Resistance testing: phenotype

DEFINITION

• 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.[3]
  • 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.[2]
  • 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).