How Can an Enzyme Activity Test Help Diagnose Pompe Disease?

One test that doctors use to diagnose Pompe disease, a rare genetic disease caused by mutations in the GAA gene, is an enzyme activity test.

GAA provides cells with the instructions necessary to make an enzyme that plays a role in breaking down a complex sugar molecule called glycogen. Mutations in this gene mean that cells don’t have enough of the enzyme. This leads to glycogen accumulation, damaging cells and causing disease symptoms.

What is an enzyme activity test?

An enzyme activity test measures the amount of active enzyme in a sample. If your doctor suspects that you may have Pompe disease, you likely will be asked to undergo a test of GAA enzyme activity. A sample taken for test could be a small bit of blood, muscle, or skin.

How do I give a sample?

The type of sample your doctor uses often depends on how the laboratories in your area most commonly do the test.

Blood draws for an enzyme activity test are very simple and take only a few minutes. A few drops of blood are usually sufficient. A finger stick (or a heel stick for infants) is all that is necessary. However, your doctor might want to test for multiple conditions. So it is not uncommon for them to take a few milliliters of blood from the arm, especially in adult patients.

Preparation for a muscle or skin biopsy is slightly more complex. This is because it involves a small surgery. Your doctor will first numb the area. He or she will then use a surgical blade or a large needle to collect the sample. Depending on how the doctor obtains the sample, you may need one or two stitches to close the wound. After that, your doctor will bandage the site and give you care instructions. You will have to keep the area clean and watch for signs of infection. The doctor will send the samples to a laboratory for testing.

What do the results mean?

It takes a few days to a few weeks for the results to become available.

The laboratory will compare the enzyme activity from your samples with control samples from people with normal levels of GAA activity.

A muscle biopsy sample is generally large enough that clinicians can also stain it for glycogen. If large amounts of glycogen are present, in combination with low levels of GAA activity, this may indicate Pompe disease.

If you have very low levels of GAA activity, your doctor will discuss whether you need to undergo additional tests to confirm a Pompe diagnosis.

In case your doctor diagnoses you with Pompe, he or she will discuss the next steps regarding disease treatment and symptom management.

Last updated: July 14, 2020

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