Topic Overview

The human immunodeficiency virus (HIV) changes (mutates) often. Sometimes these changes make the virus resistant to a particular medicine or class of medicines, which means the medicine is no longer effective against the virus. When this happens, the medicine no longer controls virus growth (replication) or protects the immune system.

Resistance testing is done to determine whether resistance has caused treatment to fail and to identify antiretroviral medicines that can be used to treat the infection. There are many reasons that treatment fails, such as:

• The virus is initially resistant to one or more antiretroviral medicines, or the virus mutates and stops responding to the medicines.
• There is a change in the way your body absorbs a medicine.
• There are interactions between two or more medicines that you are taking.
• You have not taken your medicine as prescribed.

Two tests are available to detect resistance to medicines used to treat HIV infections:

• Genotyping assays detect medicine resistance mutations in the viral genes. It takes 1 to 2 weeks to get the results.
• Phenotyping assays measure the ability of viruses to grow in cells with various concentrations of antiretroviral medicines. It takes 2 to 3 weeks to get the results. Phenotyping assays are not available in some areas of Canada.

Both of these tests are done on a sample of blood taken from a vein. These tests may not be accurate if the resistant virus is less than 20% of the circulating virus.

You may be tested for infection with a resistant virus when:

• You are diagnosed with an HIV infection.
• Your viral load has not decreased by at least one-half after 4 weeks of therapy.
• Your viral load has not decreased to a undetectable level within 4 to 6 months of beginning treatment. An undetectable level is defined as less than 50 copies/mL.
• Your viral load is detectable after having been at an undetectable level.

Resistance reduces the number of treatment options in the future, so it is important to keep resistance from happening.