IgM (immunoglobulin M)

IgM is a type of antibody that the immune system produces first when it encounters a new infection. The test measures the total amount of IgM in the blood and helps the doctor assess how your defence against infections works, especially when infections are frequent or unusual. It does not, however, show which infection you have.

Who the test is intended for

• adults and children with frequent, severe or prolonged infections, such as pneumonia or sinus or ear infections;
• when the doctor checks whether the immune system is functioning normally, usually alongside IgG and IgA;
• when protein electrophoresis shows an abnormality in the gamma-globulin fraction;
• people with chronic liver disease or an autoimmune disease, when the doctor requests additional investigations;
• for monitoring a known immunoglobulin disorder or treatment with medicines that weaken immunity.

What the test reveals

Immunoglobulins are antibodies with which the body defends itself against bacteria and viruses. IgM appears first, early in an infection, and is later replaced by IgG, which lasts longer. Children are born with low IgM, which gradually rises during the first years of life, so their result is compared with age-specific limits.

Total IgM shows the amount of all IgM antibodies in the blood, regardless of the pathogen. It therefore cannot tell whether you have, for example, cytomegalovirus or toxoplasmosis. Specific tests for antibodies against individual pathogens, such as CMV IgM or Toxoplasma IgM.

What the test involves and how to prepare

A blood sample is taken from a vein, and collection takes a few minutes. No special preparation is required, and you do not need to fast unless you are also having tests that require it on the same day. Tell your doctor which medicines you take, especially those that affect the immune system.

What your result means

Reference values depend on age and method and are printed on the report. The doctor interprets IgM alongside IgG, IgA, your symptoms and other results.

• Low IgM: may be part of a congenital or acquired antibody deficiency, especially if IgG and IgA are also low. It can also occur with medicines that weaken immunity and with some blood disorders. Slightly low IgM without frequent infections often has little significance.
• Elevated IgM: most often accompanies a recent or prolonged infection. It may also be elevated in chronic liver diseases and autoimmune diseases. Less often, the cause is a greater amount of a single type of IgM (monoclonal IgM), which the doctor investigates using protein electrophoresis and other tests.
• Normal IgM: does not rule out infection or another immune disorder if the doctor suspects one.

How to arrange testing

You can have blood collected without an appointment or referral at one of our branches across Croatia (listed on the Contact). Prices for all tests and panels are available in the price list. The same blood collection can also be used to measure IgG and IgA, with which IgM is most often interpreted.

Frequently asked questions

Does elevated IgM mean that I have an acute infection?

Not necessarily. Total IgM rises in many conditions and does not identify the pathogen. If the doctor suspects a particular infection, they request IgM and IgG antibodies specifically against that pathogen.

What is the difference between IgM and IgG?

IgM is the “first line” of defence and appears early in an infection. IgG is produced later, remains for a long time and protects against becoming ill again. IgG is also the only immunoglobulin that crosses the placenta and protects the newborn.

Is low IgM dangerous?

Usually not on its own. More important is whether you have frequent or severe infections and whether other immunoglobulins are also low. In that case, your doctor may refer you to an immunologist for further investigation.

Should IgM be checked regularly?

It is not necessary in healthy people without symptoms. The test is repeated when the doctor is monitoring a known disorder or treatment.