Explanation

Coxiella burnetii phase 1 IgG (Q fever): what is measured?

Q fever is caused by the bacterium Coxiella burnetii, which comes mainly from goats and sheep and is breathed in through the air. The bacterium has two forms of its outer layer: phase 1 and phase 2. The immune system makes antibodies against both forms, but in a fixed order: first against phase 2, later against phase 1. This test measures IgG antibodies against phase 1. These are the last to appear and are the most important sign of chronic Q fever, in which the bacterium settles in, for example, a heart valve or a blood vessel wall.

What can it detect?

Signs of chronic Q fever, for example an inflammation of a heart valve or of a blood vessel wall after a past infection
A past infection with Coxiella burnetii that happened some time ago

Result

What does the result mean?

Positive result

A positive phase 1 IgG means there are antibodies against this form of the bacterium. A low titer usually fits an infection you had in the past and that cleared up on its own. Only a strongly raised and persistently high titer is suspicious for chronic Q fever: the LCI guideline (RIVM) uses a titer of 1:1,024 or higher in the immunofluorescence test for this, and further tests are then always needed before any conclusion is drawn. Chronic Q fever mainly occurs in people with a condition of a heart valve or a blood vessel, or with reduced immunity.

Negative result

A negative phase 1 IgG makes chronic Q fever unlikely. It does not rule out that you have ever had Q fever: in an acute infection the antibodies against phase 2 appear first, and phase 1 IgG then sometimes stays low or absent. Shortly after an infection this test is almost always negative, because the antibodies only appear from about the seventh to fifteenth day of illness, and phase 1 IgG is the last of them.

After an abnormal result

With a strongly raised titer, the guideline says referral to a hospital or reference center follows for further tests, for example a heart ultrasound (echocardiogram), a PET-CT and a PCR on blood, before a decision on treatment is made. Acute Q fever is a notifiable disease in group C of the Dutch Public Health Act; chronic Q fever is not.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM) on Q fever, the immunofluorescence test is the reference method; its sensitivity and specificity for IgG are close to 100 percent. The result says the most in people who have had acute Q fever or in whom chronic Q fever is suspected; in people without symptoms a single value says little. A result only has meaning together with the symptoms, the medical history and the phase 2 results, and the lab that determines the titer also determines the scale on which that titer should be read.

What can affect the result?

The level of the titer depends on the test; the cut-off of 1:1,024 applies to the immunofluorescence test and not automatically to an ELISA.
Antibodies only appear from about the seventh to fifteenth day of illness, and phase 1 IgG is the last of them to appear.
Antibodies against Coxiella can cross-react with antibodies against other bacteria, including Bartonella and Legionella.
A single result says little; an assessment usually needs two measurements a few months apart.

Who it is for

Who is the test useful for?

Useful for

Says the most in people who have had acute Q fever, and in people in whom chronic Q fever is suspected, for example with an artificial heart valve, a vascular prosthesis, an aneurysm or reduced immunity. In people without symptoms and without exposure to goats or sheep, a single result says little: antibodies are then rare 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 Dutch people had antibodies against the bacterium without being ill.

RIVM and the LCI guideline (RIVM) on Q fever

Drawbacks and risks

A raised titer without symptoms quickly leads to extensive hospital tests, while chronic Q fever is rare in the Netherlands.
The result says nothing about Q fever fatigue syndrome; that diagnosis is not made with this test.

Get tested