Hepatitis B Core Antibody IgM Test

If you have been exposed to hepatitis B and want to know whether the infection is recent, active, or possibly flaring, this is the test that can answer that question. Standard hepatitis B panels detect whether the virus is present and whether you have immunity, but they do not always tell you when the infection began or whether your immune system is in the middle of an active fight.

Hepatitis B core IgM antibody (anti-HBc IgM) is the earliest specific antibody your body produces when responding to the hepatitis B virus core protein. A high level points strongly to recent infection or an acute flare. A low or undetectable level usually means the infection is old, inactive, or absent.

What This Antibody Reveals About Your Body

Anti-HBc IgM is a protein your immune system makes when B cells (the antibody-producing white blood cells) encounter the inner core of the hepatitis B virus. It is the first hepatitis B antibody that appears after infection, often before the more widely known surface antigen tests can detect anything.

Because IgM is an early-response antibody, its presence tells you something specific that other hepatitis B markers cannot: that the immune system is currently engaged with the virus, either because the infection is new or because a long-standing infection has reactivated.

Why It Matters: Acute vs Chronic Infection

The most important job of this test is distinguishing a recent (acute) hepatitis B infection from a long-standing (chronic) one. In one large study, anti-HBc IgM was positive in 230 of 255 patients with acute hepatitis B, while the more familiar surface antigen test (HBsAg) was positive in only 210. The IgM was the only specific marker in 12.3% of cases. In other words, hepatitis B can be missed by standard testing alone.

Among people who already carry hepatitis B chronically, this antibody usually sits at low or undetectable levels. When it rises, it signals that the infection is active again, a state called a flare or acute exacerbation.

The Window Period Problem

Right after a hepatitis B infection, there is a stretch of time when the surface antigen test (HBsAg) can be falsely negative. During this window, anti-HBc IgM may be the only marker showing the virus is there. This is why blood banks in some regions test for it: in one Nigerian cohort of 273 surface-antigen-negative donors, IgM testing identified hidden infections that would otherwise have slipped through.

For someone who has had a recent possible exposure and a negative standard hepatitis B panel, this test can fill that gap.

Acute Liver Failure Risk

In severe hepatitis B presentations, anti-HBc IgM provides prognostic information. A study of patients with hepatitis B related acute liver failure found that the antibody distinguishes two clinically different forms of the disease, separable by IgM titer and viral load, each with a different prognosis. People with persistent high IgM levels for more than six months after acute infection are more likely to progress to chronic liver disease.

Chronic Hepatitis B Flares

In a study of chronic hepatitis B patients with acute exacerbation, anti-HBc IgM positivity predicted clearance of the hepatitis B e antigen (HBeAg, a marker of active viral replication) with about three times the odds (OR 3.18). Every patient who lost the surface antigen within a year had markedly elevated anti-HBc IgM. So in a flare, this number can predict who is more likely to clear the virus.

Reference Ranges and Cutoffs

Anti-HBc IgM is reported as a sample-to-cutoff (S/CO) ratio or index. Different labs and assays use different scales, and there is no universal reference range. The cutoffs below come from disease-specific research studies, not from a general healthy population, and are illustrative orientation rather than absolute targets. Your own lab will report its own threshold for positivity.

Source: Park et al. 2015 reported that an S/CO ratio of ≥8 distinguished acute infection from chronic flare with high sensitivity and specificity in IgM-positive patients. Lall et al. 2020 found a cutoff of 20.5 S/CO performed similarly. A Taiwanese study suggested local populations may need an even higher index (2.4 to 2.5 on a different assay scale) for accurate acute diagnosis.

What this means for you: a positive result does not automatically mean a brand new infection. The level matters, and the result must be interpreted alongside the surface antigen, viral load, and liver enzyme tests. Always compare your result against your specific lab's reference within the same lab over time.

Tracking Your Trend

For most people, a single anti-HBc IgM measurement is enough to answer a specific clinical question, like whether a recent illness is acute hepatitis B or whether a known chronic infection is flaring. This is not a marker most people retest routinely the way they would cholesterol.

That said, if you have known hepatitis B and your anti-HBc IgM was elevated, retesting in 3 to 6 months can show whether the immune flare is settling. If you tested positive after a possible recent exposure, retesting alongside surface antigen and surface antibody at 3 and 6 months helps confirm whether the infection has resolved or persisted. Reliable IgM autoantibody measurements have shown good stability within an individual over a year in healthy adults, so changes you see are likely real and not test noise.

What to Do If Your Result Is Positive

A positive anti-HBc IgM should not be interpreted in isolation. The decision pathway is to order, in parallel: hepatitis B surface antigen (HBsAg), hepatitis B surface antibody (anti-HBs), total hepatitis B core antibody, hepatitis B e antigen (HBeAg), hepatitis B viral DNA load, and a liver function panel including ALT (a liver enzyme called alanine aminotransferase) and AST (aspartate aminotransferase). Together, these tests place your result in the right context.

If your IgM is high and your surface antigen and viral load confirm an active infection, this is a finding that warrants involving a hepatologist or infectious disease specialist quickly. If your IgM is borderline or low and you have known chronic hepatitis B, your gastroenterologist or hepatologist can use this information to decide whether the immune system is in a flare that may benefit from treatment changes.

When Results Can Be Misleading

• Chronic hepatitis B with a flare: a meaningful share of people with chronic infection in an acute exacerbation have detectable IgM and can be misclassified as having a brand new infection if only the simple positive/negative result is used.
• Lab assay differences: in a Taiwanese study, the manufacturer's standard cutoff index of about 1.2 produced too many false positives for acute infection, and the appropriate cutoff had to be raised to 2.4 to 2.5 for that population.
• Coinfections: if you have hepatitis B and your IgM is unexpectedly negative during an acute hepatitis episode, the liver injury may be from another virus (such as hepatitis A, hepatitis delta, or cytomegalovirus) layered on top of a chronic carrier state.
• Window period: very early in infection, even IgM may not yet be detectable, so a single negative result soon after a known exposure does not rule out infection.

How This Test Compares to Standard Hepatitis B Testing

A standard hepatitis B panel typically includes the surface antigen (HBsAg), the surface antibody (anti-HBs), and total core antibody. Anti-HBc IgM is a separate, phase-specific test. The total core antibody tells you whether you have ever been infected with hepatitis B. The IgM version tells you whether that infection is recent or active right now. These are different questions, and one test cannot substitute for the other.