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

If you start taking antiretroviral medications early in the course of your infection, before your CD4+ cell counts have dropped and your viral load has increased, you may cycle through all the available medications. In this case, you may have exhausted all the available medications and have no options left when your viral load and CD4+ counts are at their worst.

Drug resistance testing is done to determine whether resistance is the cause of treatment failure and to identify antiretroviral medications that can be used to treat the infection. There are many reasons for treatment failures, such as:

Initial viral resistance to one or more antiretroviral medications.
Altered absorption or metabolism of the medications.
Interactions between medications.
Not following the prescribed medication schedule.

There are two tests available for testing for drug resistance in HIV:
Genotyping assays detect drug resistance mutations in the viral genes. It takes 1 to 2 weeks for the results.

Phenotyping assays measure the ability of viruses to grow in various concentrations of cells with antiretroviral medications. It takes 2 to 3 weeks for 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 drug-resistant virus is less than 20% of the circulating virus.

You may be tested for drug resistance when:

You develop symptoms of HIV infection. The symptoms may be similar to an upper respiratory tract infection, the flu, or another viral illness.

Your viral load has not decreased by least one-half after 4 weeks of therapy.

Your viral load has not been suppressed to a undetectable level with 4 to 6 months of initiating treatment. An undetectable level is defined as less than 50 copies/mL (by bDNA or RT-PCR test method).

Your viral load has been detected after having been at an undetectable level.