean they can pass HIV on if, for example, they have unprotected sex or share injecting equipment.
Blood can be taken from the arm with a syringe and the sample sent to a laboratory, with results taking a day or so, possibly a week. Alternatively a small sample of blood is taken during a finger prick test (called a ‘rapid test’), with the result ready within a minute (some rapid tests take up to 20 minutes).
There are also oral HIV tests that get a result from saliva (antibodies are in the spit of an infected person but this doesn’t mean you can catch HIV from saliva; you can’t).
Signs of HIV infection cannot be detected in the blood immediately – this usually happens within four weeks of infection (sometimes longer). Different tests take different lengths of time before they can detect a recent infection. If your risk was in the last three months tell the person testing you as it may affect the type of test they use. Whenever the risk was, don’t put off testing. If your risk was recent the testing centre will probably advise you to have a test immediately, followed by a second one a few weeks later (this will pick up any recent infection the first test might have missed).
In the past people were advised to wait three months (12 weeks) between taking a risk and testing; this is no longer necessary as tests are now more advanced and can detect signs of infection much sooner.
Very occasionally it can take up to three months for antibodies to appear in the blood, so a HIV negative result is only totally accurate if three months have passed between the test and the last time a risk was taken. However, a negative result four to eight weeks after taking a risk is a very good sign that HIV infection hasn’t happened. But to be absolutely sure another test should be taken around eight weeks later; the testing centre will advise you.
Important: if a risk of HIV was taken in the last 72 hours it is possible to take PEP (Post-exposure Prophylaxis) to help stop an infection happening.
Where to test
HIV tests are available free and confidentially from:
- sexual health clinics
- HIV testing centres such as Terrence Higgins Trust Fastest centres (where rapid tests are available)
- a GP /family doctor.
Private clinics will test for a fee and home sampling kits are for sale (you collect a blood or saliva sample at home, then send it to a lab who inform you of the result).
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