The human immunodeficiency virus (HIV) changes (mutates) often. Sometimes these changes make the virus resistant to a particular medicine or class of medicines, which means the medicine is no longer effective against the virus. When this happens, the medicine no longer controls virus growth (replication) or protects the immune system.
Resistance testing is done to determine whether resistance has caused treatment to fail and to identify antiretroviral medicines that can be used to treat the infection. There are many reasons that treatment fails, such as: