We used an adaptation of an
enzyme-linked
immunoadsorbent assay (ELISA) to determine serum levels of
IgM,
IgG and
IgA rheumatoid factors (RF) in 50 patients with classic or definite
rheumatoid arthritis (RA) according to the ARA criteria, balanced for positive or negative-routine
Latex-RF reaction. A control group of 50 young normal subjects and a reference group of 44 patients with other
connective tissue diseases (
OCTD) were also studied. We confirmed the high sensibility of the method, together with its good specificity and reproducibility. For the
IgM RF a very significant correlation was found between ELISA results and
Latex-RF titration (p less than 0.001). Many
Latex-RF negative RA patients had high ELISA levels of
IgM RF, suggesting that this assay reveals, at least in part, hidden or non-agglutinating
IgM RF. Among the
OCTD group only some SLE cases, mainly
Latex-RF positive, had enhanced
IgM RF on ELISA. Considered quantitatively,
IgG RF did not play a significant diagnostic role for RA (p greater than 0.05), because they were also found, with widely dispersed values, in normal subjects, and because the mean increase in RA patients was relatively small. Interestingly,
IgA RF were above the normal range in many RA patients, both
Latex-RF positive or negative. The mean values differed significantly from those of controls (p less than 0.005), and a correlation was observed between
IgA RF levels and
IgA containing
immune-complexes. Normal
IgA RF values were observed in SLE patients, even if
Latex-RF positive, suggesting that their increase in RA patients is not the mere expression of a polyclonal B cell activation.