Arthritis Test Results
Erythrocyte Sedimentation Rate (ESR, Sed rate)
This blood test indicates the degree of inflammation in the body. In people with inflammatory diseases such as rheumatoid arthritis, lupus, and polymyalgia rheumatica, the sed rate may be elevated.
Limitations: While a high sed rate indicates inflammation, it does not identify the source of inflammation and it can result from other causes. Therefore, a positive test result is not useful in making a specific diagnosis. However, because sed rate may rise and fall with the activity of inflammatory disease, doctors may use the test to monitor the disease and its response to treatment.
C-Reactive Protein (CRP)
This blood test measures the levels of another marker of inflammation, called C-reactive protein. Levels of the protein rise when there is inflammation in the body. A high CRP could indicate an inflammatory disease such as rheumatoid arthritis, lupus, or vasculitis. Many doctors consider it to be a better test than the sed rate for measuring inflammation. A high CRP in a patient with rheumatoid arthritis suggests that there is significant inflammation in the body. Like the sed rate, CRP levels rise and fall with activity of inflammatory disease, so CRP testing may be useful to monitor disease activity and how well a patient is responding to treatment.
Limitations: Like the sed rate, the CRP test only tells you that there is inflammation. It does not tell you what is causing the inflammation. There could be many causes of a high level, including infections and tumors. Even obesity can increase CRP levels, so a doctor should consider a patient's body mass index when using CRP levels to monitor disease activity in rheumatoid arthritis. Furthermore, a low CRP level doesn't always mean that inflammation is not present. CRP levels are not always increased in people with RA or lupus.
Joint Fluid Tests
The blood is not the only bodily fluid that holds clues to the type of arthritis you have. Sometimes doctors order tests to analyze samples of joint fluid, also called synovial fluid. Doctors draw joint fluid from the joint space with a needle; in a person with arthritis, the presence of bacteria suggests that joint inflammation is caused by bacteria. The presence of crystals can confirm a diagnosis of gout, a condition in which uric acid, a waste product, deposits as crystals in the joint tissue, causing intense pain and inflammation. Other types of crystals may indicate different diagnoses.
Limitations: Although joint fluid tests may help rule out a diagnosis of rheumatoid arthritis (for example, if a diagnosis of gout is confirmed), they cannot confirm an RA diagnosis. Analyzing the joint fluid may eventually be helpful in detecting markers of joint destruction in rheumatoid arthritis, but at present this is investigational and not widely done.