Explanation

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

Cat scratch disease is caused by the bacterium Bartonella henselae. People pick it up through a scratch, a bite or saliva from a cat; cats often carry the bacterium without being ill themselves. This test measures IgG antibodies against the bacterium. IgG appears later than IgM, usually a few weeks after infection, and can then remain detectable for months to years.

What can it detect?

An immune response to Bartonella henselae, the cause of cat scratch disease
Support in explaining a long-lasting swollen, painful lymph node after contact with a cat
Rarely: support in investigating an inflammation of a heart valve or of the eye in people with matching symptoms

Result

What does the result mean?

Positive result

A positive IgG result means that your immune system has had contact with Bartonella henselae. This fits cat scratch disease, but does not say when that infection was: IgG can remain detectable for a long time. Many people with cats have antibodies without ever having been ill, so without matching symptoms the result has little meaning. Cat scratch disease usually involves a swollen, painful lymph node near the scratch or bite, which goes away on its own, on average after about six weeks.

Negative result

A negative IgG result makes cat scratch disease less likely, but does not rule it out. In the first weeks after infection, IgG is often not yet detectable, and not everyone makes a clearly measurable immune response. With a strong suspicion, the test can be repeated after a few weeks, or the hospital can do other tests, such as a PCR on tissue or pus from the lymph node.

After an abnormal result

With a positive result and a matching swollen lymph node, the GP usually chooses to wait and see, because the node goes away on its own; 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. Cat scratch disease is mainly diagnosed on the clinical picture: a swollen lymph node after contact with a cat. Serology can support this, but is not conclusive, partly because antibodies against Bartonella can cross-react with antibodies against other bacteria such as Coxiella burnetii and Chlamydia. In people without symptoms and without contact with cats, a positive result is more often wrong than right.

What can affect the result?

IgG antibodies remain detectable for a long time after a past infection and say nothing about an active infection.
Cross-reactions with antibodies against Coxiella burnetii and Chlamydia, among others, occur.
In the first weeks after infection, the result can still be negative.
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 lymph node that stays swollen and painful for more than a few weeks after a scratch or bite from a cat, or with an inflammation around the eye after contact with cat saliva. 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 if you have a lot of contact with cats: antibodies are then common without disease.

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. Children and young adults with a cat are the largest group, and more cases are seen in autumn and winter. About 22 in 100 Dutch cats have the bacterium in their blood and about half of the cats studied have antibodies.

RIVM

Drawbacks and risks

Antibodies occur in many people with cat contact; without matching symptoms, a positive result can lead to unnecessary further testing or unnecessary antibiotic use.
A negative result can give false reassurance, because the test misses some infections.

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