Resistance tests

Resistance tests can show which drugs are unlikely to work.

UK treatment guidelines recommend a resistance test before changing treatment.

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.

There are two main types of these blood tests. See Figure 8 below.

Figure 8: Types of resistance tests

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.

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.

Checking the changes in your virus gives 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. With drug resistance, this information is just as important.

Phenotype tests (‘fold’ changes)

A phenotypic resistance test adds each increasing concentrations of a drug in a test tube that contains your HIV. It shows how sensitive or resistant you are and how active each drug is.

Results are given in terms of how much drug is needed to have the same effect as a regular dose on non-resistant HIV.

For example, 10-fold resistance to a drug means 10 times as much drug is needed to get the same antiviral 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 only 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 compares results from a genotype test to those in a large database of matched phenotype results.

This test is therefore not really a phenotype test but can still be useful.