The HBsAg test is a fundamental blood screening tool used to detect the presence of the Hepatitis B surface Antigen (HBsAg). This antigen is a specific protein found on the outer shell of the Hepatitis B virus (HBV). Detecting this protein in a person’s blood confirms a current Hepatitis B infection, indicating the virus is actively present and replicating. The test serves as the primary gateway for diagnosing HBV infection, whether acute or chronic.
The Role of the HBsAg Test in Identifying Hepatitis B
The Hepatitis B surface Antigen (HBsAg) is a lipoprotein component of the viral envelope. Once the virus infects liver cells, it produces and sheds large quantities of this antigen into the bloodstream, which the HBsAg test measures. The presence of HBsAg signals an active infection and confirms the virus is present. The HBsAg test is utilized as a standard diagnostic tool in various medical scenarios, including routine screening.
Common Screening Scenarios
The test is often ordered during pregnancy to prevent mother-to-child transmission. Screening is also recommended for individuals starting immunosuppressive therapies, as these drugs can trigger viral reactivation. People who have had high-risk exposures, such as accidental needle sticks or unprotected sexual contact, also undergo this testing.
If HBsAg is detected, it confirms the person is contagious, regardless of whether symptoms are apparent. This test is the initial step in a comprehensive hepatitis B panel. The HBsAg protein is typically detectable in the blood approximately two to ten weeks after initial exposure.
Understanding HBsAg Test Results
Interpreting the HBsAg test result is straightforward, reported as either negative or positive.
Negative Result
A negative HBsAg result means the Hepatitis B surface Antigen was not detected in the blood sample, indicating the person does not have a current, active Hepatitis B infection. However, a negative result does not guarantee protection from future infection. Further testing checks for the Hepatitis B surface Antibody (anti-HBs). If this antibody is present, the person is immune, either from successful vaccination or recovery from a past infection. If both the antigen and the antibody are negative, the person is susceptible, and vaccination is usually recommended.
Positive Result
A positive HBsAg result definitively indicates an active Hepatitis B infection is present. The person is currently infected, the virus is replicating, and they are contagious. This active infection could be acute (recent) or chronic (long-standing). The HBsAg test alone cannot differentiate between these two stages. If the surface antigen persists for more than six months, the infection is classified as chronic Hepatitis B. The presence of HBsAg is the necessary trigger for ordering additional blood work.
Next Steps Following a Positive Result
Following a confirmed positive HBsAg result, a comprehensive evaluation is performed to determine the infection stage and assess liver health. This requires a panel of secondary blood tests.
Secondary Testing
Secondary tests include the Hepatitis B e Antigen (HBeAg) and its antibody (anti-HBe), which help determine the level of viral replication and infectivity. Measuring the Hepatitis B viral DNA (HBV DNA) level quantifies the amount of virus circulating in the blood. Liver function tests, such as Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST), check for inflammation or damage. Non-invasive assessments, like FibroScan, may also be used to look for evidence of liver scarring or cirrhosis.
Patients with a positive HBsAg result are typically referred to a specialist, such as a gastroenterologist or hepatologist, for ongoing management. Specialists use the secondary test results to decide if antiviral treatment is necessary and to monitor for complications like liver cancer. Regular monitoring of the viral load and liver function is required, even if treatment is not immediately started. Guidance on preventing transmission is an essential part of the follow-up. Infected individuals are advised to avoid sharing personal items, and non-immune sexual partners and household contacts should be tested and vaccinated.