Resistance testing: phenotype
- Phenotypic resistance assays use a patient’s HIV RNA plasma-derived gene sequences (RT, PR, integrase and envelope) to measure HIV replication at different concentrations of antiretroviral drugs.
- Interpretation of phenotype is based on two important clinical cutoffs: (1) lower cutoff or the value above which clinical response is diminished (partial susceptibility); and (2) upper cutoff or value above which no clinical response is expected.
- Phenotypic testing compares the activity of a patient’s HIV to a control strain with known susceptibility to a specific drug.
- Phenotypic co-receptor tropism assays detect CCR5, CXCR4, or dual/mixed (D/M)-tropic virus. Viral tropism is the ability of HIV to bind to different co-receptors on target cells to achieve viral entry. Phenotypic co-receptor tropism assay is used to assess activity of CCR5 antagonist (maraviroc).
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