Explanation

Anaplasma phagocytophilum IgM: 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). This test measures IgM antibodies against the bacterium. IgM is the type of antibody the body makes first, and could therefore point to a recent infection. With Anaplasma, however, IgM is not very reliable: the US CDC advises against using IgM as a sign of a recent infection.

What can it detect?

A possible recent immune response to Anaplasma phagocytophilum, only interpretable together with IgG and the clinical picture

Result

What does the result mean?

Positive result

A positive IgM result may fit a recent infection with Anaplasma, but does not prove it. IgM tests for this bacterium often give a positive result without an infection, partly due to cross-reactions with antibodies against other pathogens, and IgM can remain detectable long after an infection. A recent infection only becomes clear from a clear rise in IgG in a second blood draw 2 to 10 weeks later, or from a PCR on blood in the first week of symptoms.

Negative result

A negative IgM result makes a recent infection less likely, but does not rule it out. Not everyone makes measurable IgM, and shortly after the symptoms start, the result can still be negative. 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 in IgG, 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 limit values for IgM against Anaplasma, and the test is not very reliable: according to the US CDC (Centers for Disease Control and Prevention), IgM is not a reliable way to show anaplasmosis and should not be used as a sign of a recent infection. The strongest evidence is a clear rise in IgG between two blood draws, or a PCR on blood in the first week. In the Netherlands, where anaplasmosis is rarely diagnosed, a single positive IgM in someone without matching symptoms is usually wrong.

What can affect the result?

Falsely positive IgM results are common; the result can only be interpreted together with IgG and the clinical picture.
IgM can remain detectable for a long time after an infection and then says nothing about the time of infection.
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

Only meaningful together with IgG, with fever, headache and muscle pain in the weeks after a tick bite. Not useful as a stand-alone test, as a general check after a tick bite or in people without symptoms: a positive result is then usually wrong.

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 IgM is often wrong for this bacterium and can lead to unnecessary worry, unnecessary further testing or unnecessary use of antibiotics.

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