Immunoglobulin M (IgM)
The first antibody the body makes when it meets a new infection, so a raised IgM points to a recent or current one. It is also why 'IgM' antibody tests are used to show an acute infection, as opposed to IgG which shows past exposure.
What it measures
Immunoglobulin M is the largest antibody and the first one produced when the immune system encounters a new threat, appearing early in an infection before IgG takes over for lasting immunity. This timing is the key to its usefulness: a specific IgM antibody test (for example against a particular virus) indicates a recent or current infection, whereas the equivalent IgG indicates past infection or immunity. Total IgM measures the overall amount and is read with IgG and IgA. A raised total IgM most often reflects a recent or ongoing infection, but a persistent, marked, single-type (monoclonal) rise can point to a specific blood condition (Waldenström macroglobulinaemia). A low IgM can be part of an antibody deficiency. In newborns, a raised IgM can signal an infection acquired before birth, since IgM does not cross the placenta.
What a high value can mean
- Recent or acute infection — the commonest cause; IgM rises early.
- Chronic infection or inflammation, autoimmune disease — a broad rise.
- Some liver conditions (e.g. primary biliary cholangitis) — a characteristic IgM rise.
- Monoclonal IgM (a single type) — Waldenström macroglobulinaemia, a specific blood condition; investigated with protein electrophoresis.
- In a newborn — a congenital (before-birth) infection.
What a low value can mean
- Antibody (immune) deficiency — can be part of a broader immune deficiency.
- Protein loss or some medications and blood conditions.
When to discuss with a doctor
A raised total IgM is most often explained by a recent or ongoing infection and is interpreted with the clinical picture. Where a specific infection is in question, a targeted IgM antibody test (positive suggesting recent infection) is the relevant test. A persistent, marked monoclonal IgM should prompt specialist assessment for a blood condition, and a low IgM as part of recurrent infections warrants immune evaluation. IgM is read together with IgG and IgA. Mediora.AI shows IgM alongside the other immunoglobulins so the pattern — acute versus long-term — is visible; the interpretation belongs with your doctor.