Explanation

Coxiella burnetii phase 2 IgM (Q fever): 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. The outer layer of the bacterium occurs in two forms, phase 1 and phase 2. This test measures IgM against phase 2: the type of antibody that the immune system makes first and that in Q fever usually appears first of the four antibodies, on average between the seventh and fifteenth day of illness. IgG comes after that and stays present longer.

What can it detect?

An immune response that may fit acute Q fever
A reason for targeted further testing in case of fever or pneumonia after contact with goats or sheep

Result

What does the result mean?

Positive result

A positive phase 2 IgM may fit acute Q fever. The LCI guideline (RIVM) is clear about this: a raised phase 2 IgM on its own has a low positive predictive value and must be confirmed, either by IgG antibodies appearing in a second sample or by a positive PCR. Phase 2 antibodies can also persist for a long time, so the result may also belong to an old infection. For the notification criteria, however, a positive phase 2 IgM together with matching symptoms does count as probable acute Q fever.

Negative result

A negative phase 2 IgM 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 or plasma is the better test. If the first result is unclear, the guideline advises sending in a second blood sample after 14 days. In some people, phase 2 IgG also appears earlier than phase 2 IgM.

After an abnormal result

With a positive result and matching symptoms, the doctor usually has a second blood sample tested after about two weeks for a rise in IgG antibodies, 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 the laboratory must report it to the GGD (municipal public health service) within one working day, even if it is a probable result.

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 for IgM and a specificity of 95 to 100 percent. An ELISA for phase 2 IgM has a sensitivity of about 85 percent. According to the guideline, detecting phase 2 antibodies once is not suitable for demonstrating an acute infection, because it does not distinguish between acute and past infection. The order in which the antibodies appear also varies greatly from person to person.

What can affect the result?

Antibodies can only be detected from about the seventh to fifteenth day of illness (window period).
Phase 2 IgM can remain detectable for a long time after a past infection.
Cross-reactions with antibodies against other bacteria, including Bartonella and Legionella, occur.
An ELISA misses about 15 in 100 infections that the immunofluorescence test does find.

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, and then always together with IgG and with a second blood sample. In pregnant women and in people with a heart valve or vascular condition, there is extra reason to think of Q fever. Not useful as a standalone test in people without symptoms: Q fever has become rare in the Netherlands, and a positive result is then more often false than true.

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 notifications in 2009. After vaccination of goats and sheep, this fell back to a few dozen notifications per year, as before the outbreak. In a national study in 2006 and 2007, about 2.4 in 100 people in the Netherlands had antibodies against the bacterium without being ill.

The RIVM and the RIVM/LCI guideline on Q fever

Drawbacks and risks

A standalone positive phase 2 IgM counts as probable Q fever and leads to a notification to the GGD, while the result is often not caused by an acute infection.
Unnecessary worry and unnecessary antibiotic use with a result that does not fit the symptoms.

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