Explanation

Mycoplasma pneumoniae IgM: what is measured?

Mycoplasma pneumoniae is a bacterium that causes respiratory infections. Symptoms usually start gradually, with a dry cough and feeling unwell, and can last from a few days to more than a month; 5 to 10 percent develop bronchitis or pneumonia. According to the LCI guideline (RIVM), IgM antibodies are detectable about 2 to 3 weeks after infection, about a week after symptoms start. This test measures those IgM antibodies.

What can it detect?

Possible recent infection with Mycoplasma pneumoniae, with matching respiratory symptoms (together with IgG)

Result

What does the result mean?

Positive result

A positive IgM may fit a recent infection with Mycoplasma pneumoniae, especially with symptoms that started gradually with a dry, persistent cough. It does not prove that infection: according to the LCI guideline (RIVM), 20 to 50 percent of healthy children have positive antibodies, and IgM can remain detectable for a long time, especially in people who have had the bacterium more often. Most infections clear up on their own; a cough can, however, last for weeks after a respiratory infection.

Negative result

A negative IgM makes a recent infection less likely, but does not rule it out. In the first week of symptoms, IgM often cannot yet be detected. In adults who have had the bacterium before, the body sometimes makes no IgM in a new infection, only IgG.

After an abnormal result

With a positive IgM, the GP assesses the symptoms and can have antibodies measured again after 1 to 2 weeks or have a PCR done. Whether antibiotics are needed depends on the symptoms and the physical examination, not on the antibodies alone. A Mycoplasma pneumoniae infection is not notifiable; only with several cases in an institution may a notification to the GGD (municipal public health service) be needed.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM), a PCR on mucus from the airways (sputum or a throat swab) is the best way to show an infection at this moment. The IgM result is hard to interpret: antibodies are common in healthy people, IgM can remain present for a long time alongside IgG, and in a new infection sometimes only IgG is made. Repeating it after 1 to 2 weeks can sometimes help to decide whether the symptoms belong to an infection with this bacterium.

What can affect the result?

In the first week of symptoms, IgM often cannot yet be detected.
IgM can remain detectable long after an infection, so a positive result does not always belong to the current symptoms.
In a new infection in adults, IgM is sometimes missing.
Positive antibodies are common in healthy people, especially in children.

Preparation

Do not have your blood taken in the first week of symptoms; before that, the antibodies are often not yet measurable.

Who it is for

Who is the test useful for?

Useful for

Useful with respiratory symptoms that fit this bacterium, such as a dry cough that has lasted more than a week or a mild pneumonia, especially in children and young adults, and if the symptoms started at least a week ago. With symptoms, a PCR on mucus from the airways says more. Without symptoms, the test is not useful, because positive antibodies then often have no meaning.

How common is it?

In the Netherlands, Mycoplasma pneumoniae is one of the most common bacterial causes of respiratory infections and pneumonia, with a wave of more infections about every 4 to 7 years. In the winter of 2023 to 2024, the number of positive laboratory results rose from 24 per week in October to 186 per week in December, the highest since 2005. Most infections occur in children and young adults aged 5 to 20.

LCI guideline Mycoplasma pneumoniae infection; RIVM (weekly virological reports 2023-2024)

Drawbacks and risks

A positive IgM can wrongly be seen as the explanation for symptoms, and so lead to unnecessary antibiotics.
A negative IgM in the first week of symptoms can wrongly reassure you.

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