Strategies for third line HIV therapy

May 2008     View PDF     En español

Drug resistance

Drug resistance occurs when HIV can reproduce in the presence of drugs. Cross resistance occurs when HIV that reproduces in the presence of a particular drug can also reproduce in the presence of other drugs in the same class. For instance, HIV that’s resistant to Sustiva (efavirenz) is usually cross resistant to both Viramune (nevirapine) and Rescriptor (delavirdine). When a person develops drug resistance or cross resistance, it can severely limit effective the options of HIV drugs.

Two kinds of tests help determine if your HIV has become resistant. Both require that your HIV level be at least 1,000 for best results. The genotype test finds out which drugs your HIV has become resistant to. The phenotype test is used to find out the degree of resistance. However, both can fail to detect resistance because drug-resistant HIV may not be present in the blood sample, or may be present at too low a level for detection.

It’s important to emphasize two points. The first is that resistance tests give the most meaningful results when they’re done while you’re on HIV therapy. This way, the results are likely most relevant to the drugs you’re taking. The other is that the results are only part of the story when considering other drugs. Considering your history and experiences with other regimens is critical to choosing the best drugs to use for your next regimen.