IgM rheumatoid factor in Lyme disease: correlation with disease activity, total serum IgM, and IgM antibody to Borrelia burgdorferi.

We tested the sera of 50 patients with Lyme disease for IgM-rheumatoid factor (IgM-RF) using a sensitive ELISA. Levels of IgM-RF greater than 3 SD above the mean of normal subjects were found in 2 of 15 patients with erythema chronicum migrans, 7 of 10 with neurologic abnormalities, and 7 of 25 with Lyme arthritis (p = 0.038). Only 2 of these sera were positive by latex agglutination. In contrast, none of the 23 control patients with osteoarthritis, ankylosing spondylitis, or Reiter's syndrome had positive tests. The levels of IgM-RF correlated with disease activity (p = 0.002), total serum IgM levels (p = 0.002), and specific IgM antibody titers to Borrelia burgdorferi (p = 0.006). IgM-RF reactivity was absorbed with heat aggregated IgG (HAGG), but the titer of specific IgM antibody was insignificantly affected by this procedure. Thus, small amounts of RF are produced at certain times in many patients with Lyme disease, and IgM-RF production appears to be linked to the specific IgM response.