Inclusion Body Myositis – Blood Tests

Blood tests for creatine kinase and the anti-cN1A (NT5C1A) antibody can be helpful in making the diagnosis of inclusion body myositis. Muscle damage can also increase levels of what are often referred to as “liver enzymes.”


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.

Vial of blood for testing

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 some patients do have significantly higher CK levels. In some other muscle diseases, such as Duchenne muscular dystrophy or polymyositis, the creatine kinase level can be up to 50 times higher than the upper limits of normal. However, it is rare for someone with IBM to have a level more than 15 times greater than the upper limit of normal. Levels higher than this do not absolutely rule out the diagnosis of IBM, but they should prompt a serious consideration of alternative diagnoses.


“Liver Enzymes” May Be High in Inclusion Body Myositis due to Muscle Damage

People with inclusion body myositis may have elevated levels of enzymes such as alanine transaminase (ALT) and aspartate transaminase (AST), which are often called “liver enzymes.” These enzymes are released into the blood by damaged liver cells, so high levels of ALT and AST may be a sign of liver disease. However, these enzymes are also found in muscle, and may be elevated in IBM due to ongoing muscle damage, even when the liver is healthy. This is important to remember, because liver disease may be mistakenly diagnosed in someone with IBM.


Antibody Testing can Help Confirm a Diagnosis of Inclusion Body Myositis

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. This name is usually abbreviated, but there are several different abbreviations. The use of “anti-cN1A” is becoming common, but “anti-cN-1A” and “anti-NT5C1A” (Cytosolic 5‘-NucleoTidase 1A) are also used. 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-cN1A antibody makes the diagnosis of inclusion body myositis highly likely.

It is rare for healthy adults to have a positive anti-cN1A antibody test. The test is sometimes positive in some other autoimmune disease, such as dermatomyositis, systemic lupus erythematosus, and Sjogren’s syndrome. However, these diseases can usually be distinguished from IBM based on the patient’s history and examination.

Frequently Asked Questions regarding the NT5C1A antibody test:

Artistic depiction of antibodies

Artistic depiction of antibodies

Can the anti-cN1Aantibody 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. You could still have IBM even if the antibody test is negative, and a positive result would just tell you what you already know.

However, if you are seeing an inclusion body myositis specialist, your doctor may suggest the anti-cN1A antibody test to provide more information for research.

Does a positive antibody test mean that my prognosis is worse?

Maybe. Some 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-cN1A 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.


Video Discussion of the Anti-cN1a (NT5C1A) Antibody Test for Inclusion Body Myositis


by Kevin Dooley, MD

Revised April 30, 2024

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