Does this test have other names?

HBsAg

What is this test?

This test looks for hepatitis B surface antigens in your blood. The test is used to find out whether you have a recent or long-standing infection from the hepatitis B virus (HBV).

HBV has proteins called antigens on its surface that cause your immune system to make antibodies. Hepatitis B surface antigens can be found in your blood within several weeks after the infection starts. They are one of the earliest signs of a hepatitis B infection.

HBV is 1 of 5 hepatitis viruses. The others are hepatitis A, C, D, and E. Most hepatitis infections are caused by these 5 viruses. HBV is spread through blood, seminal fluid, and vaginal secretions. It can take several months to develop symptoms of hepatitis B after you become infected. The virus causes an infection in the liver. In most cases, this virus clears up on its own within 6 months. But in a small portion of adults and a larger portion of children, the virus doesn't go away. This is especially true for newborns. This is called having a chronic infection. It may lead to liver cell damage, scarring, cirrhosis, or liver cancer.

Hepatitis B surface antigens are an early sign of an acute infection, and they are also present during chronic, or long-term, infection.

Why do I need this test?

You may need this test if your healthcare provider believes you have a liver infection caused by HBV. You may need this test if you have symptoms of hepatitis B. Symptoms often start slowly. Many people have no symptoms or only feel like they have a mild case of the flu. You may not have symptoms until the infection becomes severe or chronic.

The most common symptom is extreme tiredness. Other symptoms may include:

• Nausea

• Loss of appetite

• Muscle aches

• Fever

• Yellowed skin and eyes (jaundice)

• Dark-colored urine

• Belly (abdominal) pain

• Swelling and confusion. This is in extreme cases.

You may also have this test if you have a history that puts you at risk for being in contact with the virus. Risk factors for hepatitis B infection include:

• Having sex with someone infected with the virus

• Living in close contact with someone who has the virus

• Being a man who has sex with men

• Being a child born to a mother who has the virus

• Sharing needles for IV (intravenous) drug use

• Working in a healthcare center where you are exposed to blood

• Getting a blood transfusion or organ transplant. This is less common with active screening.

You may also have this test several times if you've already been diagnosed with hepatitis B to see whether your infection is getting better.

What other tests might I have along with this test?

Your healthcare provider may order other blood tests to look for HBV. These tests can look for antigens on the surface, envelope, and core of the virus, as well as the antibodies to these antigens. The symptoms of all 5 hepatitis infections are much the same. So this blood test is often done along with other hepatitis blood tests to tell your provider which type of virus and what stage of infection you may have.

Your healthcare provider may also order a series of blood tests called a hepatitis B monitoring panel to see if your infection is getting better.

What do my test results mean?

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

Normal results are negative or nonreactive, meaning that no hepatitis B surface antigen was found.

If your test is positive or reactive, it may mean you are actively infected with HBV. In most cases, this means that you will recover within 6 months. If you recover, you will have immunity from the virus and will not be able to pass the virus to others. A positive test may also mean you have chronic hepatitis B infection. If you don't recover in 6 months, the virus may stay in your blood and cause liver problems. You can also infect others. Your healthcare provider may give you medicines if you don't recover after 6 months.

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

What might affect my test results?

Other factors aren't likely to affect your results.

How do I get ready for this test?

You don't need to prepare for this test. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.