Detection of autoantibodies against citrullinated protein in anti-cyclic citrullinated peptide 2-positive early arthritis sera by microarray surface plasmon resonance imaging. Sensorgrams for the binding of 20 anti-CCP2-positive early arthritis sera to four peptide sets are shown. The different citrullinated peptides are depicted in different shades of red, and the arginine control peptides are shown in different shades of green. The different colors the four different citrullinated peptides illustrate the data listed in Table 1. Widely reactive autoantibodies against citrullinated protein (ACPA)-positive serum from a rheumatoid arthritis patient (X) and a mixture of two monoclonal ACPAs (Y) were used as controls.