Blood tests are available which detect whether the HIV in your body is resistant to anti-HIV drugs. These tests are a relatively recent addition to HIV care. In the coming years, we can expect to learn more about the best way to use them, and for technology to improve to make them a more accurate measurement tool.

At the moment, it's recommended that drug resistance tests are used when a person is first diagnosed with HIV. It is also recommended that everybody who is about to to start anti-HIV treatment for the first time should have a resistance test. You should also have a resistance test whenever you change treatment.

Resistance tests are also recommended to help guide the choice of treatment in women who are pregnant, and in children.

Using and interpreting resistance tests

Results should be interpreted by someone who is experienced in their use. Test results should be considered alongside a full treatment history, rather than in isolation. This is because drug resistance is not the only reason why HIV treatment can fail – missed doses, poor absorption and drug interactions are other possible causes to consider.

Resistance tests are now of value if your viral load is above 200 copies – it used to be the case that you had to wait until your viral load was 1000 copies or more before they could produce accurate results.

Resistance tests will also be more accurate if done while you are still taking a failing combination rather than after you've stopped it.

Which tests to use

There are two main methods of testing for HIV drug resistance:

• Genotypic tests which look for specific mutations in HIV's genes that are known to be linked with resistance to anti-HIV drugs.
• Phenotypic tests which measure the concentration of a drug required to reduce viral replication by a set amount. When resistance to a drug begins to develop, higher levels of that drug will be required to stop HIV growing.

There is no clear indication that one type of test is more useful than another at the moment – each has its pros and cons.