Explanation

Aspergillus antigen (galactomannan): what is measured?

Galactomannan is a sugar-like substance from the cell wall of the mold Aspergillus. When the mold grows in the body, this substance ends up in the blood. Unlike an antibody test, this test therefore does not measure the immune response, but a piece of the mold itself. The laboratory reports the result as an index; in the guidelines, an index of 0.5 or higher counts as positive. The test was developed to detect invasive aspergillosis early in people with a severely weakened immune system.

What can it detect?

Invasive aspergillosis: a mold infection that spreads from the lungs into the body in people with a severely weakened immune system
A reason for further testing, such as a CT scan of the lungs, with persistent unexplained fever during a period with few white blood cells

Result

What does the result mean?

Positive result

A positive result means that galactomannan was found in your blood. In someone with a severely weakened immune system and persistent fever, this is, according to the SWAB guideline, a reason for further testing, usually a CT scan of the lungs. In people with a normal immune system, invasive aspergillosis is so rare that a positive result is almost always a false positive. False-positive results are known to occur, among other things due to certain antibiotics, other molds and food.

Negative result

A negative result makes invasive aspergillosis less likely, but does not rule it out, certainly not in people who are already using antifungal medicines: these lower the result. One negative measurement is therefore usually not enough; with an ongoing risk, a repeat measurement says more, and with an ongoing suspicion, material from the lungs can be examined. In people with a normal immune system, a negative result adds nothing, because the chance of this infection is already virtually zero.

After an abnormal result

With a positive result in someone with a weakened immune system, the SWAB guideline calls for further testing, usually a CT scan of the lungs and often examination of material from the lungs, and treatment with antifungal medicines is considered. In someone with a normal immune system, an abnormal result is first checked before any action is taken. Aspergillosis is not notifiable in the Netherlands.

Reliability

How reliable is the test?

The test was developed and studied in people with a severely weakened immune system, for example after a stem cell transplant or during chemotherapy with few white blood cells, and is most meaningful in them. In people without symptoms or with a normal immune system, the meaning of a positive result has hardly been studied. How useful a result is depends strongly on how high the chance of the infection already was beforehand: in a healthy adult that chance is virtually zero, so a positive result is then almost always wrong.

What can affect the result?

False-positive results occur, among other things due to certain antibiotics, other molds and food products.
Antifungal medicines lower the result, so an infection can be missed.
The test was made for people with a severely weakened immune system; with a normal immune system, the predictive value is very low.
One measurement says little; a repeat measurement, assessed together with a CT scan, says more.

Who it is for

Who is the test useful for?

Useful for

Most meaningful in people with a severely weakened immune system, for example during chemotherapy with few white blood cells or after a transplant, usually with persistent unexplained fever. Not useful in people with a normal immune system, not even with respiratory symptoms or a suspected mold problem at home: the infection then hardly ever occurs and an abnormal result is almost always a false positive.

How common is it?

Invasive aspergillosis occurs almost only in people with a severely weakened immune system, such as after a stem cell transplant, during chemotherapy with few white blood cells, after an organ transplant or in seriously ill patients in intensive care. In people with a normal immune system, the infection is exceptionally rare, even though almost everyone breathes in traces of the mold every day. The RIVM monitors this mold mainly because of increasing resistance to antifungal medicines.

SWAB guideline on invasive fungal infections and RIVM

Drawbacks and risks

In people with a normal immune system, a positive result is almost always a false positive and can lead to unnecessary hospital testing, including a CT scan with radiation exposure.
Treatment with antifungal medicines has many side effects and interactions with other medicines; starting it based on a result without symptoms is undesirable.

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