Resistance testing is now performed fairly routinely. Resistance testing shows whether or not a given drug is effective against your HIV virus. It may tell you whether you have been infected with a drug-resistant strain of HIV, and also if you are resistant to drugs you are currently taking, or have taken in the past and then initially start to benefit from these medications. Resistance is the ability of HIV to overcome the effects of anti-HIV medication. The best way to avoid drug resistance developing is to make sure that you never miss any of your tablets.
Every time HIV reproduces, or replicates, there is a possibility that a mistake is made. These mistakes or mutations produce changes in the new generation of virus, which may make the virus resistant to the anti-HIV drugs you are taking. The best way to avoid this is to be on a fully supressive drug regime. This means being on the right combination of drugs and making sure that you never miss taking your tablets.