Longitudinal study of the presence of hidden 19S IgM rheumatoid factor in juvenile rheumatoid arthritis.
Complement-fixing hidden 19S IgM rheumatoid factor (RF), i.e., 19S IgM RF that can be detected in the IgM-containing fraction by the hemolytic assay after separation of the serum by acid gel filtration, was evaluated serially (3 or more evaluations) over a 4-year period in 26 children with juvenile rheumatoid arthritis (JRA) correlating its presence with disease activity. Six children with continually active disease had consistently elevated hidden RF titers (26 evaluations), and the titers of 4 children with inactive disease remained insignificant (12 evaluations). Sixteen children had disease with variable activity over this period. Of these, 11 (43 evaluations) demonstrated positive correlation between disease activity and hidden RF titers. Five patients (8/20 evaluations) did not show correlation of disease activity with hidden RF titers. Thus, the presence of hidden RF correlated with disease activity in 93 of 101 evaluations. When hidden RF titers and the erythrocyte sedimentation rate were determined simultaneously on 40 occasions, the presence of hidden 19S IgM RF correlated significantly better (p less than 0.001) with disease activity.