[Measurement and comparison of the rheumatoid factors class IgG, IgM and IgA in chronic juvenile rheumatoid arthritis].

The rheumatoid factor (RF) was studied in 35 sera from 23 children with juvenile rheumatoid arthritis (JRA). The immunoglobulin class of RF was investigated and also its reactivity to both human and rabbit IgG. The RF of IgG class (IgG-RF) was more frequently positive than the IgM-RF and the IgA-RF. Nevertheless, against human IgG we found IgM-RF in the 51% of sera and IgA-RF in 48%, and against rabbit IgG in 65 and 37% respectively. All classes of RF were more frequent in rheumatoid patients than in normal controls (p less than 0.0005) although the IgA-RF increase was not significant in some groups. The specificity of the 5 RF types was always very high (92-100%). The sensibility ranged between 71% (IgG-RF against rabbit IgG) and the 37% (IgA-RF against rabbit IgG). Most sera simultaneously contained more than one class of RF. Against human IgG, the 37% had 2 classes. When we used rabbit IgG, the 31% had 3 classes and the 62% had 2 classes. The correlation of every RF class each other generally was very high (p less than 0.001). A correlation was also present in the seronegative and the systemic group, when we separately studied every clinic form. The ELISA allows detect positive IgG-RF and IgA-RF in seronegative cases by agglutination tests, therefore the seronegative concept must be reconsidered. The correlation among different RF classes are, frequently, very closed.