Abstract: The invention relates to a method of identifying a subject suspected of having or being susceptible to an autoimmune disease, such as rheumatoid arthritis (RA), comprising: contacting a sample of bodily fluid obtained from the subject with (i) a binding pair member having a binding affinity for citrullinated tenascin (cTNC) or a fragment thereof or (ii) a cTNC peptide; determining in a sample of bodily fluid obtained from the subject the presence or amount of (i) a citrullinated peptide derived from tenascin or (ii) an anti cTNC antibody; comparing the presence or amount of (i) the citrullinated peptide derived from tenascin or (ii) the anti cTNC antibody with a pre-defined threshold value; and assigning a diagnosis of RA or a future likelihood of developing RA when the presence or amount of (i) cTNC or (ii) an antibody against cTNC is detected or exceeds the threshold; and associated kits, peptides, binding members and uses thereof.