No test, however, will tell whether resistance is actually occurring. With a viral typing test, we may know that an individual is at risk for developing resistance to certain antivirals, but the actual emergence of resistance must be inferred from viral load testing and the monitoring of the individual's overall health. The reverse transcriptase inhibitors have differing resistance patterns. AZT resistance emerges gradually because a combination of several viral mutations are required for high level resistance. The risk of developing AZT resistance increases with time on the drug.
Time allows the virus to form the specific set of mutations required.
On the other hand, both 3TC and nevirapine have a single point mutation that confers high level resistance to the drug. This means only one viral mutation is required to highly reduce the effectiveness of these drugs. In fact, early testing of nevirapine revealed the rapid development of resistance when the drug was used alone. Nevirapine is now used only in combination with other antivirals. Some researchers are suggesting that the use of both 3TC and nevirapine be avoided when prescribing a first-time antiviral combination.