Fab-linked glycans as biomarker for the transition from a pre-disease “at-risk-phase” to rheumatoid arthritis; AAV or Sjögren syndrome

A method of determining whether an antibody sample comprises an anti-modified protein antibody (AMPA), the method comprising:

• contacting antibodies of the antibody sample with a peptide or protein that comprises a modified protein epitope, wherein the modified protein epitope comprises citrulline, homo-citrulline, or acetylated lysine;
• contacting antibodies of the antibody sample with a molecule that binds an N-linked glycan on a Fab-portion of an antibody;
• and
• determining whether an antibody with an N-linked glycan on a Fab-portion of the antibody has bound to the modified protein epitope in the peptide or protein so as to determine whether the antibody sample comprises AMPA.