Not surprisingly, academic laboratories frequently underestimate genotypic resistance. 1 Questions also exist about the relative difficulty in amplifying DNA from virus of different HIV-1 clades. Because a drug-resistant virus can grow out rapidly and become the dominant virus within a few weeks under pressure of drug treatment, it is important to identify these minority populations. Therefore, a negative resistance assay provides little reassurance to the clinician or patient. Some physicians may find this impractical, even unethical.
Low specificity is also a concern with both types of assays.
By comparison, pneumococcal resistance to penicillin is defined as at least a 33-fold increase in the concentration of drug required to inhibit bacterial growth. If HIV drug resistance phenotyping standards were applied to penicillin, many patients would be denied a safe, effective, and inexpensive antibiotic. A more biologically rational definition of resistance incorporates the relationship between drug sensitivity and drug concentration in the patient. Point mutations affect HIV drug sensitivity differently for different drug classes. False-positive results also can occur with genotypic drug resistance testing.