The emergence of drug resistance is a major factor contributing to the failure of antiretroviral therapy in HIV-infected patients. Drug resistance can be detected by genotypic or phenotypic assays, both having distinct advantages and disadvantages. Results from genotypic and phenotypic testing are helpful in excluding from the subsequent regimen drugs to which the resistance is identified, and both tests predict virologic response to salvage therapy in patients who have failed a previous regimen. Resistance testing is likely to be beneficial as an aid in selecting a salvage regimen.

At entry, patients are randomized to Arm A (sequencing) or Arm B (phenotyping) and have a resistance test drawn while still receiving the current regimen even though regimen failure is suspected. The test results are available between Weeks 1 and 4, inclusive. There are weekly visits for the first 4 weeks after entry to monitor viral load and maintenance of the current failing (prestudy) regimen. If virologic failure is confirmed, a new regimen is chosen and prescribed at the first visit after resistance test results are available. [AS PER AMENDMENT 12/6/00: If the resistance assay fails to yield results, another regimen is chosen and prescribed based on the patient's medical and medication history.] If virologic failure is not confirmed, the current drug regimen is not changed. Otherwise, on-site study visits occur every 4 weeks until Week 24 and then every 8 weeks thereafter through Week 48. [AS PER AMENDMENT 12/6/00: on-site study visits occur every 4 weeks until Week 24 and then every 8 weeks thereafter]. Medical resource use is assessed at baseline and then every 8 weeks through Week 48. Quality of life is assessed at baseline and then every 16 weeks through Week 48.