Phenotypic assays may be difficult to interpret due to the fact that thresholds used to define susceptibility may not necessarily reflect the appropriate drug concentrations. Testing should be performed in the presence of the antiretroviral agents in question, since discontinuation of therapy often results in the proliferation of wild type virus that may deceptively suggest susceptibility. The usual recommendation is a 2-week allowance. Interpretation of results in patients who have received prior antiretroviral agents is difficult. Resistance to previously used drugs may be present even if resistance assays indicate susceptibility, so that prior drug history and outcome are important factors in regimen selection.
Results are most accurate with failure of early regimens.
These are compared in Table 2-3. Interpretation requires knowledge of mutational changes, eg, expertise. Technician experience influences results. Assesses total effect, including mutations, mutational interactions. Reproducibility is good.