Drug-resistance testing identifies which, if any, HIV medicines won’t be effective against a person’s HIV. Drug-resistance testing is done using a sample of blood. People with HIV should start taking HIV medicines as soon as possible after their HIV is diagnosed. But before a person starts taking HIV medicines, drug resistance testing is done. Drug resistance test results help determine which HIV medicines to include in a person’s first HIV regimen. Once HIV treatment is started, a viral load test is used to monitor whether the HIV medicines are controlling a person’s HIV. If viral load testing indicates that a person’s HIV regimen isn’t effective, drug-resistance testing is repeated. The test results can identify whether drug resistance is the problem and if so, can be used to select a new regimen.