BRD2 and hepatitis B virus infection: Although NAT assays are not optimal reference standards for HBsAg, given the complex relationship between viral kinetics of HBV DNA and levels of HBsAg, NAT assays are nevertheless useful markers of viremia and disease activity to guide treatment, as well as the detection of occult hepatitis B. Occult hepatitis B (OHB) is defined as the presence of HBV DNA in serum or liver tissue with undetectable HBsAg [57].