The hepatitis B virus (HBeAg) antigen test is a serological immunoassay that measures the presence of HBeAg protein, a soluble antigen secreted by the hepatitis B virus during active replication within liver cells. The presence of HBeAg in the blood indicates high viral replication activity, elevated viral DNA levels, and an increased risk of transmission to others. The disappearance of this antigen and the appearance of antibodies against it (HBeAb) is a positive sign of reduced viral activity.