If your doctor suspects you have arthritis, he or she may have your blood drawn to determine which type of arthritis you have. In people with osteoarthritis, blood tests are not usually abnormal, but with other types of arthritis, including rheumatoid arthritis, certain tests will help with a proper diagnosis.
Here is a review of the different types of blood tests and markers used to diagnose arthritis and other inflammatory conditions.
What Blood Markers Are Used to Diagnose Rheumatoid Arthritis?
Rheumatoid factors are a variety of antibodies that are present in 70% to 90% of people with rheumatoid arthritis (RA). Rheumatoid factor (RF), however, can be found in people without RA or with other autoimmune disorders. In general, when no rheumatoid factor is present in someone with RA, the course of the disease is less severe.
Another test for rheumatoid arthritis that measures levels of antibodies that bind citrulline modified proteins (anti-CCP) is more specific and tends to be elevated in patients with rheumatoid arthritis or in those about to develop rheumatoid arthritis. The presence of anti-CCP antibodies can be used to predict which patients will get more severe rheumatoid arthritis.
Are There Tests to Determine Inflammation?
Yes. The erythrocyte sedimentation rate (ESR) reflects the degree of inflammation in the body. In healthy people, the ESR is low and it climbs with inflammation. It doesn't point to any particular disease, but is a general indication of the amount of inflammation in the body. In lupus and polymyalgia rheumatica, the ESR often correlates with disease activity.
C-reactive protein (CRP) levels are an even better indication than ESR of the amount of inflammation present. In people with rheumatoid arthritis, if the CRP is high, it suggests that there is significant inflammation or injury in the body.
Both CRP and ESR levels are used to monitor disease activity and to monitor how well someone is responding to treatment.
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 -- can also have decreased levels of complement in the blood. In some cases of lupus, particularly those with kidney disease, complement levels are used to follow the illness because they may rise and fall according to the activity of the disease.