Abstract: The present invention provides a method of diagnosing rheumatoid arthritis (RA), comprising: testing a sample from a subject for the presence or absence of antibodies against oxidised collagen II; wherein the presence of antibodies against oxidised collagen II in the sample is indicative of RA in the subject. The present invention also provides a method for identifying whether a subject responds to a disease modifying anti-rheumatic drug (DMARD) and related methods of treating RA, a method for identifying whether a subject responds to an anti-TNF biologic and related methods of treating RA, and a method of diagnosing osteoarthritis (OA).