Explanation

Mycoplasma pneumoniae IgG: what is measured?

Mycoplasma pneumoniae is a bacterium that causes respiratory infections, from a mild flu-like illness with a dry, persistent cough to pneumonia. After an infection, the immune system first makes IgM antibodies and, 2 to 3 weeks later, IgG antibodies, which can remain present for more than a year. This test measures those IgG antibodies. According to the LCI guideline (RIVM), a past infection does not protect against a new infection.

What can it detect?

A past infection with Mycoplasma pneumoniae
Together with IgM and a second blood draw: an indication that recent respiratory symptoms were caused by this bacterium

Result

What does the result mean?

Positive result

A positive result means that you have had an infection with Mycoplasma pneumoniae at some point. This is common: according to the LCI guideline (RIVM), 20 to 50 percent of healthy children have positive antibodies. A single IgG does not show when the infection took place and therefore does not simply explain current symptoms. A clear rise between two blood draws 1 to 2 weeks apart can, however, point to a recent infection; in a new infection, the body sometimes makes only IgG.

Negative result

A negative result means that no IgG antibodies were found. In the first weeks of an infection, IgG cannot yet be detected, because it only appears 2 to 3 weeks after IgM; so a negative result does not rule out a recent infection. After a long time, the antibodies can also disappear again.

After an abnormal result

With a positive IgG and symptoms, 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. Antibodies are hard to interpret, because they are common in healthy people and remain present for a long time, and because the pattern changes as someone has had more infections. A single IgG result therefore says little; repeating it after 1 to 2 weeks can sometimes help.

What can affect the result?

A single IgG does not show when the infection took place.
In the first weeks of symptoms, IgG often cannot yet be detected.
Positive antibodies are common in healthy people, especially in children.
After a blood transfusion or treatment with immunoglobulins, antibodies from donors can be measured for a while.

Who it is for

Who is the test useful for?

Useful for

The result mainly says something together with IgM and a second blood draw, with a long-lasting cough or pneumonia whose cause is unclear. With symptoms that have just started, a PCR on mucus from the airways says more. In people without symptoms, a positive IgG changes nothing, because antibodies are common.

How common is it?

In the Netherlands, Mycoplasma pneumoniae is one of the most common bacterial causes of respiratory infections and pneumonia. About every 4 to 7 years there is a wave with more infections; 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 IgG can wrongly be seen as the explanation for long-lasting symptoms, while it often belongs to an old infection.
A negative IgG in the first weeks of symptoms can wrongly reassure you.

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