Protein arginine deiminase 4 (peptidylarginine deiminase 4 (PAD4)), is a guanidino-modifying enzyme which catalyzes the conversion of specific arginine residues to citrulline in a calcium-dependent manner. PAD4 activity is increased in rheumatoid arthritis (RA), producing citrulline-containing proteins (CCP) that induce the formation of arthritogenic autoantibodies. In addition to anti-CCP autoantibodies, 23-45% of RA patients also produce autoantibodies specific for the PAD4 enzyme itself. Cayman’s PAD4 Autoantibody ELISA Kit is an immunometric assay which can be used to measure anti-PAD4 autoantibodies of any isotype (IgM, IgG, and IgA) in human plasma and serum without prior sample purification. Affinity-purified anti-PAD4 antibody isolated from the plasma of a patient with RA is used as the standard. One unit is approximately equal to 1 ng of anti-PAD4 Ig protein. The standard curve spans the range of 15.6-1,000 U/ml, with an LLOQ of 31.3 U/ml.