Blood tests are available which detect whether the HIV in your body is resistant to anti-HIV drugs.

It's recommended that drug resistance tests are used when a person is first diagnosed with HIV in case they have been infected by a virus which is already resistant to one or more drugs. It is also recommended that everybody who is about to start HIV treatment for the first time should have a resistance test. You should also have a resistance test whenever you are changing treatment because your viral load is detectable.

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

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 can now be done if your viral load is above 200 copies/ml – it used to be the case that you had to wait until your viral load was 1000 copies/ml 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. Once you stop taking a drug, the sensitive viruses start to grow rapidly as they are usually more ’fit’ than the resistant viruses. The test may give the wrong answer as it may not be possible to detect the smaller number of resistant viruses.

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.