Resistance tests

Resistance tests can show which drugs you have developed resistance to and which drugs are unlikely to work.

UK treatment guidelines recommend that everyone changing treatment should have a resistance test.

You generally need to have a viral load over 500-1000 copies/mL to produce a reliable result. You also need to have blood taken while you are still using your failing combination.

Types of resistance tests

There are two types of these blood tests.

Genotype tests (mutation changes)

A genotypic resistance test looks at the structure of your virus and how it has changed from normal ‘wild type’ virus.Different changes are associated with resistance to different drugs.

By checking the changes in your virus to these known mutations you get a good idea of which drugs are unlikely to work.

Although this test does not register very low levels of resistance, it can still be vital as a guide to choosing drugs for your next combination.Results should take about a week.

Although genotype tests cannot predict which drugs will work, they can predict which drugs will not and with multidrug resistance, this information is just as important.

Phenotype tests (‘fold’ changes)

A phenotypic resistance test adds each drug to your HIV in a test tube. It shows how sensitive or resistant you are and how well each drug is working. Results are given in terms of how much more resistant your virus is compared to a sensitivevirus.

For example, 10-fold resistance to a drug means you would need 10 times as much drug to get the same anti-HIV effect.

Interpreting phenotype tests is complicated. Sometimes it is not clear at what level individual drugs remain active, and each drug can be different.

Phenotype tests are recommended in the UK guidelines when genotype results alone do not provide a clear result. Phenotype resistance tests are 3-4 times more expensive than genotype tests. They take longer to get results – usually 2-4 weeks – because the tests cannot be run in your own clinic and it takes time for the virus to grow.

Virtual phenotype tests

The ‘Virtual Phenotype’ test, available in some clinics, uses results from a genotype test and compares this to a large database of matched phenotype results.

Fig 8. Types of resistance tests

Note: Resistance tests can only detect resistance to drugs that you are currently taking or have recently been taking. A virtual phenotype’ test compares results from your genotype test to a large database of phenotype results to predict your phenotype.