Blood Tests to Diagnose Arthritis
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 your doctor decide which type you have.
This article reviews 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 can be found in people without RA or with other autoimmune disorders, however. In general, when no rheumatoid factor is present in someone with RA, the course of the disease is less severe. A new test for rheumatoid arthritis that measures levels of antibodies that bind citrulline modified proteins (anti-CCP) is more specific and tends to be only elevated in patients with rheumatoid arthritis or in patients 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 (sed. rate or ESR) is an indication of the degree of inflammation in the body. It is actually a measurement of the speed with which red blood cells fall in a test tube of blood. When the inflammation in the blood goes up, these inflammatory substances attach to red blood cells and the cells fall faster. In healthy persons, the sed. rate 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) also indicates the amount of inflammation present. It is thought to be a better test than the sed. rate for certain diseases. In patients 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 Signs Indicate Connective Tissue Disorders Like Lupus?
The ANA is a blood test that is used in the evaluation of lupus or other connective tissue disorders. It measures the presence of abnormal antibodies. When the ANA is positive, it indicates that someone may have an autoimmune disorder, but alone it can’t make the diagnosis. If the ANA is negative, it is very likely that you don’t have lupus.
The ANA profile is a series of tests consisting of an ANA and other related abnormal antibodies. This may be done if the ANA is found to be positive or possibly at the same time as the ANA. This profile helps look for diseases such as Sjögren’s syndrome, lupus, drug-induced lupus, polymyostitis, and scleroderma.
What Does the Presence of HLA-B27 Indicate?
HLA-B27 is a genetic test. In patients 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 from muscle 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 Complement?
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 go up with worsening and down with improvement of lupus disease activity.
What Are Cryoglobulins?
Cryoglobulins are antibodies that may be high in a variety of different diseases, including rheumatoid arthritis, lupus, Sjögren’s syndrome, Waldenstrom’s macroglobulinemia, multiple myeloma, lymphoproliferative disorders, hepatitis B and C, and other infections.
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