The distribution of class-specific rheumatoid factors is similar in rheumatoid and pre-illness sera.

The occurrence of IgM, IgG, and IgA class rheumatoid factors (RF) was compared by means of ELISA in 21 sera from patients with established rheumatoid arthritis (RA) and in 22 pre-illness sera from subjects who 4 months to 5 years later developed seropositive rheumatoid arthritis. The positive cases were virtually confined to three groups of about equal size: positive for all three RF classes, positive for IgM-RF and IgG-RF, and positive for IgM-RF only. Thus, the distribution of the RF isotypes was similar in rheumatoid and pre-rheumatoid sera.