What Does Being HBsAg Positive Mean?

Hepatitis B surface antigen (HBsAg) is a protein that appears in the blood when you have a hepatitis B infection. A HBsAg blood test can detect this antigen.

A positive HBsAg result means you have an active hepatitis B virus (HBV) infection and are contagious. HBV is spread through contact with infected body fluids, such as blood and semen.

This article discusses the HBsAg test and other tests used to confirm hepatitis B infections. It also explains how hepatitis B infections are treated and provides prevention tips.

What Is the HBsAg Test?

HBsAg test is a blood test that detects hepatitis B surface antigen (HBsAg). HBsAb is a protein on the surface of the hepatitis B virus. High levels of HBsAg typically indicate an active hepatitis B virus infection.

HBsAg is a part of the outer layer of a hepatitis B virus cell. The center of the cell contains the viral DNA, the genes it uses to replicate. Surrounding the DNA is a protein called hepatitis B core antigen (HBcAg).

HBsAg surrounds HBcAg and is 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.

Other Tests Used to Diagnose Hep B

The HBsAg test is used in conjunction with other tests blood tests to identify an HBV infection. Other serologic markers for hepatitis B include: 

• Hepatitis B surface antibody (anti-HBs): This test indicates immunity from HBV infection, either due to recovery after an infection or from vaccination.
• Total antibody to hepatitis B core antigen (anti-HBc): This test measures antibodies that show if you have ever had an active HBV infection. Anti-HBc appears at HBV infection onset and persists for life. 
• IgM antibody to hepatitis B core antigen (IgM anti-HBc): This test shows whether an acute HBV infection has occurred in the past six months.  

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.

Who Should Get Tested?

As of 2023, the Centers for Disease Control and Prevention recommends all adults should be tested for hepatitis B at least once in their life.

Your blood may be screened for HBV for many different reasons. HBV screenings are used to diagnose an active or chronic hepatitis B infection and identify individuals who may benefit from vaccination.

Routine HBV screenings are performed:

• During pregnancy
• Before donating blood or tissue
• Prior to starting immunosuppressive therapy
• In individuals who are at higher risk for HBV

Hepatitis B screening is used to identify acute or chronic hepatitis B infections and those who would benefit from vaccination.

How Is the Test Done?

A HBsAg test is performed by a phlebotomist using a venous blood draw. Blood is usually taken by inserting a needle into a vein at the inside of the elbow or back of the hand. The blood is collected into an airtight tube and sent to a lab to be analyzed.

No special preparation is needed prior to the test. You do not need to fast prior to the test, so it can be taken at any time of the day. It is always a good idea to drink water prior to getting blood drawn to ensure you are well hydrated. 

The HBsAg test focuses on the presence or absence of this biomarker. If HBsAg is detected, the test is positive. If no HBsAg is detected, the test is negative.

What a Positive HBsAg Means

In most instances, a positive HBsAg means a person has an active or recent hepatitis B infection. The body produces antibodies to HBsAg as part of the immune response to infection.

HBsAg 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).

A recent hepatitis B vaccination (HepB) can also cause a positive HBsAg test result. If you received a dose of the HepB vaccine within 30 days of your test, your healthcare provider may recommend waiting another month before retesting.

Most labs identify a range between 0.05 and 250 international units per milliliter (IU/ml). The precise level correlates to your status as someone who's not exposed, who has been vaccinated or is in a recovery stage of an infection, or who is in a currently active state of infection.

While it is most important to know if you are HBsAg positive or negative, there are certain situations that may cause this test to be falsely negative or falsely positive.

Hepatitis B Treatment

There is no specific treatment for an acute HBV infection. The hepatitis B virus is usually self-limiting (naturally resolving without treatment) and will clear up on its own within four to six months.

Infections that last longer than six months are considered chronic. The World Health Organization recommends treating chronic HBV infections with antiviral medication. Viread (tenofovir) or Baraclude (entecavir) are considered the strongest oral antiviral drugs used for hepatitis B infections. 

Preventing the Spread of Hepatitis B

Hepatitis B can be prevented through education, vaccination, and testing.

Hepatitis B is spread through contact with blood, semen, and other body fluids from an infected person. Hepatitis B exposure can occur through sex, childbirth, or sharing needles, razors, or medical equipment (like a glucose monitor).

Hepatitis B cannot be passed through kissing, sharing eating utensils, sneezing, coughing, hugging, breastfeeding, or through food or water.

Vaccination

Hep B vaccination can help prevent the spread of hepatitis B. The vaccine is routinely given to all infants within 24 hours of birth. Follow-up doses are required throughout childhood and adulthood.

In Newborns

HBsAg testing is routinely performed during pregnancy. This is to identify and treat infants at risk for contracting hepatitis B during vaginal or C-section delivery.

Prompt treatment in the first 12 hours of life can prevent hepatitis B transmission from mother to child. This includes two shots: hepatitis B immune globulin (HBIG) and the first dose of the hep B vaccine.

Other Prevention Strategies

Hepatitis B prevention strategies in medical settings include screening blood products, sterilizing needles, syringes, and other equipment, and wearing sterile gloves.

Other ways to prevent the spread of hepatitis B include:

• Avoid sharing personal hygiene products such as toothbrushes, razors, or medical equipment (like a lancet or glucose monitor).
• Do not use IV drugs or share needles or drug-preparation equipment.
• Make sure only new or sterilized needles are used during tattoos or body piercings.
• Only use a licensed provider for cosmetic injections.
• Use condoms during sexual contact.
• Wear disposable gloves if you come in contact with blood or open wounds.