Replication is monitored at various drug concentrations and compared to a reference wild type virus. This assay is comparable to conventional in vitro tests of antimicrobial sensitivity, in which the microbe is grown in serial dilutions of antiviral agents. Results are reported as the IC 50 for the test strain relative to that of a reference or wild type strain. The interpretation was previously based on a fixed ratio such as 4x to define resistance, meaning resistance is 4-fold greater than that of the reference strain. The newer method is to individualize by drug.
Virtual phenotype is a prediction of the phenotype of the test strain based on genotypic analysis.
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