Explanation

Q fever, Coxiella burnetii IgM: what is measured?

Q fever is caused by the bacterium Coxiella burnetii, which mainly comes from goats and sheep and is breathed in through the air. This test measures IgM antibodies: the type of antibody that the immune system makes first. In Q fever, antibodies appear on average between the seventh and fifteenth day of illness, usually in the order phase 2 IgM, phase 2 IgG, phase 1 IgM and phase 1 IgG. Which phase this test measures is not stated in its name; it is usually IgM against phase 2.

What can it detect?

An immune response that may fit acute Q fever
A sign that you have been in contact with Coxiella burnetii in the preceding weeks

Result

What does the result mean?

Positive result

A positive IgM result may fit acute Q fever, but is not proof on its own. According to the LCI guideline (RIVM), a raised IgM against phase 2 alone has a low positive predictive value and must be confirmed, for example because IgG antibodies still appear in a second blood sample or because the PCR is positive. In addition, IgM can persist for a long time after a past infection, so a positive result can also belong to an old infection. Acute Q fever often causes fever, headache, muscle pain and sometimes pneumonia, but in about 60 in 100 infected people it causes no symptoms.

Negative result

A negative IgM result does not rule out acute Q fever in the first week of illness, because the antibodies only appear from about the seventh to fifteenth day of illness. In that early period, a PCR on blood is the better test. If the first result is not clear, the guideline advises sending in a second blood sample after 14 days. With symptoms that have lasted for months, a negative IgM says little about an infection longer ago.

After an abnormal result

With a positive result and matching symptoms, the doctor usually has a second blood sample tested after about two weeks to see whether IgG antibodies appear, and sometimes has a PCR done. Acute Q fever is a notifiable disease in group C of the Dutch Public Health Act: the doctor or laboratory must report it to the GGD (municipal public health service) within one working day, even with a probable result. People who have had acute Q fever are then followed up with blood tests (serology) to detect chronic Q fever in time.

Reliability

How reliable is the test?

The LCI guideline (RIVM) on Q fever names the immunofluorescence test as the reference method, with a sensitivity of 100 percent and a specificity of 95 to 100 percent for IgM. An ELISA for phase 2 IgM has a sensitivity of about 85 percent. A single detected antibody is not suitable for proving an acute infection, because the test does not distinguish between an acute and a past infection. In people without symptoms and without contact with goats or sheep, the chance of Q fever is so small that a positive result is more often wrong than right.

What can affect the result?

Antibodies are only detectable from about the seventh to fifteenth day of illness (window period).
IgM can remain detectable for a long time after a past infection and then wrongly seem to point to something recent.
Cross-reactions with antibodies against other bacteria, including Bartonella and Legionella, occur.
The result must always be assessed together with the symptoms and the first day of illness; laboratories use different tests and scales.

Who it is for

Who is the test useful for?

Useful for

Useful with fever, headache, muscle pain or pneumonia in someone who has been near goats, sheep or an infected farm, or during an outbreak. In pregnant women and people with a heart valve or blood vessel condition, there is extra reason to consider Q fever, because the infection can be more severe in them. Not useful as a general check-up in people without symptoms: Q fever has become rare in the Netherlands, and the chance of a false-positive result is then greater than the chance of a real infection.

How common is it?

Q fever is rare in the Netherlands. Between 2007 and 2010 there was a large outbreak around dairy goat farms, with 2,354 reports in 2009. After vaccination of goats and sheep, this fell back to a few dozen reports a year, as before the outbreak. In a national study in 2006 and 2007, about 2.4 in 100 Dutch people had antibodies against the bacterium without being ill.

RIVM and the RIVM/LCI guideline on Q fever

Drawbacks and risks

A single positive IgM result is often not caused by an acute infection and can lead to unnecessary worry, unnecessary antibiotic use and a report to the GGD (municipal public health service).
The result says nothing about long-lasting tiredness: Q fever fatigue syndrome is not diagnosed with an IgM test.

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