Figure 8. Method for Calculating Level of Phenotypic Resistance

This graph shows the method for calculating the level of phenotypic resistance of a single antiretroviral medication. The antiretroviral drug is tested on a patient's HIV isolate and a laboratory reference (wild-type strain). The IC50 represents the concentration of the antiretroviral drug required to cause 50% inhibition of HIV replication. The fold change is calculated by dividing the IC50 of the patient's isolate by the IC50 of the wild-type laboratory strain. As shown, as the curve shifts to the right, a higher concentration of drug would be required to inhibit HIV replication and thus the strain of HIV would be more resistant. The further the curve shifts to the right (for the patient's HIV strain tested), the greater the level of resistance.

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