Explanation

Bartonella henselae IgM (cat scratch disease): what is measured?

Cat scratch disease is caused by the bacterium Bartonella henselae, which enters through a scratch, bite or saliva from a cat. This test measures IgM antibodies: the type of antibody the immune system makes first. IgM appears in the first weeks and then usually disappears again, while IgG comes later and stays much longer. As a result, IgM can point to a recent infection.

What can it detect?

An immune response that fits a recent infection with Bartonella henselae
Support for a suspicion of cat scratch disease with a newly developed, swollen and painful lymph node after contact with a cat

Result

What does the result mean?

Positive result

A positive IgM result can fit an infection from a few weeks ago. The result on its own is not proof: IgM is prone to false-positive results and to cross-reactions with antibodies against other bacteria. The result only counts if there are symptoms that fit cat scratch disease, such as a swollen, painful lymph node near a scratch or bite, sometimes with fever and headache.

Negative result

A negative IgM result does not rule out cat scratch disease. IgM has often already disappeared by the time the swollen lymph node is noticed, because the node only appears about two weeks after the scratch or bite and can last for weeks to months. IgM is also not yet detectable in the first days after infection. With an ongoing suspicion, IgG or testing in hospital, such as a PCR on material from the node, is more useful.

After an abnormal result

With a positive result and matching symptoms, the GP assesses the picture further; a swollen lymph node usually goes away on its own, so waiting often follows. With a severe course, a weakened immune system or symptoms of the heart or eye, consultation with or referral to the hospital follows. Cat scratch disease is not notifiable in the Netherlands.

Reliability

How reliable is the test?

There are no agreed limit values for these antibodies, and how well a single result shows or rules out an infection has been studied only to a limited extent. IgM against Bartonella is detectable only briefly and is not very reliable: the test misses infections in which IgM has already dropped and also gives positive results in people without the disease. In someone without symptoms and without contact with cats, a positive result means virtually nothing, because the chance of the disease was already very small. The diagnosis of cat scratch disease rests mainly on the clinical picture.

What can affect the result?

IgM is detectable only briefly and has often already dropped by the time the swollen lymph node is noticed.
Cross-reactions with antibodies against Coxiella burnetii and Chlamydia, among others, occur.
False-positive IgM results also occur with other infections and with autoimmune diseases.
Laboratories use different tests and cut-off values; results cannot simply be compared with each other.

Who it is for

Who is the test useful for?

Useful for

Can be useful with a recently developed, swollen and painful lymph node after a scratch or bite from a cat, and then preferably together with IgG. In people with a weakened immune system, the infection can be more severe and testing is needed sooner. Not useful as a general check in people without symptoms, not even with a lot of contact with cats, because the chance of a false-positive result is then greater than the chance of a real infection.

How common is it?

The RIVM estimates that in the Netherlands at least 300 to 1,000 people a year get cat scratch disease, more than 2 per 100,000 inhabitants per year. It mainly affects children and young adults with a cat, and more cases are seen in autumn and winter. In about 1 in 3 patients, fever, headache and feeling generally unwell occur in the first days to weeks.

RIVM

Drawbacks and risks

IgM tests give a false-positive result relatively often; without matching symptoms, this can lead to unnecessary further testing and unnecessary antibiotic use.
A negative result says little, because IgM has often already dropped by the time the symptoms are noticed.

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