HBsAg (hepatitis B surface antigen): what a positive or negative result means

Hepatitis B is a liver infection caused by a virus spread through blood and body fluids. HBsAg is a protein on the surface of the virus. Lab Tests Online UK explains that a positive result means the virus is present in your blood, that it is the earliest sign of infection and can appear before symptoms develop.

Unlike most single blood tests, hepatitis B testing uses a panel of markers: proteins, antibodies and viral genetic material that together build a picture of your status. Your doctor usually requests only those that are relevant to your situation.

Reasons for testing include symptoms of hepatitis (jaundice, dark urine, pale stools, fever, tiredness and upper right abdominal pain), possible exposure through unprotected sex, sharing needles or needlestick injury, chronic liver disease, pregnancy, being born to a mother with hepatitis B, monitoring treatment, and checking the response to vaccination. The NHS lists the ways hepatitis B spreads and says that in the UK hepatitis B vaccine is given to babies as part of the 6-in-1 vaccine, with additional vaccinations for babies born to mothers with hepatitis B.

The NHS says adults need vaccination only if at high risk, for example travel to a high-risk country, liver or kidney disease, HIV, injecting drug use, multiple sexual partners, men who have sex with men, or jobs such as healthcare or prison work.

No fasting or other preparation is needed; a nurse or doctor takes blood from a vein in the arm. If a positive HBsAg is found, a second sample and further markers are usually requested to confirm it and to assess activity.

Lab Tests Online UK lays out the most common patterns. If HBsAg, anti-HBs and anti-HBc are all negative you have not had hepatitis B and are not protected; vaccination may be discussed. If anti-HBs is positive and anti-HBc negative, you are protected through vaccination. If HBsAg is negative with anti-HBs and total anti-HBc positive, you had hepatitis B in the past and have cleared it. If HBsAg and anti-HBc IgM are positive, you may have a recent infection, and other tests plus symptoms decide whether it is acute or chronic.

HBsAg positive for more than six months means chronic infection. With HBeAg positive, the virus is actively replicating and treatment is often recommended; with HBeAg negative and anti-HBe positive, an inactive carrier state is possible, which still needs regular monitoring because infection can reactivate. In some people HBsAg eventually disappears on its own and anti-HBs appears, termed functional cure.

The NHS says most people with hepatitis B have no symptoms or mild ones, the infection usually lasts 1 to 3 months, and it usually clears on its own without treatment. If it lasts longer than 6 months it is chronic, there is no known cure, and it is usually treated with antivirals and regular check-ups with a liver specialist. Resting, staying hydrated, asking about safe painkiller doses and avoiding alcohol help the liver, and household contacts and sexual partners should be offered testing and vaccination.

A positive result is not a reason for shame, and chronic hepatitis B is manageable with specialist care. Ask the clinic about informing partners; staff can help.

After a positive HBsAg you are normally referred to a liver or infection specialist for HBV DNA, liver tests and scans. Pregnant women with hepatitis B are offered measures to protect the baby. Seek urgent help for jaundice with confusion, vomiting blood or severe abdominal pain; call 999 or attend A&E.