7S IgG rheumatoid factor and hidden 19S IgM rheumatoid factor in juvenile chronic arthritis.

Hidden 19S IgM rheumatoid factor (RF) and 7S IgG RF were assayed in 24 children with juvenile chronic arthritis (JCA) and 26 controls, in whole sera and in the IgM and IgG containing fractions of the serum after separation by acid gel filtration. When non-fractioned serum was used, IgM RF detection was positive in 4.2% by latex fixation test and 4.2% by ELISA. IgG RF was positive in 91.7% by ELISA. In the IgM containing fraction of the serum, the positivity for IgM RF was 41.7% by LFT and 54.2% by ELISA. In the IgG containing fraction the positivity for IgG RF was 100%. These results suggest toward the need of previous serum dissociation before hidden 19S IgM RF is assayed and demonstrate high prevalence of 7S IgG RF.