Rheumatoid arthritis (RA) is a chronic autoimmune disease that primarily affects the joints, but can affect other parts of the body. Diagnosing and managing RA involves clinical evaluation by a rheumatologist, as well as several different laboratory tests that require blood work. The results of these tests may be used in two ways:
The doctor and healthcare team use the results of these tests to guide treatment options for each patient. In turn, understanding how the results of blood tests used to monitor RA and its treatment can help patients better able to manage their RA.
Rheumatoid factor (RF) was the first autoantibody to be discovered in people with RA. (Autoantibodies develop in response to the body’s own tissue, and are characteristic of autoimmune diseases, such as RA.) Despite the name, however, RF is not specific to RA, and there are many factors that can impact RF lab results. About 20% of those with confirmed RA will not have an abnormal RF test, while 5% of people who do not have RA will have an abnormal RF test. Negative levels do not exclude the disease, and positive levels do not guarantee the diagnosis.
The normal range of RF is from 0-20 IU/ml. RF above 20 IU/ml is not considered enough to diagnose RA, as there other reasons the RF level may be elevated. Some conditions and medical procedures that can raise RF levels include: other autoimmune diseases, certain chronic infections, diabetes, bacterial endocarditis, cancer, normal aging, vaccinations and transfusions. It’s important to note that once the RF level is elevated, it will often remain so even if the disease goes into remission.
Another test which is ordered when rheumatoid arthritis is suspected is the anticyclic citrullinated peptide (anti-CCP). The normal level of anti-CCP is less than 20 Units. (At Hospital for Special Surgery, anti-CCP is reported in Units. Some labs report this same result using a different measurement notation, that is, as less than 20 EU/ml.) A level above 20 suggests the possibility of RA. As with rheumatoid factor, some people with positive anti-CCP antibody will not have RA, but this test is somewhat more specific for RA than the rheumatoid factor. The higher the levels of anti-CCP antibody, the more likely it is to suggest RA.
This test is 97% specific for RA if it is present. Once a patient develops a positive anti-CCP, it will usually remain positive, despite remission.
About 20% of RA patients are seronegative, meaning that their RF and anti-CCP lab results both continue to come back negative. In these cases, the physician makes the diagnosis based on physical examination and imaging.
RF and anti-CCP are not used to monitor disease activity, because they both tend to remain positive despite remission. Once the diagnosis of RA has been made and confirmed, these tests are not repeated.
Sedimentation rate (also known as erythrocyte sedimentation rate or ESR), is a crude measure of inflammation. It is calculated by measuring the rate at which red blood cells sediment in a test tube in one hour. Normal levels for men range from 0-15 mm/hr to 0-20mm/hr and for women 0-20 mm/hr/ to 0-30mm/hr, depending on age – higher for people over the age of 50). The ESR rate is not specific for RA, and there are many factors that can interfere with the results, such as bad processing, an infection, and aging in patients over the age of 50.
C-reactive protein is another measure of clinical inflammation. The normal measurement is less than 1.0 in many labs. This test, however, can be influenced by factors such as obesity and infection and is not specific to RA.
Both ESR and C-reactive protein are non-RA-specific measures of inflammation. Both tests are used to test disease activity; when they are high, this suggests that the disease is very active (assuming no other causes for high results, such as infection, are present). The healthcare team orders these labs regularly to monitor the patient’s disease and check how his or her medications are working.
A complete blood count (CBC) test looks at red and white blood cell counts. Below are the normal measurements in our lab – other labs may well have their own set of normal values.
The CBC tests help to inform the healthcare team about side effects of treatment and any secondary consequences of RA, such as anemia. If the patient’s hemoglobin levels indicate anemia, this will be further investigated looking for its cause.
A complete metabolic panel is used to monitor kidney and liver function, in order to assess whether changes to medication must be made or whether they are working well. A complete metabolic panel measures sodium (Na), potassium (K), chloride, glucose, creatinine (a measure of kidney function), and AST and ALT (markers of liver function).
Both the CBC and the complete metabolic panel are used to monitor disease activity as well as side effects and efficacy of medication.
With these laboratory tests as a guide, the healthcare provider may need to make adjustments to the patient’s medications and RA treatment. Laboratory tests provide important information in the diagnosis, management and treatment of rheumatoid arthritis. By becoming informed about the normal values for tests, as well as their own numbers, patients with RA can better communicate with the health care team and gain a better understanding about some of the information that is used in developing and monitoring their treatment plans.
Learn more about the HSS Early RA Support and Education Program, a free support and education group, developed specially for people recently diagnosed with RA and early RA.
Summary by Lysa Petrsoric, MPH, MSW, April 20, 2015
Edited by Nancy Novick.