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Resistance tests
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 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 when ever you change treatment.
An exception to this is where HIV infection is very recent. Resistance tests are recommended for people who start treatment within six months of contracting HIV. Within this period, any transmitted drug resistant strains may be detected by the test.
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
Resistance tests are a complex new development in HIV care. Results should be interpreted by someone who is experienced in their use, who is likely to be at the centre which performs the analysis of your blood sample sent for resistance testing, and not at your HIV treatment centre. 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 may be unreliable if your viral load is below 1,000 copies. You may need special advice about the results of a drug resistance test if you are infected with a type of HIV called non-B subtype. These subtypes are found more commonly in most parts of the world, particularly outside Europe and North America.
Most HIV-positive people in the UK who contracted HIV in Africa will be carrying a non-B subtype of HIV, as well as an increasing proportion of those who have contracted HIV through heterosexual sex.
Resistance tests will also be more accurate if done while you are still taking a failing combination rather than after you've stopped it. This is because when you stop your current drugs, drug resistant HIV will be no more likely to reproduce than drug sensitive HIV – and usually less likely. Resistant viruses that once were the most common will be out-grown by sensitive viruses until they form one of many sub-groups of HIV within the body. Resistance tests are unable to detect sub-groups which make up less than 10-20% of the total HIV in your body. Starting a drug which a sub-group of your HIV is resistant to will allow that group to grow back again, causing your treatment to fail.
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. Genotypic tests are cheaper and deliver results sooner. Changes in phenotype result from changes in genotype, so genotypic testing may produce the earliest clues about emerging resistance.
Phenotypic testing provides a quantitative guide to a drug's effects on the HIV in your body. However, these tests are more expensive and take longer to produce results.
The Virtual Phenotype™ is an interpretation system which may be used more often to analyse genotypic resistance test results in future.
It provides a 'phenotypic' result based on the matching of resistant HIV strains within a large database of genotypic and phenotypic information.