Explanation

Anaplasma phagocytophilum IgG: what is measured?

Anaplasma phagocytophilum is a bacterium that grows in white blood cells and can reach people through a tick bite. The disease is called human granulocytic anaplasmosis (HGA); it used to be called ehrlichiosis as well. This test measures IgG antibodies against the bacterium. IgG often cannot yet be detected in the first week of symptoms and then remains detectable for months, sometimes years, so the test mainly shows contact with the bacterium, not whether it is still present now.

What can it detect?

Past contact with the bacterium Anaplasma phagocytophilum or a related bacterium
Support in investigating anaplasmosis with fever after a tick bite, preferably together with a second blood draw a few weeks later

Result

What does the result mean?

Positive result

A positive result means that IgG antibodies against Anaplasma 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: IgG can remain detectable for months to years. One positive result is therefore no proof of a recent infection; that only comes from a clear rise in the amount of antibodies in a second blood draw 2 to 10 weeks later. Anaplasmosis usually causes flu-like symptoms with fever, headache and muscle pain, which often go away by themselves after a few days.

Negative result

A negative result makes a past infection with Anaplasma 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 anaplasmosis. With persistent or severe symptoms after a tick bite, 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, not on the result alone. Anaplasmosis is not notifiable in the Netherlands.

Reliability

How reliable is the test?

There are no agreed Dutch limit values for these antibodies, and the meaning of a single result is limited. According to the US CDC (Centers for Disease Control and Prevention), anaplasmosis 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. Antibodies against Anaplasma and Ehrlichia cross-react with each other. In the Netherlands, where the disease is rarely diagnosed, a positive result in someone without matching symptoms is more often old or wrong than a sign of an active infection.

What can affect the result?

IgG antibodies remain detectable for months to sometimes years 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 related bacteria, such as Ehrlichia, 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, especially if low white blood cells or platelets have also been found, and then preferably with a second blood draw a few weeks later. People with a severely weakened immune system have a higher chance of a severe course. Not useful as a general check after a tick bite or in people without symptoms: a positive result is then usually old or wrong and changes nothing.

How common is it?

How often anaplasmosis occurs in the Netherlands is not known exactly; the RIVM (Dutch National Institute for Public Health and the Environment) only makes the diagnosis very occasionally. The bacterium is found in 2 to 3 out of 100 ticks caught in nature. According to the LCI guideline (RIVM) on Lyme disease, other tick-borne pathogens, such as Anaplasma, play no role of clinical significance in the Netherlands.

RIVM; LCI guideline (RIVM) on Lyme disease

Drawbacks and risks

A positive result without matching symptoms can lead to unnecessary worry, unnecessary further testing or unnecessary use of antibiotics.
A negative result in the first week of symptoms can give false reassurance.

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