Lab test rheumatoid arthritis
by Nathan Wei, MD, FACP, FACR
Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.
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A patient who is being evaluated for rheumatoid arthritis will usually have the following lab tests performed:
•A complete blood count is a test measuring red blood cells, white blood cells, and platelets. Blood consists of these components suspended in a thick, colorless fluid called plasma. Automated machines rapidly count the cell types.
The white cell count is normally between 5,000-10,000. Increased values suggest inflammation or infection. Exercise, cold, and stress will temporarily elevate the white cell count. Normal values for the red cell count vary with gender. Males normally have values around 5-6 million per microliter. Females have a lower normal range at 3.6-5.6 million red cells per microliter.
Hemoglobin, the iron containing component of red cells which carries oxygen, is also measured in a complete blood count. The normal hemoglobin value for males is 13-18 g/dl. Normal for females is 12-16 g/dl. The hematocrit measures the percent of total blood volume which is red cells. Normal value for males is 40-55%, and the normal value for females is 36-48%. Generally, the hemoglobin times 3 equals the hematocrit. Decreased values indicate anemia.
The MCV, MCH, MCHC are red cell measures which indicate the size and hemoglobin content of individual red cells. These indices give clues as to the probable cause of an existing anemia. Platelets are components which are important in clot formation. Many drugs decrease the platelet count or affect platelet function. Normal values range from 150,000-400,000.
The process of inflammation can cause changes in the blood count. The red cell count may go down, the white cell count may go up, and the platelet count may be elevated. While anemia may accompany inflammatory arthritis it may be caused by other things such as blood loss or iron deficiency. Only when other causes have been ruled out can a doctor interpret blood abnormalities as a sign of inflammation.
The percent and absolute number of each type of white blood cell is called the differential. Neutrophils are increased in bacterial infections and acute inflammation. Lymphocytes are increased in viral infections. Monocytes are increased in chronic infections and eosinphils are increased in allergies. Basophils, which are generally 1 or 2% do not usually increase.
•The chemistry panel is a series of tests which are used to evaluate overall health. The tests include heart risk indicators, diabetes indicators, as well as tests for kidney, liver, and thyroid function. For example, a patient with a high creatinine level may have a problem with the kidneys. Creatinine is a waste product found in the blood. Certain types of inflammatory arthritis can affect kidney function. Certain arthritis drugs can affect kidney function too. Uric acid is another test of the blood chemistry panel which, if elevated, may be indicative of gout.
•The erythrocyte sedimentation rate is a test determining how fast red blood cells settle to the bottom of a column in one hour. When inflammation is present, the body produces proteins in the blood which make the red cells clump together. Heavier cell aggregates fall faster than normal red cells. For healthy individuals, the normal rate is up to 20 millimeters in one hour. Inflammation increases the rate significantly. Since inflammation can be caused by conditions other than arthritis, the sed rate test alone is not diagnostic.
•Rheumatoid Factor is an antibody found in patients with rheumatoid arthritis. Rheumatoid factor was discovered in the 1940's and became a significant diagnostic tool in the field of rheumatology. 80% of RA patients have RF in their blood. Usually, the higher concentration of RF, the more severe the rheumatoid arthritis. RF can take many months to show up in a patients blood. If tested too early in the course of the disease, the result could be negative and retesting should be considered at a later date. There are also patients with all the signs and symptoms of RA but are seronegative for RF. Some doctors suspect another disease masquerading as RA in these cases. RF can occur in response to inflammatory of infectious diseases other than RA, though usually in these cases, the amount is lower. Rheumatoid factor is an antibody and there are three types of rheumatoid factor: IgM, IgG, and IgA.
•Patients with certain rheumatic diseases, especially lupus, make antibodies to the nucleus of cells. These antibodies are called antinuclear antibodies and are tested for by placing a patient's blood serum on a microscope slide containing cells with visible nuclei. A substance containing fluorescent dye is added which binds to the antibodies. Under a microscope the abnormal antibodies can be seen binding to the nuclei. Over 95% of patients with lupus have a positive ANA test. 50% of rheumatoid arthritis patients are positive for ANA. Patients with other diseases also can have positive ANA tests.
•Patients may also have thyroid function tests performed if there is the suspicion of thyroid disease. Thyroid disease can aggravate joint symptoms.
•C reactive protein is another inflammatory marker that tends to go up quicker and come down quicker than the sed rate
•Anti-CCP is a test that seems to have more specificity than the rheumatoid factor. If the anti-CCP is positive in a patient with suspicious joint symptoms, the diagnosis of RA is relatively certain. Anti-CCP is often associated with rheumatoid factor positivity. The combination of a positive anti-CCP with a positive rheumatoid factor makes the diagnosis of RA about 100%.
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