RA diagnostic marker of negative ACPA and application of RA diagnostic marker

The invention discloses an RA (rheumatoid arthritis) diagnostic marker of a negative ACPA (anti-cyclic citrullinated peptide antibody) and application of the RA diagnostic marker and provides application of serine/threonine kinase 3, namely STK3, or a segment thereof to preparation of a reagent used for diagnosing an RA disease with the negative ACPA. The screened autoantigen STK3 is used as the RA diagnostic marker, wherein the specificity of the RA diagnostic marker reaches 89.45%, the sensitivity reaches 20.69%, and the AUC can reach 0.6006; and the expression and detectability of the autoantibody anti-STK3 in an RA patient with the negative ACPA are further confirmed by use of Western blot detection, and the clinical conversion potential of the detection marker is prompted.