Pros and Cons to Testing For HIV-Drug Resistance

SAN FRANCISCO — All official guidelines on HIV treatment either make blanket recommendations for drug-resistance testing or at least suggest that the clinician consider such testing depending on the patient's circumstances, Brad Hare, M.D., said at a meeting on HIV management sponsored by the University of California, San Francisco.

But deciding whether to use genotypic or phenotypic assays can be difficult, said Dr. Hare, a physician in the positive health program at the university.

Genotypic drug-resistance assays identify the presence of specific mutations in the HIV genome. Drug resistance is then inferred using an algorithm or a database analysis Phenotypic assays, on the other hand, use viral isolates or recombinant virus derived directly from the patient's plasma. The analysis derives from a culture-based system, and the concentration of a specific drug needed to inhibit viral replication can be quantified.

In general, genotypic testing holds the edge early in a patient's disease, before the virus has a chance to develop complex patterns of resistance. Phenotypic testing tends to be better late in a patient's infection, when the patient is likely to be experiencing more regimen failure as a result of virus with complex mutations. (See box.)

Both tests may be required in complicated patients to get the optimal information for management.

Source: Dr. King