Based on a 187-plex antibody-based assay, 61 and 55 proteins were differentially expressed in pSS and RA, compared to HC. All proteins were upregulated in pSS patients, except FGF-4. Based on 4-plex and 6-plex biomarker signatures, which both included IL-4, IL-5 and clusterin, achieved accurate prediction of an individual’s group membership for at least 94% of cases. Here, FGF4 is linked to rheumatoid arthritis.