A person who develops joint swelling and pain may undergo blood tests that can be used to either support or rule out a diagnosis of rheumatoid arthritis.
No single lab test can definitively diagnose the disease, but several lab tests can detect biological markers of RA—measurable characteristics of the blood that are associated with rheumatoid arthritis.
The most useful biological markers for the diagnosis of RA are called rheumatoid factor and anti-CCP. Tests for these as well as several other biological markers are described below.
Rheumatoid factor. Rheumatoid factor is an antibody (protein) found in the blood. An elevated level of rheumatoid factor along with symmetrical joint pain (pain in the same joints on both sides of the body) is an indicator for rheumatoid arthritis.
While a helpful tool, this test alone cannot confirm or eliminate a diagnosis of rheumatoid arthritis. In fact, research shows:
- As few as 30% of people with early rheumatoid arthritis test positive for rheumatoid factor1
- About 80% of people with established rheumatoid arthritis test positive for rheumatoid factor1
- A small number of healthy people have elevated levels of rheumatoid factor
Elevated rheumatoid factor may also indicate another rheumatic disease, such as Sjögren syndrome, or other medical conditions, such as chronic lung disease and hepatitis C.
Anti-CCP antibody test. According to the American College of Rheumatology, anti-CCP antibodies are found in 60% to 70% of people with rheumatoid arthritis.1 These antibodies are directed against cyclic citrulinated peptides (CCP) and can be present before rheumatoid arthritis symptoms ever develop. A positive anti-CCP test is considered quite predictive for having RA.
C-reactive protein (CRP). When there is inflammation in the body, the liver produces a protein called CRP. A high CRP level indicates an inflammatory condition, including infection. For this reason, a high CRP level is not necessarily a strong indicator of RA specifically but supports an RA diagnosis. In addition to using CRP lab test for diagnosis, doctors often use this test to monitor a patients’ response to treatment.
Erythrocyte sedimentation rate test (ESR). Similar to CRP, this test also screens for inflammation. The faster red blood cells (erythrocytes) fall to the bottom of a tube filled with the patient’s blood, the greater the degree of inflammation. Because inflammation is a factor in many conditions, this test is not specific for rheumatoid arthritis and by itself does not diagnose a patient, but can support the diagnosis. It is also used to see how active the condition is.
Anti-nuclear antibody (ANA). People with autoimmune disorders produce antinuclear antibodies (ANA) that attack healthy cells. ANA can be associated with rheumatoid arthritis but is more common in other autoimmune disorders, particularly systemic lupus erythematosus (SLE). The ANA lab test detects the presence of ANA in the blood.
The 14-3-3η (eta) protein. This is the newest blood test to be used in the diagnosis of rheumatoid arthritis. People with RA seem to have elevated levels of the 14-3-3η protein in their blood. While not considered a standard test, the 14-3-3η test may be used with other lab tests in the early diagnosis of RA.
In addition to some or all of these tests, a doctor may order a comprehensive metabolic panel to evaluate the patient’s kidney and liver function.