Terminology

Researchers make a distinction between two types of resistance: genotypic and phenotypic resistance.

Specific mutations are associated with resistance to individual antiretroviral drugs (i.e. M184V is associated with 3TC). Viral strains with drug-specific mutations are said to have genotypic resistance to that drug.

Phenotypic resistance is a measure of the actual impact of resistance on a drug's antiviral effect. If the virus is significantly less susceptible to a drug when actually exposed to it, it is said to have phenotypic resistance to that drug. (Viral 'susceptibility' or 'sensitivity' is the degree to which the virus is suppressed by a drug, i.e., the extent to which that drug is still effective against it.) Resistance is rarely an all-or-nothing phenomenon; different mutations (or combinations of mutations) confer varying degrees of drug resistance, ranging from minimal to very high.

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