A study of IgM rheumatoid factors in a middle-aged population of Northern Norway.

A random selection of 8807 blood samples was analysed for the presence of IgM rheumatoid factor (RF). The sera were obtained from an epidemiological survey of rheumatoid arthritis in a middle-aged population of Northern-Norway. The prevalence of IgM-RF was 1.36% for the total population, but a prevalence of IgM RF in healthy individuals between 0.48-0.49% was found with no significant difference between males and females. The majority of RF positive sera were low titred. Only 11.4% of the persons possessing serum IgM RF suffered from classical or definite rheumatoid arthritis. Eighty-one per cent of the IgM RF positive individuals converted to seronegativity in the course of 3.5 years.