The association between mutations in the hepatitis B surface antigen (HBsAg) gene and the occurrence of occult HBV (OBI) in patients has not been studied adequately to determine if the two are correlated. The current study was aimed to investigate HBsAg mutations, the genotype of HBV and co-infection with HCV in OBI in the central part of Iran to determine any possible associations.

In this study, 3700 plasma samples were examined for the presence of HBsAg, anti-HBc and HBV-DNA. All HBsAg^–/anti-HBc⁺/HBV-DNA⁺ samples were regarded as OBI. The genotype of HBV was identified using Gap-PCR and RT-PCR was used to determine possible co-infection with HCV. Finally, direct sequencing was performed to analyse mutations within the surface antigen gene of HBV in occult versus acute HBV infection.

Of the 3700 patient samples analysed, 352 (9.5%) cases were determined to be HBsAg^–/anti-HBc⁺ in which HBV-DNA was detected in 57 (16.1%), these latter patients were classified as OBI. All of the patients studied carried the D genotype. Direct sequencing of the S-gene from occult and acute HBV patients revealed one silent and one glycine to arginine mutation but the acute HBV patients showed an additional mutation (alanine to threonine). All the mutations were outside the range of the ⍺-determinant. Furthermore, none of the OBI patients were co-infected with HCV.

The absence of conformational mutations in the ⍺-determinant of HBsAg confirmed that this antigen could be detected by commercial Elisa kits and therefore was not responsible for false negatives during blood screening. However, it can be concluded that suitable amounts of HBsAg were not expressed by HBV in the OBI patients to be detected by Elisa. Low level expression of HBsAg might be related to the D genotype of the virus. Furthermore, our results suggest that OBI is not related to co-infection with HCV.