Explanation

Ehrlichia chaffeensis IgG: what is measured?

Ehrlichia chaffeensis is a bacterium that grows in white blood cells (monocytes) and causes the disease human monocytic ehrlichiosis (HME). The bacterium is spread by the lone star tick, a tick found mainly in the southeastern and south-central United States; the disease does not occur in the Netherlands. This test measures IgG antibodies against the bacterium. IgG often cannot yet be detected in the first week of symptoms and can then remain detectable for many months.

What can it detect?

Past contact with Ehrlichia chaffeensis or a related bacterium
Support in investigating ehrlichiosis with fever after a tick bite or a stay in the southeastern or south-central United States

Result

What does the result mean?

Positive result

A positive result means that IgG antibodies against Ehrlichia were found. This says that your immune system has had contact with this or a related bacterium at some point, not that you are ill now: the antibodies can remain detectable for many months. Antibodies against Ehrlichia and Anaplasma cross-react, so in someone who has never been to the United States, a positive result more likely fits contact with a related bacterium or a wrong result. A recent infection only becomes clear from a clear rise in a second blood draw 2 to 10 weeks later.

Negative result

A negative result makes a past infection with Ehrlichia less likely. In the first week of symptoms, IgG often cannot yet be detected, so a negative result shortly after the fever starts does not rule out ehrlichiosis. With fever after travel, the doctor decides based on the clinical picture, and not on this result, whether treatment is needed.

After an abnormal result

With matching symptoms and a positive result, the doctor usually arranges a second blood draw to show a rise, or a PCR on blood, often in consultation with a medical microbiologist. Whether treatment is needed depends on the clinical picture and the trip, not on the result alone. Ehrlichiosis is not notifiable in the Netherlands.

Reliability

How reliable is the test?

There are no agreed Dutch limit values for these antibodies. According to the US CDC (Centers for Disease Control and Prevention), ehrlichiosis cannot be confirmed with one antibody result; the strongest evidence is a clear rise between two blood draws, and in the first week a PCR on blood says more. Most laboratories cannot tell the different Ehrlichia species apart, and antibodies against Ehrlichia and Anaplasma cross-react. Because the disease does not occur in the Netherlands, the result only means something in someone who has been in an area where the bacterium does occur.

What can affect the result?

IgG antibodies remain detectable for many months after contact and say nothing about an active infection.
In the first week of symptoms, the result is often still negative.
Cross-reaction with antibodies against Anaplasma and other Ehrlichia species occurs.
Laboratories use different tests and cut-off values; results cannot simply be compared with each other.

Preparation

Preferably do not have your blood taken in the first week of symptoms; the antibodies are often not yet detectable then.

Who it is for

Who is the test useful for?

Useful for

Can be useful with fever, headache and muscle pain in the weeks after a tick bite or a stay in the southeastern or south-central United States, preferably with a second blood draw a few weeks later. Not useful in people who have not been in such an area, as a general check after a tick bite in the Netherlands, or in people without symptoms: a positive result is then almost always wrong or the result of a cross-reaction.

How common is it?

Human monocytic ehrlichiosis does not occur in the Netherlands; the disease is mainly reported from the southeastern and south-central United States, from the East Coast to Texas. Dutch ticks do carry the related bacterium Neoehrlichia mikurensis, in about 6 out of 100 ticks. In Europe it has occasionally been found in humans, mainly in people with a severely disturbed immune system.

RIVM; CDC (United States)

Drawbacks and risks

In someone who has not been to the United States, a positive result is almost always wrong and leads to unnecessary worry and unnecessary further testing.
A negative result in the first week of symptoms can give false reassurance.

Get tested