Overview

Drug resistance happens when disease-causing organisms change (mutate) over time. Those changes can make the organisms survive treatment that would usually kill or control them.

Because HIV mutates often, it can easily become drug-resistant. This can make HIV harder to treat. Sometimes people become infected with HIV that is already drug-resistant.

Missing doses of your HIV medicines increases your risk of drug resistance. That's why it's important to take your medicines as directed.

When should you be tested for resistance?

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

• You are diagnosed with HIV.
• You start treatment.
• Your viral load is not going down as expected.
• Your viral load is detectable after having been at an undetectable level.

Two tests can find a resistant virus:

• A genotyping assay detects medicine resistance changes in the viral genes. This is used most often.
• A phenotyping assay measures how well viruses grow in cells with various concentrations of antiretroviral medicines. This test is rarely done. It may be used if your doctor thinks you may have a virus that is resistant to a lot of medicines.

Both of these tests are done on a sample of blood taken from a vein. These tests may not be accurate if you have not been taking your medicines for a while.