Having an accurate, timely diagnosis is a critical aspect of care. There are reliable tests to help confirm the diagnosis of a person with Duchenne. The following tests are typically used to confirm a preliminary diagnosis.
Creatine Kinase (CK) Blood Test
If your family doctor suspects that you or your child has a medical problem in the muscles, it is likely that your family doctor will order a CK (creatine kinase, sometimes referred to as a CPK or creatine phosphokinase) blood test as a first step in the diagnostic process.
CK is an enzyme that normally lives inside muscles. In healthy muscle, the membrane surrounding the muscle is intact, keeping the CK inside. In Duchenne, a lack of dystrophin keeps the muscle from tolerating the constant muscle movement of everyday activities, giving rise to tiny tears in the damaged muscle membrane. When this happens, CK leaks out of the muscle and into the blood.
The CK level of a person who does not have Duchenne is usually less than 200 units/liter. People with Duchenne often have CK levels 10 to 100 times the normal range. Elevated CK levels indicate a problem with the muscles (rather than a problem with the nerves, for example), although a high CK does not confirm a diagnosis of Duchenne, and is not genetic testing.
If you/your child have an elevated CK test, your doctor should offer you testing that can give a diagnosis. The two types of tests that are used to make a diagnosis are genetic testing and muscle biopsy.
Genetic testing (using blood cells or muscle cells) is the best way to obtain exact genetic information for a conclusive Duchenne diagnosis. DNA-based genetic testing is used to find mutations (changes) in the gene that Duchenne.
There are different genetic testing methods that find different types of mutations. You may need to have more than one genetic test to find a change. If a person with Duchenne has a complete set of genetic tests, it is very likely that the lab will find a mutation. If the lab finds a mutation, most children will not need to have a muscle biopsy. Learn more about genetic testing and PPMD’s free genetic testing program, Decode Duchenne.
If genetic testing is not able to accurately provide a diagnosis, a muscle biopsy should be performed. A muscle biopsy (a surgical procedure that involves taking a small sample of muscle for analysis) looks for changes in the amount and look of dystrophin protein, and is usually done by immunohistochemistry of slides of the muscle (putting a stain on the muscle, then looking at the muscle cells under a microscope for evidence of dystrophin) or by a Western blot test of the muscle protein (a chemical process that tests for the chemical presence of dystrophin). This type of testing can usually tell whether a person has Duchenne or Becker muscular dystrophy. Because this is a surgical procedure, it more invasive than a blood test.
Making Informed Choices
Every primary care provider has a different amount of experience with Duchenne and with genetic testing. Some will know how to help you make informed choices about the pros and cons of muscle biopsy and genetic testing, and others may not. Many primary care providers will refer you to a neuromuscular or genetics specialist for diagnosis and testing and for long term management. If you have any concerns after meeting with the specialist, a second opinion may be useful. Informing yourself is always a good way to help promote good health care for your family.
Genetic counselors are excellent resources for assisting with the decision of whether to test for a diagnosis, getting a second opinion, the best tests to choose and how to understand the results. If you have questions, please contact one of Parent Project Muscular Dystrophy’s board-certified genetic counselors.