What they are

Genotypic resistance tests are blood tests which look for changes in HIV's genes associated with the development of resistance. These genetic changes are called mutations, and for most anti-HIV drugs the mutation patterns which emerge as resistance develops have already been discovered.

When and why

Tests which measure resistance to anti-HIV drugs have been used in research laboratories for some time. Being able to monitor resistance reliably in HIV treatment centres could make a significant contribution to patient care. Identifying drugs to which the virus in your body is resistant, and so will not work for you, could help you decide which drugs to eliminate and which you might consider taking.

However, these tests are a new development and they are not yet fully validated. This suggests that they should be interpreted by someone who is experienced in using them, and their results considered alongside a full treatment history. Remember that resistance is not the only reason why drugs fail - poor absorption, drug interactions, lack of potency and missing doses are other possible causes. Talk to your doctor about the potential role of each of these factors before starting a new combination.

Resistance tests will be more accurate if performed while you are still taking a failing combination rather than after you have stopped it. This is because when you stop your drugs, resistant viruses will be no more likely to reproduce than sensitive viruses. Most resistance tests are less able to detect viruses which form a minority, e.g. 10-20% of the overall HIV population in your body. In addition, resistance tests may be unreliable if your viral load is less than 500-1,000 copies. For more detail on resistance testing see Testing for resistance in Anti-HIV therapy: Resistance.