The HBsAg test is used along with other tests to confirm hepatitis B and whether or not you’re contagious.
What Is Hepatitis B?
Hepatitis B is a serious liver infection caused by the hepatitis B virus (HBV). For some people, hepatitis B infection becomes chronic, meaning it lasts more than six months. Having chronic hepatitis B increases your risk of developing liver failure, liver cancer or cirrhosis—a condition that causes permanent scarring of the liver.
Most people infected with hepatitis B as adults recover fully, even if their signs and symptoms are severe. Infants and children are more likely to develop a chronic hepatitis B infection.
Hepatitis B Symptoms
Signs and symptoms of hepatitis B, ranging from mild to severe, usually appear about one to four months after you’ve been infected. They may include:
Abdominal painDark urineFeverJoint painLoss of appetiteNausea and vomitingWeakness and fatigueYellowing of your skin and the whites of your eyes (jaundice)
Hepatitis B Causes
The hepatitis B virus is passed from person to person through blood, semen, or other body fluids.
With a positive HBsAg, transmission routes for infection include:
Sexual contact: You may become infected if you have unprotected sex with an infected partner whose blood, saliva, semen, or vaginal secretions enter your body. Sharing of needles: HBV is easily transmitted through needles and syringes contaminated with infected blood. Sharing intravenous (IV) drug paraphernalia puts you at high risk of hepatitis B. Accidental needle sticks: Hepatitis B is a concern for healthcare workers and anyone else who comes in contact with human blood. Mother to child: People who are pregnant and are infected with HBV can pass the virus to their babies during childbirth. However, the newborn can be vaccinated to avoid getting infected in almost all cases. Talk to your healthcare provider about being tested for hepatitis B if you are pregnant or want to become pregnant.
What Is HBsAg?
HBsAg is cleared within four to six months in self-limited infections (infections that resolve by themselves). It can be detected in the blood during both acute infections (infections that come on suddenly) and chronic infections (infections that last for longer than six months).
Along with levels of HBsAg, symptoms of the individual, and other factors, there are additional antibodies that can be tested to distinguish between acute and chronic infections.
At the center of the hepatitis B virus is DNA, which contains the genes the virus uses to replicate itself. Surrounding the DNA is a protein called hepatitis B core antigen (HBcAG), which cannot be detected with blood tests.
Surrounding this is HBsAg, which is actually part of the “envelope” that protects the virus from attack by the body’s immune system. However, the immune system is good at getting through this envelope in order to kill the virus. When it does, remnants of surface antigen protein are left in the blood-like debris, which lab tests can detect.
Your body can produce antibodies to any of these antigens once you are exposed to the virus. These antibodies develop at different stages of the infection.
Screening Tests for Hepatitis B
Your blood may be screened for HBV for many different reasons. There are several types of test, but the three generally included are the HBsAg, the antibody to HBsAg (also called HBsAb), and the antibody to hepatitis B core antigen (HBcAb).
These tests allow the healthcare provider to know whether you could benefit from vaccination, or if you have active or chronic hepatitis B and need counseling, care, or treatment.
You may be routinely screened if you are pregnant, are donating blood or tissue, need immunosuppressive therapy, or have end-stage renal disease. You will also be screened if you are in groups that are at higher risk for HBV.
