Hepatitis B Tests HBsAg/anti-HBs
Hepatitis B Surface Antigen
The picture on the left is an electron micrograph, a picture so blown up you can see the virus particles. The hepatitis B virus has an envelope coat. This coat surrounds the viral DNA or genetic code.
The immune system's antibodies can recognize a small bit on the surface of the envelope coat. This is like seeing a button and knowing that it is found on a shirt. However, not every shirt is being worn - most shirts may be sitting in the closet.
Hepatitis B is unusual in that for every complete virus, there are probably thousands of empty envelope coats. However, each empty coat is being made by a hepatitis B virus. Just knowing the coat is around means that complete virus must be there too. When hepatitis B surface antigen (HBsAg) is found in the blood, it means the virus envelope coat is around and therefore the hepatitis B virus is also present. However, if the infection is gone, you may see the body's immune memory of the infection, namely antibody to hepatitis B surface antigen (anti-HBs).
Learning points
- HBsAg positive: hepatitis B is around, currently infected. HBsAg is on the surface of the envelope of the virus.
- HBsAg is more sensitive that HBV DNA PCR. That is to say, if the viral load is negative, HBsAg can still be positive and the virus is still around.
- anti-HBs positive: the body has seen hepatitis B envelope before, either through infection or vaccine. When this is positive, you are immune to infection.
- Exceptions: hepatitis B can very rarely still be around when HBsAg is negative. This is called "occult hepatitis B".
It is important to be familiar with all these hepatitis B proteins because all of our tests and many of our new treatments are based on the structure of the virus. If you want to learn more about these specific tests, you can go on to learn about the details about HBeAg/anti-HBe or HBV DNA or viral load testing.
You can move on to learn about chronic hepatitis B infection or go back to the main page on hepatitis B tests.