(Last updated 5/6/2014; last reviewed 5/6/2014)
Once a person becomes infected with HIV, the virus begins to multiply (make copies of itself) in the body. As HIV multiplies, it sometimes mutates (changes form) and produces variations of itself. Variations of HIV that develop while a person is taking HIV medicines can lead to drug-resistant strains of HIV. HIV medicines that previously controlled the person’s HIV are no longer effective against the new, drug-resistant HIV. In other words, the person’s HIV continues to multiply. Drug resistance can cause HIV treatment to fail.
Because drug-resistant HIV can spread from person to person, some people are initially infected with a drug-resistant strain of HIV.
Medication adherence means taking HIV medicines every day and exactly as prescribed. Skipping HIV medicines allows HIV to multiply, which increases the risk that the virus will mutate and produce drug-resistant HIV.
As a result of drug resistance, one or more HIV medicines in a person’s HIV regimen may no longer be effective.
Cross resistance is when resistance to one HIV medicine causes resistance to other medicines in the same HIV drug class. (HIV medicines are grouped into drug classes according to how they fight HIV.) As a result of cross resistance, a person’s HIV may be resistant even to HIV medicines that the person has never taken. Cross resistance limits the number of HIV medicines available to include in an HIV regimen.
Drug-resistance testing is done to identify which, if any, HIV medicines won’t be effective against a person’s strain of HIV. Drug-resistance testing is done using a sample of blood.
Drug-resistance testing is done when a person first begins receiving care for HIV infection. Resistance testing should be done whether the person decides to start taking HIV medicines immediately or to delay treatment. If treatment is delayed, resistance testing should be repeated when treatment begins.
Drug-resistance testing done before a person starts taking HIV medicines for the first time can show whether the person was initially infected with a drug-resistant strain of HIV. The results can also guide the choice of HIV medicines to include in a person’s first HIV regimen.
After treatment is started, drug-resistance testing is also done if viral load testing indicates that a person’s HIV regimen isn’t controlling the virus. If drug-resistance testing shows that the HIV regimen isn’t effective because of drug resistance, the test results can be used to select a new HIV regimen.