Abstract
BACKGROUND: Occult hepatitis B infection (OBI) is characterized by the detection of hepatitis B virus (HBV) DNA in serum (usually HBV DNA < 200 IU/mL) or the liver but negativity for hepatitis B surface antigen (HBsAg). The diagnosis of OBI relies on the sensitivity of assays used in the detection of HBV DNA and HBsAg. HBsAg assays with inadequate sensitivity or inability to detect HBV S variants may lead to misdiagnosis of OBI in people with overt HBV infection. CASE SUMMARY: We report a HBsAg-negative but hepatitis B envelope antigen-positive patient who had a significant HBV DNA level. The patient was initially diagnosed as having OBI. However, sequence analysis revealed a unique insertion of amino acid residues at positions 120-124 in the S protein, which affects the formation of a disulfide bond that is associated with the formation of a loop. It is well known that there is an overlap between the S protein and Pol protein. We found that this new insertion site occurred in polymerase/reverse transcriptase domain, indicating that this insertion might be involved in HBV pathogenicity. The patient was finally diagnosed with a false OBI. CONCLUSION: An insertion of amino acid residues at positions 120-124 of the S protein affects the formation of immunodominant epitopes and results in negative HBsAg levels.