There are a number of blood tests the doctor may choose. The following is a list of some of these. Unless otherwise noted, all of these tests require taking a sample of blood by using a needle. Some blood tests require that you do not eat or drink, except for water, for about eight hours before the test. Because exercise can affect some test results, you might need to limit exercise for several days before the test to avoid falsely high results. Certain medications may also affect the results of these tests, so be sure to check with your doctor if you are taking any type of medications at all, including aspirin or other over-the-counter medications.

Aldolase is an enzyme found especially in the liver and skeletal muscles. When the liver or muscles are damaged, the cells release their contents (including aldolase) into the bloodstream. Since muscle weakness can be caused by problems with either the nerves or the muscles, this test identifies weakness caused by muscular problems. Aldolase will not change when weakness is caused by neurological problems.

Normal ranges may vary slightly based on a particular laboratory’s calibrations. There are also slight differences in normal ranges based on age and gender. Generally, however, normal adult findings are between 1.0 to 7.5 units per liter.

Antinuclear Antibodies (also known as ANA) is a screening blood test to determine if you have an autoimmune disease. Antibodies are an important part of the immune system. They fight infections, viruses, and other foreign substances that can cause disease. When this protective system turns toward fighting the body’s own tissues, an autoimmune disease is present, and the ANA test will be positive. A positive ANA will not, however, identify the specific disease.  Moreover, a negative test does not rule out myositis.

Creatine Kinase (also known as CK, or Creatine Phosphokinase [CPK]) is an important diagnostic blood test for myopathies. CK is a type of protein called an enzyme that is especially active in skeletal muscle, heart tissue, and the brain. When muscle tissue is damaged, the cells release their contents into the bloodstream, causing elevated CK levels in the blood.

CK levels in the blood can vary with a number of factors, including gender, race, age, activity, health status, testing method, and more. Higher levels of serum CK can indicate muscle damage from chronic disease or acute muscle injury. In myositis, it is not uncommon for CK levels to far exceed the upper limit of normal. If the CK test indicates muscle damage, more tests will be needed to find exactly where the muscle damage occurred.

CK levels are often used to evaluate the progress of disease after treatment. This is not a reliable measure of disease activity, however, and normal enzyme levels do not completely rule out the possibility of muscle inflammation. Myositis patients sometimes wonder why they feel better or worse than their CK levels indicate. Levels may lag behind the improvement or worsening of the disease, and they may be affected by activity or other factors.  Additionally, in advanced disease, serum muscle enzyme levels can be persistently low in the setting of major muscle weakness.

Sedimentation Rate (also called erythrocyte sedimentation rate [ESR] or sed rate) measures swelling and inflammation of the muscles. Doctors use the sed rate to watch the progress of the muscle inflammation. This test is not specific to a particular disease but simply identifies the presence and severity of the inflammation. It can be used to monitor the effect of treatment.

Sed rate results are reported in the distance in millimeters (mm) that red blood cells fall in a test tube in one hour (hr). The normal range varies based on laboratory equipment, age, gender, pregnancy, infection, and other factors. The sed rate will be higher with more severe inflammation.

Myositis autoantibodies are an important diagnostic and predictive tool. They are identified through a special blood test that tests for a whole panel of autoantibodies that are related to myositis diseases.