<prompt> The following is a marker of acute hepatitis B infection-DNA polymeraseHepatitis core antigenAnti HBsIgG to core antigen  <response>: HBV-DNA can be measured by PCR in the blood. Viral loads are usually in excess of 105 copiesmL in the presence of active viral replication, as indicated by the presence of e antigen. In contrast, in individuals with low viral replication, who are HBsAg- and anti-HBe-positive, viral loads are less than 105 copiesmL. The exception is in patients who have a mutation in the pre-core protein, which means they cannot secrete e antigen into serum . Such individuals will be anti-HBe-positive but have a high viral load and often evidence of chronic hepatitis. These mutations are common in the Far East, and those patients affected are classified as having e antigen-negative chronic hepatitis. They respond differently to antiviral drugs from those with classical e antigen-positive chronic hepatitis. Measurement of viral load is impoant in monitoring antiviral therapy and identifying patients with pre-core mutants. Specific HBV genotypes A-H can also be identified using PCR. In some settings, these may be useful in guiding therapy, as genotype A tends to respond better to pegylated interferon-alfa compared to genotypes C and D. Ref Davidson edition23rd pg 875 