Inclusion Body Myositis – Blood Tests
Blood tests for creatine kinase and the NT5C1A antibody can be helpful in making the diagnosis of inclusion body myositis.
Elevated Creatine Kinase can be a Clue that a Patient has a Muscle Disease
Creatine kinase (CK), also known as creatine phosphokinase (CPK), is an enzyme found in high levels in tissues that consume energy rapidly, such as skeletal muscle, heart muscle, and the brain. When these tissues are injured, creatine kinase is released into the blood. Serum creatine kinase is a routine test that can be performed in most medical laboratories. High levels of creatine kinase can indicate a recent heart attack or stroke, but these levels are only temporarily high. Strenuous exercise can also cause a temporary rise in CK levels. Muscle diseases like inclusion body myositis can cause CK levels to be high for many years.
Creatine kinase is often checked to see if a patient has a muscle disease, and it can provide the first clue that diseases like inclusion body myositis should be considered in the diagnosis. Inclusion body myositis patients usually have normal or slightly elevated creatine kinase levels, but in some cases can be up to 10 times normal. This is in contrast to some other muscle diseases, such as Duchenne muscular dystrophy or polymyositis, which can have levels up to 50 times normal.
Antibody Testing can Help Confirm a Diagnosis of Inclusion Body Myositis
Recently it has been discovered that some inclusion body myositis patients have antibodies that can help diagnose the disease. These antibodies are known by the long name of anti-cytosolic 5’ –nucleotidase 1A, or anti-CN1A or anti-NT5C1A for short. The antibody test is positive in only about half of IBM patients, so a negative test does not mean a patient does not have inclusion body myositis. However, a positive test makes it likely that IBM is the correct diagnosis.
In a person with typical symptoms and examination findings, a positive test for the anti-NT5C1A antibody makes the diagnosis of inclusion body myositis highly likely.
It is rare for healthy adults to have a positive NT5C1A antibody test. The test is sometimes positive in some other autoimmune disease, such as dermatomyositis, systemic lupus erythematosus, and Sjogren’s syndrome (~10-25% positives). However, these diseases can usually be distinguished from IBM based on the patient’s history and examination.
Polymyositis is often confused with inclusion body myositis. The antibody test can help separate the two conditions, as the test is rarely positive (<5%) in polymyositis.
Frequently Asked Questions regarding the NT5C1A antibody test:
Can the anti-NT5C1A antibody test replace the muscle biopsy?
No. However, a positive antibody test does help strengthen the diagnosis, and can be particularly helpful if a biopsy is not planned or desired, or if the biopsy does not give a definite answer.
If I already have a firm diagnosis of IBM after thorough evaluation and biopsy, would getting the antibody test still be useful?
The antibody test would not change your diagnosis. A negative result doesn’t necessary mean that you don’t have IBM, and a positive result would just tell you what you already know.
Does a positive antibody test mean that my prognosis is worse?
Maybe. Some recent studies seem to show more severe disease and faster progression in those with a positive antibody test compared to those with a negative test. However, the answer is not completely clear, and there is a lot of overlap between the two groups.
Does this antibody cause the muscle damage in IBM?
We don’t know if the antibody contributes to the muscle damage in IBM. This is an area of research.
What more can you tell me about the anti-NT5C1A antibody test?
This is a specialized test that cannot be performed in most labs, and it may take several weeks to obtain results. The test is not standardized, and the technique can vary from lab to lab. Also, each lab has to decide how to report results. For example, how strong does the test reaction have to be for the test to be considered positive? If the result is weakly positive, or moderately positive, is that considered positive?
Although much work needs to be done, the anti-cytosolic 5’ –nucleotidase 1A antibody test is already useful.
by Kevin Dooley, MD
Reviewed December 18, 2018
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