"The treatment goal of antiretroviral therapy is to achieve and maintain an undetectable viral load. Patients on antiretroviral therapy who do not achieve this goal can develop drug resistance to one or more drugs in the regimen. Poor medication adherence is the most likely reason for virologic failure and the development of drug resistance. Development of new drugs and clinical availability of HIV resistance tests has given providers more options for treatment experienced patients with extensive drug resistance. Evaluating patients with extensive drug resistance requires knowledge of previous antiretroviral drug regimens, previous drug resistance tests, and interpretation of those test. Managing treatment experienced patients can be complex and may require consultation with an HIV expert"--