Utility of rheumatoid factor in the diagnosis of juvenile rheumatoid arthritis.

Rheumatoid factor is commonly used by clinicians to assess children with possible juvenile rheumatoid arthritis. To assess its usefulness, we reviewed the case histories of patients in whom latex agglutinating rheumatoid factor was determined during 1981 to 1982 at our institution. A total of 437 charts were available for review. There were 11 patients with positive tests for rheumatoid factor, of whom five had juvenile rheumatoid arthritis, all polyarticular. A total of 426 children had negative results, of whom 100 had juvenile rheumatoid arthritis. This yields a sensitivity of 4.8% and a specificity of 98%. We then estimated the prevalence of juvenile rheumatoid arthritis in three clinical settings: a primary practitioner's office, a tertiary children's hospital walk-in clinic, and a pediatric rheumatology center. The predictive values and marginal benefits for rheumatoid factor were then calculated in those settings using Bayes' theorem. In the two general outpatient settings, the primary practitioner's office and tertiary walk-in clinic, the positive predictive values were 0.7% and 0.5%, respectively; marginal benefits were 0.4% and 0.3%, respectively. Rheumatoid factor testing appeared to be of some benefit in the pediatric rheumatology center with a positive predictive value of 72.5% and marginal benefit of 22.5%. In no case was rheumatoid factor testing helpful in establishing a diagnosis of juvenile rheumatoid arthritis or in ruling it out. Testing for rheumatoid factor is a poor screening procedure for juvenile rheumatoid arthritis in the general situations in which it is more likely to be requested and of supportive diagnostic value only in the highly restricted population of older children with polyarticular arthritis.