Resistance testing can be genotype or phenotype testing (see below). Resistance tests are used to determine which drugs will work best against your virus. There are several types of resistance tests available. If your HIV viral load is greater than 1000 or if you are considering changing your anti-HIV therapy, it is recommended that you have resistance testing.
Genotype tests analyze the genetic makeup of your virus. They look for changes (mutations) in HIV’s enzymes that can make it harder for drugs to work effectively. Your test result will list any mutations found.
Each drug is associated with a mutation or mutations that can make that drug less effective. Some HIV drugs don’t stop working unless several mutations are present.
We still don’t know everything about these mutations, or which combinations of them are most problematic. Because of this, it can sometimes be difficult to figure out how to make treatment decisions based on genotype results.
The phenotype test cultures (grows) your virus in a laboratory. It is then placed in test tubes containing samples of the various HIV drugs. If a certain drug is not able to control the virus, more of that drug is added to the test tube. Depending on how much drug is needed, the lab can determine how resistant the virus is to the drug.
Phenotypic resistance results are reported as susceptible, sensitive, or less susceptible. Susceptible means that the drug will probably work well. Sensitive means that the drug will work as expected in the average person and less susceptible means the drug will probably not work very well for you.
Phenotype test results are often easier to interpret than genotype tests.
This is a genotype test that goes one step further – it uses phenotype data from many patients to predict whether your virus will be sensitive or resistant to each of the HIV drugs.
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