How useful is the rheumatoid factor? An analysis of sensitivity, specificity, and predictive value

The rheumatoid factor (RF) is frequently ordered in an effort to detect disease, yet its diagnostic utility has not been thoroughly examined. To determine the test's sensitivity, specificity, positive predictive value, and negative predictive value, we analyzed tests ordered in our institution.

We performed a retrospective analysis of all 86 patients with a positive RF over a 6-month period identified consecutively soon after the test was ordered. A similar analysis was applied to 86 seronegative patients selected at random from a total seronegative population of 477 during the same period. The patients represented the primary care and subspecialty practices and inpatient wards of a 504-bed university teaching hospital.

A positive RF result was strongly associated with rheumatoid arthritis or another rheumatic disease. For rheumatoid arthritis, sensitivity = 0.28 and specificity = 0.87, while for any rheumatic disease, sensitivity = 0.29 and specificity = 0.88. The positive predictive values for rheumatoid arthritis and any rheumatic disease were 0.24 and 0.34, respectively, and the negative predictive values were 0.89 and 0.85, respectively. Seropositive patients were slightly older (55 vs 49 years old), but the incidence of false-positive RFs among the elderly (69%) was not significantly higher than among younger patients (65%). The cost per true-positive RF result was $563.

In this study, most positive RF results were not helpful since the majority represented false-positive results. The low positive predictive value of the RF casts doubt on the utility of the RF in the diagnostic evaluation of patients. Contrary to traditional clinical expectations, the diagnostic utility of the RF may be greatest when it is negative. However, the subset of patients with seronegative rheumatic disease reduces the test's power to exclude such disorders even when the RF is negative. Given the test's limitations, clinicians should reconsider their expectations when ordering an RF. The utility of the RF may improve if it is ordered more selectively.