Table 7:

Definitions of response to hepatitis B nucleoside antiviral agents

Primary non-response	Less than 2-log10 IU/mL decrease in HBV DNA measured at 6 months of treatment, most commonly related to non-adherence to medication
Genotypic resistance	Mutation of HBV DNA polymerase known to decrease the efficacy of the antiviral agent
Phenotypic resistance	Defined by an in vitro assay demonstrating decreased inhibition of viral replication in the presence of the specific mutation in the polymerase gene
Viral breakthrough	Increase in HBV DNA of 1 log10 IU/mL or greater above the nadir, measured in two consecutive samples 1 month apart, occurring after the first 3 months of therapy; this is commonly because of genotypic resistance, but it may also be the result of lack of adherence
Clinical or biochemical breakthrough	A rise in ALT from its nadir during treatment associated with a rise in HBV DNA of 1 log10 IU/mL or greater; it may also be the result of either genotypic resistance or lack of adherence

ALT = alanine aminotransferase; HBV DNA = viral load