Testing Options Late-onset Pompe Disease (LOPD)

The journey to a diagnosis of Pompe disease can be long and complicated.

Very often it can involve visiting several different doctors and having to undergo a variety of tests because symptoms can affect many areas of the body, including the muscles, lungs, and heart, and because there are many different types of tests that can be performed. Doctors often need to first rule out other diseases that share similar symptoms with Pompe disease; this is referred to as a differential diagnosis.

Testing Options

Your doctor will most likely start with a thorough physical exam, but will also order tests that can include the following:

  • Tests for muscle weakness and function
  • Tests for breathing ability

Other tests your doctor may perform include a urine test (urinalysis), muscle imaging/biopsy, sleep tests, electrocardiogram, or chest x-ray (mostly in children)

The tests above can suggest a diagnosis of LOPD, but only 2 tests can diagnose LOPD definitively: a test for enzyme activity and a genetic test for gene mutation.

Testing for GAA enzyme levels:

  • Testing for GAA enzyme activity is the definitive test for diagnosing Pompe disease. This test detects the amount of GAA enzyme activity a patient has. People with LOPD can have variable reductions in the activity of GAA enzyme, anywhere from 2% to 40% of normal, although the range may vary depending on the lab that is performing the test
  • Enzyme activity can be measured in skin cells (fibroblasts), muscle tissue, and blood. Skin cells and muscle biopsies are not used as often as blood tests because they are invasive, inconvenient, and time-consuming
  • The methods listed below can be used to test for GAA enzyme activity. The dried blood spot (DBS) test is commonly used due to its accuracy and convenience.
  • Mixed white blood cells (leukocytes)
  • Purified white blood cells (lymphocytes)
  • Dried blood spot (DBS) sample, which is blood dried on filter paper for analysis

Genetic test for gene mutation:

A diagram showing autosomal recessive inheritance. Silhouettes of parents and a pair of their chromosomes illustrate that they both have a copy of the variant GAA gene. Silhouettes of four children and a pair of their chromosomes illustrate the possibilities of being an affected child, an unaffected child, or an unaffected child who is a carrier.

This figure shows the pattern of inheritance for Pompe disease. Patients with Pompe disease inherit a gene variant from each of their parents. If someone has 1 copy of the altered gene, they will not develop the disease; they are considered a “carrier.” Only when a person has 2 copies of the altered gene will the disease occur. This is referred to as autosomal recessive inheritance.

A blood or saliva test can also provide more information about the specific genetic defect in the GAA gene that causes less enzyme to be produced; this is called mutation analysis or genotyping. Changes in the GAA gene can sometimes give information on how severe the disease will be (i.e., how much enzyme deficiency there is and how severe the symptoms are likely to be) but not always. Genotyping may be useful when:

  • A patient’s GAA enzyme activity is near normal levels and their symptoms are mild
  • Identifying a family member(s) who may be a carrier(s) of an altered GAA gene (see Figure 1)

If your doctor thinks it is appropriate, he or she may also order a multigene test panel to investigate the possibility of Pompe and other inherited disorders simultaneously.