Summary: Resistance to anti-HIV drugs
- Resistance develops when HIV mutants emerge which can reproduce in the presence of a drug.
- These mutants form the basis of the new virus population, because they can reproduce despite the presence of the anti-viral drug.
- Resistance is one of the main reasons why drugs stop working.
- Virus populations can develop resistance to more than one drug. This is why combination therapy may fail.
- Once resistance to one drug has emerged, this virus population may also be resistant to drugs that you have not taken. This is called cross-resistance.
- The chance of resistance is reduced if you take every dose of the anti-HIV drugs prescribed to you. This is because missing doses causes blood levels of the drug to fall, and this makes it easier for mutant viruses to reproduce.
- The more often you miss doses, the more likely it is that resistant viruses will emerge.
- Another thing which encourages the development of resistance is continuing to take drugs after treatment begins to fail, i.e. when viral load begins to go up or CD4 cell count. However, resistance to different drugs emerges at different speeds.
- People who inject drugs may develop resistance more quickly than people who do not inject.
- Tests have been developed to detect which drugs you are resistant to and your level of resistance to them. British and European guidelines recommend the use of resistance testing to guide choice of therapy in people starting or changing treatment and in recently infected individuals.
- Several trials have shown resistance testing with expert interpretation is beneficial when therapy is changed, though the patient groups who may benefit most are not well defined.
- The case for resistance testing to guide therapy in people who have been infected for several years is less strong than in people who are recently infected because tests taken in later disease are unlikely to detect minority resistant viruses. Storing samples from early in infection is desirable where possible.
- Resistance testing may also be useful in pregnant women and newborn infants.
- Transmission of drug-resistant virus appears to be increasing in the United Kingdom and the United States.
- Problems remain with standardisation of these tests and interpretation of results. However, the best type of resistance test is not known at present.
