An HBsAg (Hepatitis B Surface Antigen) test is used to detect the presence of the Hepatitis B virus in the blood. HBsAg is the earliest marker of active Hepatitis B infection, which can be either acute or chronic. A positive result means the person is infected and may be able to transmit the virus to others. Doctors usually recommend this test when symptoms such as jaundice, fatigue, abdominal pain, dark urine, or loss of appetite suggest a possible liver infection, or during routine screening in pregnancy and before surgeries.

For the HBsAg test, a small blood sample is collected from a vein, and no fasting or special preparation is required. This test helps diagnose ongoing Hepatitis B infection and is often followed by additional tests like HBeAg, Anti-HBs, and liver function tests (LFT) to assess the stage and severity of the disease. Early detection is important to prevent complications such as chronic liver disease, cirrhosis, or liver cancer.