Hepatitis B infection is suspected based on medical history, signs and symptoms, and physical examination. If liver function tests (blood tests) are abnormal, your doctor may suspect infection with hepatitis B virus. But abnormal liver test results can come due to many conditions that affect the liver. The diagnosis of hepatitis B is confirmed based on specific hepatitis B virus blood tests known as hepatitis 'markers' or 'serology.' The blood test can also help to differentiate acute from chronic infection.
These markers are antigens formed by the hepatitis B virus and antibodies produced by the immune system to fight the virus. The three antigens that are commonly tested include - the surface antigen (HBsAg), the core antigen (HBcAg) and the e antigen (HBeAg).
HBsAg and anti-HBs: If hepatitis B surface antigen (HBsAg) is detected in blood test, it indicates that the patient is currently infected with the virus. HBsAg can be detected about four weeks after initial exposure to the virus. If the person recovers from acute hepatitis B infection HBsAg clears in about four months after the onset of symptoms. These people form antibodies to HBsAg (anti-HBs) which gives immunity against subsequent hepatitis B viral infection. Similarly, after successfully vaccination the person forms anti-HBs in the blood.
If the virus is not cleared during an acute episode the person develops chronic hepatitis B. Chronic hepatitis B is diagnosed if HBsAg is present in the blood for at least six months. In cases of chronic hepatitis B, HBsAg continues to be detected for many years, and anti-HBs do not appear.
Detection of IgM antibodies directed against the hepatitis B core antigen (anti-HBc IgM) can be used to diagnose an acute hepatitis B infection. In later stages of the infection, IgG antibodies (anti-HBc IgG) develop and they persist for life. This may or may not cause chronic infection in the individual and he may recover too. .
HBeAg, anti-HBe, and pre-core mutations
Hepatitis Be antigen (HBeAg) is detected by blood tests when the hepatitis B virus is actively multiplying. When antibody to HBeAg (anti-HBe) is found (also called HBeAg seroconversion), it indicates a more inactive state of the virus and a lower risk of transmission. If the hepatitis B virus undergoes a structural change, called a pre-core mutation, virus does not produce HBeAg, even though it is actively reproducing. In these people even though no HBeAg is detected in the blood, the hepatitis B virus is still active and it is infective.
Hepatitis B virus DNA
Blood test for detection of levels of hepatitis B virus DNA (HBV DNA) in the blood is the best marker of hepatitis B virus reproduction. Presence of hepatitis B virus DNA indicates that the virus is actively multiplying in the person. HBV DNA is found soon after infection, but if the infection is eliminated over time it is not detected. In cases of chronic hepatitis, HBV DNA continues to be high for many years and may decrease if the immune system is able to control virus multiplication. Level of HBV DNA in blood is sometimes referred to as the 'viral load'.