To the Editor:
Rheumatoid factor (RF) and anticitrullinated protein antibodies (ACPA) are important biomarkers in the diagnosis of rheumatoid arthritis (RA), but leave a gap of about 30–50% seronegative RA, which drives the demand for novel biomarkers. In 2011 a novel autoantibody system, anticarbamylated protein (anti-CarP) antibodies, was described in the sera of patients with RA1. In contrast to enzyme-mediated protein citrullination, carbamylation is a chemical reaction whereby cyanate converts lysine into homocitrulline1, a protein modification that is chemically similar to citrulline (1 CH2 longer side chain)2. Most studies on anti-CarP antibodies have used an ELISA based on carbamylated fetal calf serum (Ca-FCS) and accordingly a complex mixture of carbamylated proteins as the antigen(s)1,2. Although many clinical observations have been reported, precise information on the antigenic targets of anti-CarP antibodies is limited. Importantly, anti-CarP antibodies have been detected in both ACPA-positive and ACPA-negative RA patients, suggesting that they might represent an important test in the diagnosis of RA1,2. A recent metaanalysis estimated the sensitivity, specificity, and OR of …
Address correspondence to Dr. M. Mahler, Inova Diagnostics, 9900 Old Grove Road, San Diego, California 32131-1638, USA. E-mail: mmahler{at}inovadx.com or m.mahler.job{at}web.de.