Hidden 19S IgM rheumatoid factor in synovial fluid.

Synovial fluid (SF) of 16 adult patients with rheumatoid arthritis (RA) and 9 children with juvenile rheumatoid arthritis (JRA) was investigated for the presence of hidden 19S IgM rheumatoid factor (RF). SF was incubated with 500 IU hyaluronidase/g of SF, dialyzed against acetate buffers at decreasing pH, centrifuged and subjected to chromatography on a Sephadex G-200 column. The resulting IgM-containing fraction of SF from 5 seropositive patients (3 adult RA, 2 JRA patients) demonstrated RF activity by the complement-dependent hemolytic assay. Hidden 19S IgM RF was not found in the IgM-containing fractions of SF from 13 adult seronegative RA patients. Hidden 19S IgM RF was shown in the IgM-containing fraction of SF in 4 of 7 children with seronegative JRA.