Genotypic resistance testing helps doctors make better treatment decisions for their patients when existing anti-viral therapies are not working effectively. If drug resistance is found, a new treatment regimen may be chosen.

Genotypic resistance testing is ordered when viral load values (a measure of how much HIV is in your body) rise steadily during therapy, indicating treatment failure and the possibility of resistance. The test may also be ordered before the start of therapy for an acute infection if a drug- resistant virus is suspected, so that immediate, appropriate therapy is possible.

The test result identifies the viral mutations. These are described by a combination of letters and numbers, for example K103N. Based on the test result, your doctor will identify whether a given mutation is one known to cause drug resistance. Not all mutations cause drug resistance. Some mutations are very common, and resistance is known to certain drugs and/or drug combinations. This is important information for physicians to help them predict which treatment regimen will work best for each patient.

Genotypic resistance testing, although relatively new, is available in large laboratories and reference centers.

The test is not good at detecting “minority” mutations, which affect less than 20% of the virus population.

Genotypic resistance testing works best on blood samples with a viral load of at least 1,000 copies per milliliter of blood. If your viral load is very low, the test probably won’t work. It may not detect resistance to all drugs.