Laboratory tests

The diagnosis of RA can be made more certain by documenting the presence of synovitis--inflammation of the synovial membrane. Synovitis can be detected by withdrawing a small amount of fluid. Other causes of synovitis must still be considered, however. Although X-rays of affected joints are not useful during the early stages of RA, those taken more than six months after the onset of active disease can show the characteristic narrowing of the joint space and the bony erosions that point to a diagnosis of RA.

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A blood test to check for the presence of rheumatoid factor is also useful. However, this test is not definitive because elevated levels of rheumatoid factor can be found in people with other autoimmune diseases, as well as a number of unrelated disorders. Other common blood abnormalities indicative of RA are mild anemia, an elevated sedimentation rate (a nonspecific sign of inflammation), and a low white blood cell count.