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Epidemiological and genetic aspects of IgM rheumatoid factors.

The occurrence of IgM rheumatoid factors (RF) was studied in a random sample of 8807 persons aged between 20 and 50 years in Tromsø, North Norway. Seropositivity for IgM RF was defined as a Waaler titre of 40 or more. A total prevalence of IgM RF of 1.36% was found, and a prevalence between 0.48-0.94% was found among the healthy persons, with no sex difference. Approximately 50% of IgM Rf positives are thus healthy. Only 11% of those with IgM RF suffered from rheumatoid arthritis. The majority of RF positive sera from healthy persons were low titred, and 81% of them converted to seronegativity in the course of 3.5 years. A low titred IgM RF appears rather harmless, while a high titre indicates a specific disease process. No association between IgM RF and DR4 could be found in healthy persons. The frequencies of Gm-allotypes a, b, e, f-n and x in healthy, RF positive individuals did not differ from the RF positive patients with RA, suggesting that the Gm-allotypes are not involved in the genetic pre-disposition for RA.