PRTN3 and rheumatoid arthritis: In addition, the studies of preclinical RA samples have been discordant in the order of antibody appearance: IgA RF and IgM RF were the first antibodies in a Swedish cohort55, whereas ACPA4 and ACarPA56 preceded IgM RF in a Dutch cohort, and again IgG ACPA preceded RF (no ACarPA analysis included) in American military57.