Disclosed herein are methods of diagnosing Rheumatoid arthritis in a subject comprising determining whether the subject is immunologically reactive with N-acetylglucosamine-6-sulfatase and/or filamin-A, wherein immunological reactivity of the subject to one or more of N-acetylglucosamine-6-sulfatase or filamin-A, as compared to an appropriate control, indicates the subject has rheumatoid arthritis. Examples of specific assays and kits for use with the methods are also disclosed.