Why is Immunoglobulin M (IgM) High? Causes, Symptoms, and What to Do

A raised (elevated) IgM means your blood contains more immunoglobulin M, the antibody your immune system makes first when it meets a new germ. By far the most common reason is a recent or ongoing infection, such as glandular fever (mononucleosis); less often it points to rheumatoid arthritis, a hereditary hyper-IgM syndrome, or a blood cancer such as Waldenström macroglobulinemia. A low IgM can come from an inherited or acquired antibody deficiency, from some blood cancers, or from other illnesses and medicines; a single low value is usually rechecked before any conclusion is drawn. Either way, one result on its own is not a diagnosis.

What the IgM test measures

IgM is an antibody, a protein made by immune cells to recognise and clump together germs. It usually forms a five-unit structure, which makes it the largest antibody. It is the first antibody made in a new immune response and the first a baby's body starts to make; it is too large to cross the placenta, so newborns do not get it from their mothers.

A quantitative immunoglobulin test (often reported as "Immunoglobulin M, Qn, Serum") measures the total amount of IgM in the blood, usually together with IgG and IgA. It is ordered to look for immune deficiency in people with repeated infections, to investigate an unexplained high blood protein, and to follow conditions that change antibody levels.

This is different from an infection-specific IgM test (for example EBV, CMV or hepatitis IgM), which looks only for antibodies against one germ. Total IgM can be normal while a specific IgM is positive, and the reverse.

Reference range

Ranges vary by laboratory, method and age (the SI figures are the mg/dL values divided by 100), so compare your result with the range printed on your report. Values are normally lower in infants younger than 9 months. You may be asked not to eat or drink for 4 hours before the test.

Causes of high IgM

Common

• Recent infection – the most common cause. The immune system makes IgM first, so levels rise early in an infection.
• Infectious mononucleosis (glandular fever) – a viral illness that Epstein-Barr virus (EBV) can cause; it is listed among the causes of increased IgM.
• Rheumatoid arthritis – an autoimmune condition, in which the immune system attacks the body's own joints; it is listed among the causes of increased IgM, though infection is a far more frequent reason.

Less common or rare

• Waldenström macroglobulinemia – a rare blood cancer in which one clone of abnormal cells overproduces a single type of IgM. The average age at diagnosis is about 70, and many people have no symptoms when it is found on routine blood tests.
• Lymphoma and multiple myeloma – blood cancers that can change antibody levels.
• Hyper-IgM syndromes – inherited conditions that cause high IgM but low levels of other antibodies such as IgG.
• Schnitzler's syndrome – a rare inflammatory disease.
• Primary biliary cholangitis – a slowly progressive autoimmune liver disease, mostly in women, in which high serum IgM is a characteristic finding.

Causes of low IgM

• Selective IgM deficiency – IgM is repeatedly low while IgG and IgA are normal. Truly selective deficiency appears to be rare; many people with low IgM do not meet the strict definition.
• Broader antibody deficiencies (hypogammaglobulinemia) – such as common variable immunodeficiency (CVID) and, very rarely, agammaglobulinemia, where the body cannot make enough antibodies.
• Blood cancers – leukemia, lymphoma and multiple myeloma.
• Secondary causes – before calling a low IgM a primary deficiency, doctors rule out infections, certain medicines, protein loss through the gut (protein-losing enteropathy) or kidneys (nephrotic syndrome), genetic syndromes and thymoma.

Symptoms

A high or low IgM level does not cause symptoms by itself. Symptoms come from the underlying cause:

• With low IgM, the main concern is repeated infections such as sinusitis, ear infections, bronchitis or pneumonia.
• With high IgM, symptoms depend on the cause: fever and swollen glands with an infection; joint pain with rheumatoid arthritis; or, with Waldenström macroglobulinemia, fatigue, swollen lymph nodes, an enlarged liver or spleen, and sometimes bleeding from the skin or the lining of the nose and mouth, headache, dizziness or confusion when the blood becomes thick (hyperviscosity).

What usually happens next

• Repeat testing. Because IgM is made early in an infection, a doctor may simply recheck it later. Selective IgM deficiency is defined by IgM that is repeatedly low, so a low value is rechecked before any label is applied.
• Full immunoglobulin panel: IgG, IgA and IgM together, because the pattern matters (for example high IgM with low IgG suggests a hyper-IgM syndrome).
• Serum protein electrophoresis and immunofixation if IgM is clearly high, to see whether it is a single abnormal (monoclonal) protein.
• Complete blood count, and in some cases serum viscosity, cryoglobulins, urine protein tests and a bone marrow biopsy.
• Infection-specific antibody tests (such as EBV antibodies) when symptoms suggest a recent infection.
• For low IgM: antibody responses to vaccines and lymphocyte (B- and T-cell) counts, usually arranged by an immunologist.

Frequently asked questions

What does elevated IgM mean?

Most often it means your immune system is reacting to a recent infection. Less often it reflects an autoimmune condition, an inherited hyper-IgM syndrome, or a blood disorder such as Waldenström macroglobulinemia. Your doctor reads it alongside your symptoms, IgG and IgA, and often a protein electrophoresis.

Is a raised IgM dangerous?

The number itself is not harmful. What matters is the cause, and in most people that is an infection. A clearly high or rising IgM is worth investigating because rare causes such as Waldenström macroglobulinemia are found this way.

What does low IgM mean?

It means less IgM than the laboratory's reference range. It may reflect a selective IgM deficiency, a broader antibody deficiency, a blood cancer, or a secondary cause such as protein loss or a medicine. Because selective IgM deficiency is defined by IgM that is repeatedly low, a single mildly low result is usually rechecked, and secondary causes are looked for, before any diagnosis is made.

What causes low IgM levels?

Recognised causes include selective IgM deficiency, common variable immunodeficiency, very rare agammaglobulinemia, leukemia, lymphoma, multiple myeloma, nephrotic syndrome, protein-losing enteropathy and certain medicines.

What does EBV IgG positive, IgM negative mean?

For Epstein-Barr virus, the IgM antibody against the viral capsid (VCA IgM) usually disappears within 4 to 6 weeks, while VCA IgG stays for life. A positive IgG with a negative IgM therefore usually points to an infection in the past rather than a current one, especially if the EBNA antibody is also positive. More than 90% of adults have been infected with EBV.

What is the difference between IgG and IgM in infection tests?

IgM is made first and usually suggests a recent infection; IgG comes later and usually stays, showing past exposure. The same principle is used for many germs, but exact timing and interpretation differ by infection and test.

Do I need to fast for an IgM blood test?

Sometimes. MedlinePlus notes you may be asked not to eat or drink for 4 hours beforehand. Follow the instructions on your test order.

When to see a doctor

If your IgM is outside the range on your report, it is worth discussing with your doctor soon, especially if you have frequent or severe infections, swollen glands, ongoing tiredness, or if your IgM stays high on repeat testing. If you have sudden confusion, a severe headache or heavy bleeding, seek urgent care.

Sources

1. Quantitative immunoglobulins — MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
2. Immunoglobulin M (IgM): Function, Tests & Disorders — Cleveland Clinic
3. Macroglobulinemia — Merck Manual Consumer Version
4. Selective IgM Deficiency: Evidence, Controversies, and Gaps — Diagnostics (PubMed Central)
5. Laboratory Testing for Epstein-Barr Virus (EBV) — U.S. Centers for Disease Control and Prevention

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