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

It's recommended that you have a drug resistance test before you start HIV treatment. This will usually happen when you are first diagnosed with HIV, in case you have a strain of the virus which is already resistant to one or more drugs. Again, you should have a resistance test if 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 your 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 only be done if your viral load is detectable. They will 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 a misleading 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. This is the sort of test usually used in clinics.
• 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.