Hepatitis B Surface Antigen

Hepatitis B surface antigen (HBsAg), also called hepatitis-associated antigen or Australia antigen, appears in the sera of patients with hepatitis B virus. It can be detected by radioimmunoassay or, less commonly, reverse passive hemagglutination during the extended incubation period and usually during the first 3 weeks of acute infection or if the patient is a carrier.

Because transmission of hepatitis is one of the gravest complications associated with blood transfusion, all donors must be screened for hepatitis B before their blood is stored. This screening, required by the Food and Drug Administration's Bureau of Biologics, has helped reduce the incidence of hepatitis. This test doesn't screen for hepatitis A virus (infectious hepatitis).

Purpose

• To screen blood donors for hepatitis B.
• To screen people at high risk for contracting hepatitis B, such as hemodialysis nurses
• To aid differential diagnosis of viral hepatitis

Patient preparation

• Explain to the patient that this test helps identify a type of viral hepatitis.
• Inform him that he needn't restrict food or fluids before the test.
• Tell him that the test requires a blood sample and who will perform the venipuncture and when.
• Reassure him that although he may experience transient discomfort from the needle puncture and the tourniquet, collecting the sample takes less than 3 minutes.
• Check the patient's history for administration of hepatitis B vaccine.
• If the patient is giving blood, explain the donation procedure to him.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 10-ml red-top tube.
• Notify the blood donor if test results are positive for HBsAg.
• Report confirmed viral hepatitis to public health authorities. This is a reportable disease in most states.

Precautions

• Wash your hands carefully after the procedure.
• Remember to wear gloves when drawing blood.
• Dispose of the needle properly.

Normal findings

Normal serum is negative for HBsAg.

Abnormal findings

The presence of HBsAg in a patient with hepatitis confirms hepatitis B. In chronic carriers and people with chronic active hepatitis, HBsAg may be present in the serum several months after the onset of acute infection. It may also occur in more than 5% of patients with certain diseases other than hepatitis, such as hemophilia, Hodgkin's disease, and leukemia. If HbsAg is found in donor blood, that blood must be discarded because it carries a risk of transmitting hepatitis. Blood samples that test positive should be retested because inaccurate results do occur.

Interfering factors

• Hepatitis B vaccine (possible positive)