IgM blood test (immunoglobulin M)

Immunoglobulins (antibodies) are proteins produced by the B lymphocytes of the immune system in order to recognise and neutralise bacteria, viruses and other foreign structures. IgM is the first class of antibodies the body produces when a new infection develops — which is why a raised IgM often indicates a recently started (acute) infection, and later the body moves on to producing IgG, which stays in the body for longer. The IgM test is most often done together with IgG and IgA (the total immunoglobulin test), because a change in a single marker is hard to interpret without context. Per MedlinePlus, this test helps to assess whether the immune system is working properly — whether it produces enough antibodies or, on the contrary, produces too many of them because of chronic inflammation, liver disease or certain blood cell diseases. The test does not answer which specific disease caused the change — additional tests and a doctor's assessment are needed for that.

• When infections keep recurring often, especially if they are caused by the same bacterium or virus
• When sinus, throat, ear or airway infections keep recurring
• When diarrhoea or other infections of the digestive tract are troublesome
• When an immune system disorder (immunodeficiency) or an autoimmune disease is suspected
• When the doctor wants to assess the course of liver disease, a chronic infection or certain blood cell diseases
• When assessing whether an infection is new (acute) or has already been had earlier

The specific reference interval depends on the laboratory method, so you will find it on your own result sheet next to the result. IgM is most often assessed together with IgG and IgA rather than on its own, because a change in one marker is hard to interpret without context (MedlinePlus).

A raised IgM most often indicates a recently started (acute) infection, because this is the first class of antibodies the body produces. Raised immunoglobulins in general may also be linked with an autoimmune disease, liver disease, a chronic infection or certain cancers. Other tests, together with a doctor's assessment, help to establish the specific cause.

• A recently started (acute) infection — IgM rises first, before the body begins producing IgG
• An autoimmune disease
• Liver disease (cirrhosis or hepatitis, for example)
• A chronic infection
• Certain cancers (of the blood cells)

Symptoms that can go with it:

• Sinus, throat or ear infections
• Airway infections
• Diarrhoea and/or infections of the digestive tract
• Severe infections from microorganisms that are usually harmless

Alongside a raised result, these are the things usually assessed:

• Whether the result was assessed together with IgG and IgA, not on its own
• Whether there was an active infection or inflammation at the time of the test
• Liver markers, if liver disease is suspected
• When to repeat the test so that the change over time is visible (the IgM → IgG shift)

A low IgM: what it can mean

Lowered immunoglobulins may indicate that the body is losing protein (because of kidney disease, severe burns or poor absorption of nutrients, for example) or that it cannot produce enough of them, for instance because of kidney failure or complications of diabetes. This can increase susceptibility to infections.

• Conditions that reduce the amount of protein in the body — kidney disease, severe burns, poor absorption of nutrients (malabsorption), inadequate nutrition
• Conditions that interfere with the ability to produce immunoglobulins — complications of diabetes, kidney failure
• A congenital genetic disease, such as common variable immunodeficiency disorder (CVID)

Symptoms that can go with it:

• Frequent, recurring infections, especially ones caused by the same bacterium or virus
• Severe infections from microorganisms that are usually harmless

Alongside a low result, the questions are different:

• Whether there are conditions associated with protein loss (of the kidneys or the digestive tract)
• A general protein test — albumin, total protein
• The frequency and nature of infections in the medical history
• When to repeat the test so that the change over time is visible