Explanation

Hepatitis B surface antigen (HBsAg): what is measured?

HBsAg (hepatitis B surface antigen) is a protein on the outside of the hepatitis B virus. So the test does not measure antibodies, but part of the virus. If HBsAg is in the blood, there is an infection: either recently caught (acute) or lasting (chronic, also called carriership). As long as HBsAg can be detected, a person is considered contagious through blood and sex.

What can it detect?

Acute hepatitis B: a recently caught infection, often without symptoms
Chronic hepatitis B (carriership): HBsAg remains detectable for longer than 6 months

Result

What does the result mean?

Positive result

A positive result means that the hepatitis B virus is in your blood and that you can pass it on through blood and sex. This single test cannot show whether the infection is recent or has been there for years; other hepatitis B markers are needed for that (such as anti-HBc, HBeAg, anti-HBe and HBV DNA). In adults, the body clears a new infection by itself in about 95 out of 100 cases. If HBsAg remains detectable for longer than 6 months, it is chronic hepatitis B; this often causes no symptoms for years, but can slowly damage the liver and therefore needs monitoring by a specialist.

Negative result

A negative result means that no HBsAg has been found at this moment. According to the LCI guideline (RIVM), HBsAg can usually be detected 4 to 8 weeks after infection, sometimes as early as 1 to 2 weeks and at the latest after 11 to 12 weeks; a test shortly after a risk contact can therefore still be negative. A negative result says nothing about protection: whether you are immune through vaccination or a past infection is shown by anti-HBs. Very occasionally there is still virus in the blood while HBsAg cannot be detected.

After an abnormal result

With a positive result, the GP has additional hepatitis B markers and liver values measured and refers you to a gastroenterologist (MDL doctor) or internist (specialist in internal medicine) for assessment and possible treatment; household members and sexual partners get a test via the GGD (municipal public health service) and vaccination if needed. Hepatitis B is notifiable (group B2, Public Health Act): the doctor and laboratory report a newly diagnosed acute or chronic infection to the GGD within 1 working day.

Reliability

How reliable is the test?

HBsAg is the standard test to show a current hepatitis B infection; in the Netherlands, all pregnant women and blood donors, among others, are tested for it. A positive result is supplemented with other markers, so that the doctor can determine the stage of the infection. Within the window period of several weeks after infection, a negative result does not rule out an infection.

What can affect the result?

Shortly after infection (usually the first 4 to 8 weeks, sometimes up to 12 weeks), HBsAg cannot yet be detected.
With a simultaneous infection with hepatitis C or HIV, HBsAg can sometimes not be detected even though hepatitis B virus is present.
After a healed infection, the virus can remain in small amounts in the liver (occult infection) without HBsAg being positive.
A single measurement does not distinguish between an acute and a chronic infection.

Who it is for

Who is the test useful for?

Useful for

Useful if you have run a risk: sex without a condom with changing partners, men who have sex with men, sex work, a partner or household member with hepatitis B, injecting drugs with shared needles, or if you were born in a country where hepatitis B is common. Also with symptoms of liver inflammation (such as jaundice) or abnormal liver values. Pregnant women are already tested as standard by their midwife. If you only want to know whether a vaccination has worked, you need anti-HBs instead of HBsAg.

How common is it?

In the Netherlands, about 40,000 people carry the hepatitis B virus. In 2023, 929 infections were reported, 90 of them acute. Most infections occur in people who were born in a country where hepatitis B is common; in the pregnancy screening in 2022, about 2 in 1000 pregnant women were positive.

RIVM; LCI guideline on hepatitis B (reports 2023, PSIE screening 2022)

Drawbacks and risks

A positive result has major consequences (being contagious, a report to the GGD (municipal public health service), warning partners and household members); until the additional tests are done, this can cause a lot of worry.
Testing shortly after a risk contact can give false reassurance if no repeat test follows.

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