Anti-HBc IgM

Anti-HBc IgM are IgM-class antibodies to hepatitis B virus core antigen (HBcAg). They appear early after infection and remain in blood for several months, making the test the main laboratory sign of recent, acute hepatitis B infection. A doctor most often requests it for suspected acute hepatitis, together with HBsAg.

Who the test is for

• people with jaundice, dark urine, fatigue and raised liver enzymes (ALT, AST), when acute hepatitis is suspected;
• people whose HBsAg is positive, to distinguish a new infection from a chronic one;
• people whose HBsAg is negative but a doctor still suspects recent hepatitis B infection;
• people with chronic hepatitis B whose disease has suddenly worsened, as assessed by a doctor.

What anti-HBc IgM is and what it shows

Hepatitis B is viral liver inflammation transmitted through blood, sexual contact and from mother to child during birth. Illness usually develops two to five months after infection. Some infected adults and most young children have no symptoms, while others develop fatigue, nausea, pain under the right rib margin, dark urine and jaundice. Symptoms usually last several weeks and sometimes up to six months.

The body responds to infection with antibodies to viral core antigen (anti-HBc). IgM-class antibodies appear first, around the time symptoms begin and liver enzymes rise. They usually disappear within approximately six months, while IgG-class antibodies remain for life. The anti-HBc (total) test therefore shows that a person has ever been infected, while anti-HBc IgM indicates recent infection.

Anti-HBc IgM develops only after exposure to the virus. The vaccine contains only the viral surface antigen, so vaccination produces protective anti-HBsantibodies, not anti-HBc.

Window period

During recovery from acute hepatitis B, HBsAg disappears from blood, while protective anti-HBs antibodies appear only after some time. During this interval, called the window period (English: window period), HBsAg and anti-HBs are negative and only anti-HBc, including anti-HBc IgM in recent infection, indicates infection. This is why HBsAg alone is not measured when acute hepatitis B is suspected.

Preparation and sample

The test uses blood collected from a vein. Fasting is not required unless other tests needing it are performed the same day. If you take biotin (for example hair and nail supplements), mention it during blood collection.

How to interpret the result

Results are reported as negative (non-reactive) or positive (reactive), sometimes with a numerical index whose method-dependent threshold is printed on the report. A doctor interprets it alongside other hepatitis B markers and liver enzymes. The most common combinations are:

HBsAg	Anti-HBc IgM	Total anti-HBc	Anti-HBs	What it most often means
positive	positive	positive	negative	acute infection
negative	positive	positive	negative	recent infection in the window period
positive	negative	positive	negative	chronic infection if HBsAg persists for more than six months
negative	negative	positive	positive	past infection

Positive anti-HBc IgM does not always mean a new infection. In people with chronic hepatitis B, it can reappear, usually at a lower concentration, when disease worsens or the virus reactivates. Rarely, the result is falsely positive.

Most adults recover from acute hepatitis B without lasting effects: the virus disappears, anti-HBc IgM is lost, and total anti-HBc and anti-HBs remain in blood. Fewer than 5 % of infected adults develop chronic infection, in which HBsAg remains in blood for more than six months. The risk is much higher in children: chronic infection develops in around 90 % of newborns infected during birth and in a quarter to half of children infected between their first and fifth years. With a positive result, a doctor usually refers to an infectious diseases specialist or gastroenterologist and advises household members and sexual partners to be tested and vaccinated.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Prices for anti-HBc IgM and other hepatitis B markers are available in the service price list, with an overview of all markers in the article Hepatitis A, B and C markers.

Frequently asked questions

What does positive anti-HBc IgM mean?

Most often it means you acquired hepatitis B within approximately the last six months. In people with known chronic hepatitis B it can also mean worsening disease. A doctor assesses its meaning in your case alongside HBsAg, anti-HBs and liver enzymes.

What is the difference between anti-HBc and anti-HBc IgM?

Total anti-HBc includes all antibodies to core antigen and remains positive for life, showing that you have ever been infected. Anti-HBc IgM is present only for several months after infection and therefore indicates recent infection.

Can anti-HBc IgM be positive after vaccination?

No. Only anti-HBs develops after vaccination. Positive anti-HBc, total or IgM, means the body has been exposed to the virus.

Am I infectious if anti-HBc IgM is positive?

In acute infection the virus is in the blood and can be transmitted through blood and sexual contact. Until speaking with a doctor, avoid unprotected sex and do not share razors, toothbrushes or nail-care equipment. A doctor assesses how long you remain infectious using follow-up HBsAg results.