BACKGROUND In most cases, patients appear to recover from acute hepatitis B virus (HBV) infection and do not exhibit the surface antigen (HBsAg). Chronic carriers are positive for HBsAg but HBsAb is usually not present. After acute infection only HBsAb remains. The presence of both HBsAg and HBsAb is unusual. METHODS We report on a patient whose results were analytically and clinically discrepant--positive for HBsAg and HBsAb on one testing platform but only HBsAg on another platform. RESULTS Reasons for this result: 1) Interference from endogenous antibodies; 2) HBsAg is from one strain and HBsAb is from another: and 3) The presence of HBsAb and HBsAg. Growing evidence indicates that both may be present in many patients. Low HBsAb may be neutralized and not recognized by the solid-phase HBsAg in the assay. Likewise, low HBsAg may be neutralized by HBsAb. It remains unclear whether HBsAg is always cleared after acute infection. CONCLUSIONS Testing indicated that both HBsAb and HBsAb were present. The data shows that different testing platforms may produce different results depending on the kinetics, the exposure of the capture HBsAg and the extent of endogenous HBAg/HBsAb. We demonstrate a simple way to rule out test interference to presumptively identify true HBsAb.