Evaluation of HBsAg and Anti-HBs Coexistence: Authenticity and Contributing Factors.

The coexistence of hepatitis B surface antigen (HBsAg) and hepatitis B surface antibody (anti-HBs) is an uncommon but clinically important serological pattern, with a reported prevalence ranging from 2.8% to 35.95%. This study aimed to evaluate the authenticity of this coexistence pattern and to investigate its underlying causes. A total of 300 sera co-positive for HBsAg and anti-HBs were incubated with recombinant HBsAg (rHBsAg) of the adw, adr, and ayw serotypes, as well as with pooled clinical HBsAg-positive sera, to calculate the neutralization index (NI) of anti-HBs. In parallel, the same co-positive samples were incubated with pooled clinical anti-HBs-positive sera to determine the NI of HBsAg. Additionally, 180 sera from healthy individuals under 32 years of age with isolated anti-HBs positivity were tested against adw and adr rHBsAg and pooled clinical HBsAg-positive sera to establish neutralization thresholds. Among the 300 co-positive samples, HBsAg was confirmed positive in 100%, and anti-HBs was confirmed positive in 89.00% (267/300). The prevalence of true coexistence decreased with increasing anti-HBs concentrations: 91.55% in the 10.00-50.00 mIU/mL group, 86.11% in the 50.00-100.00 mIU/mL group, and 76.92% in the > 100.00 mIU/mL group (p = 0.025). In serotype analysis, 46.82% (125/267) of anti-HBs were neutralized by adw and adr rHBsAg but not by ayw rHBsAg. Another 49.44% (132/267) could not be neutralized by any of the three rHBsAg preparations but were neutralized by pooled clinical sera. The proportion of truly anti-HBs positive individuals was relatively high among co-positive samples, and this proportion increased as anti-HBs concentrations decreased. The coexistence of HBsAg and anti-HBs may be attributed to coinfection with both ayr and common serotypes, or to post-vaccination infection with ayw or ayr serotypes.