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    Blood Tests to Diagnose Arthritis

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    What Tests Indicate Disorders Like Lupus?

    The ANA profile is a series of tests that measure the presence of abnormal antibodies. The profile helps your doctor look for diseases such as Sjögren's syndrome, lupus, drug-induced lupus, polymyostitis, and scleroderma.

    When the ANA is positive, it indicates that you may have an autoimmune disorder, but the test alone can't make a reliable diagnosis. If the ANA is negative, it is likely that you don't have lupus.

    What Does the Presence of HLA-B27 Indicate?

    HLA-B27 is a genetic marker. In people with inflammatory arthritis of the spine and joints (not osteoarthritis), a positive HLA-B27 test is associated with the presence of one of a group of diseases called seronegative spondyloarthropathies. This includes diseases such as ankylosing spondylitis (AS), psoriatic arthritis, and Reiter's syndrome (also called reactive arthritis). HLA-B27 is present in about 90% of people with AS, but the gene can also be seen in people with no sign of arthritis or inflammation.

    What Does It Mean if Muscle Enzymes Are High?

    When muscle enzymes -- creatine phosphokinase (CPK) and aldolase -- are high, it indicates a possible inflammatory muscle disease. Higher levels of CPK can also be seen after trauma, injections into a muscle, muscle disease due to an underactive thyroid, and while taking certain medications such as cholesterol-lowering statin drugs.

    What Are Antineutrophil Cytoplasmic Antibodies (ANCA)?

    ANCA are abnormal antibodies found in the blood in most people with Wegener's granulomatosis, a disease that affects the upper respiratory tract, lungs, and kidneys. They are uncommon in other diseases, which makes them useful in diagnosing this particular disease. ANCA levels are sometimes used to follow the course of Wegener's granulomatosis.

    What Is the Complement System?

    The complement system is made up of a network of proteins that involve the immune system and inflammation. Decreased levels of various components of complement -- C3, C4, or CH50 -- can be seen in lupus. Other diseases that involve inflammation of the blood vessels -- called vasculitis -- also often have decreased levels of complement in the blood. In lupus, complement levels are used to follow the illness, because they rise and fall according to the activity of the disease.

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