Stability of hepatitis B surface antigen over time: implications for studies using stored sera

In chronic hepatitis B, quantitative measurements of hepatitis B surface antigen (HBsAg) have been studied increasingly using stored sera to determine its usefulness in disease management. However, the stability of stored HBsAg levels has not been established. The current study determines prospectively the stability of HBsAg levels in sera stored for 1 year. Fresh sera from 105 samples were divided into 5 aliquots. The first aliquot was used to determine HBsAg at the time of blood sampling. The remaining four aliquots were stored at -20°C and thawed at 3, 6, 9, and 12 months to determine the HBsAg levels. HBsAg was measured by adopting the research protocol using the Elecsys HBsAg II assay. At baseline, age and HBV DNA showed a significant correlation with HBsAg levels (r = -0.291, P = 0.003 and r = 0.256, P = 0.003, respectively). There was no overall significant difference observed between the HBsAg levels measured at the different time points. There was high correlation between each consecutive time points from baseline to 3 months, 3-6 months, 6-9 months, and 9-12 months (r = 0.991, 0.987, 0.989, 0.993, respectively, all P < 0.001). The overall log difference in HBsAg levels between baseline and at 12 months was 0.03 (range, -0.29 to 0.85), with over 99% of samples showing no significant change (defined as >1 log) after 12 months. In conclusion, HBsAg levels remained stable in stored frozen sera for 12 months without significant changes, and could be used to quantify HBsAg accurately.