If you miss doses of your medications, HIV will multiply more easily. More mutations will occur, and some of them could cause resistance. If you have to stop taking any antiviral medication, you should stop taking all antiviral drugs at the same time. If you are not taking any antiviral drugs, the wild type virus will multiply faster than any resistant mutations. Your medications should continue working for you.
Laboratory tests can measure phenotypic and genotypic resistance.
A sample of HIV is grown in the laboratory. A normal dose of one antiviral medication is added. The growth rate of the HIV is compared to the rate of wild type virus. If the sample grows more than normal, it is resistant to the medication. It can take over a month to get the results.