Laboratory tests help identify, but do not confirm, RA

In most patients with rheumatoid arthritis (RA), the results of common laboratory testing-rheumatoid factor (RF), erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) level-are abnormal and may be used to support the diagnosis. However, a substantial proportion of patients with RA have normal test results.

Sokka and Pincus studied databases of 1892 Finnish and 478 American patients seen between 1980 and 2004. They analyzed the first recorded ESR, CRP, and RF.

The ESR at baseline was lower than 28 mm/h-a common clinical trial inclusion criterion-in 45% of the Finnish patients and 47% of the Americans; one-fourth of the Finns and almost one-third of the Americans had a normal ESR. CRP levels were normal (lower than 10 mg/L) in 44% of the Finns and 58% of the Americans. RF was negative in 38% and 37% of the Finns and Americans, respectively. Both the ESR and CRP level were normal in 44% and 58%, and all 3 test results were normal in 15% and 14% of the Finns and Americans, respectively.

The authors noted that the proportion of patients with normal laboratory test results probably is underestimated as a result of “spectrum bias” in referral patterns.