Overview

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

Here are some reasons why treatment fails.

• The virus is already resistant to one or more antiretroviral medicines when you start treatment, 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 you are taking.
• You have not taken your medicine as prescribed.

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

When should you be tested for resistance?

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

• You are diagnosed with HIV.
• Your viral load:
  • Has not decreased by at least one-half after 4 weeks of therapy.
  • Has not decreased to an 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.

Two tests can find a resistant virus:

• Genotyping assays detect medicine resistance changes in the viral genes. It takes 1 to 2 weeks to get the results.
• Phenotyping assays measure how well viruses grow in cells with various concentrations of antiretroviral medicines. It takes 2 to 3 weeks to get the results.

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.