In vitro evaluation of HSV susceptibility to antiviral drugs is based on the multiplication of a virus in the presence of increasing concentrations of antiviral drug. These techniques allow the calculation of the antiviral drug concentration leading to a 50% inhibition of viral replication (inhibitory concentration 50%, IC50). In order to discriminate between susceptible and resistant strains, IC50 thresholds have to be defined for each virus and antiviral drug pair. These values are usually determined according to the mean value obtained for susceptible viruses: three to five times the mean value of ACV-susceptible HSV for example. But these thresholds are arbitrary and the best way is to check the evolution of IC50 values of sequential isolates from a patient.
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