An HbsAg (Hepatitis B surface antigen) device typically refers to a diagnostic test device or assay used to detect the presence of the hepatitis B surface antigen in a patient's blood or serum. Hepatitis B is a viral infection that primarily affects the liver and can lead to chronic liver disease if left untreated. Diagnostic devices for hepatitis B are used to screen individuals for the presence of the virus or to confirm previous exposure to the virus.