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When you have a blood test for HIV, doctors don't look for the virus in your blood. They look for antibodies to the virus. Antibodies are particles that your immune system makes to fight an infection. They look different for every virus you're infected with.

It takes about three weeks for most people's bodies to make antibodies to HIV. So, if you have a negative result, your doctor may advise you to have a repeat test later on to make sure your result is correct. Guidelines for doctors recommend waiting at least a month before having another test.¹ Many doctors suggest waiting between three months and six months.

It's normal to feel nervous about having a test for HIV. You'll usually be offered some counseling before your test to make sure you understand what's going to happen and that you're ready to take it.

There are a few different tests for HIV.

Most clinics use an HIV test called the ELISA test (which stands for enzyme-linked immunosorbent assay). To have this test you need to have a sample of blood taken from a vein, usually from the inside of your elbow.

The ELISA test is good at detecting antibodies to HIV in your blood. But the test isn't always accurate. You'll be asked to come back to the clinic in one week or two weeks to get your results. Some clinics offer same-day testing, where you get your result later the same day. If you get a positive result, doctors will do another test (on the same blood sample) to be sure. Most clinics use the Western blot test to double check the result.

The Western blot test is more expensive than the ELISA test and takes longer to do.

It's very rare for someone to have a negative result and for this to be wrong. So, if you get a negative result and it's at least three months since you thought you might have been in contact with HIV, you probably don't have the virus.¹

There are several home HIV test kits you can buy. But only one test has been approved by the U.S. Food and Drug Administration (FDA). It's called the Home Access Express HIV-1 Test System. The FDA has looked at this home kit and says that it is very accurate at diagnosing HIV.²

To do this test you have to stick your finger with a needle and put a drop of blood onto a special piece of paper that comes in the kit. You then mail the paper to a laboratory with a confidential and anonymous personal identification number. The laboratory does the HIV test and you have to call for the result.

The test uses both the ELISA test and the Western blot test. So the results you get are the same as you would get if you go to a clinic.

You'll get counseling over the phone about the test and your result. But it might not be the same type of counseling you would get at a clinic.

These are tests that give you a result in only 30 minutes. But they are more expensive than the standard tests. They are often done in emergency rooms when it's important to get a result quickly.

If you get a positive result, you'll need to have a Western Blot test to confirm the result.

You can get an HIV test at most hospitals, sexually transmitted infections clinics, family planning clinics and doctors' offices. There are also special HIV testing clinics in many states.

You don't have to give your real name to get it done. No one needs to know if you have had the test or what the result is.

If you get a positive HIV blood test it means that you have been infected with HIV. It doesn't mean that you have AIDS.

Once you're infected with HIV, the virus starts to attack your immune system, the cells that protect you from infections. But it takes many years for this damage to affect your health. And there are drugs that can help you stay healthy for longer.

See What treatments work for HIV infection?

Getting a positive test result can be a huge shock. And you'll probably have lots of questions. Your doctor will be able to answer these for you. He or she will also tell you about the people and organisations you can turn to for advice. To learn more, see What should I do if I get a positive result?

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.