Anti–cyclic citrullinated peptide testing for the diagnosis of rheumatoid arthritis and the quest for improved sensitivity and predictive value

Abstract

In this issue of Arthritis & Rheumatism, van der Linden and colleagues (1) present comparative studies of different anti–cyclic citrullinated peptide (anti-CCP) tests in patients from the Leiden Early Arthritis Clinic. These studies are important for selecting the most clinically useful anti-CCP tests and for better understanding the role of autoantibodies to citrullinated epitopes in the pathogenesis of rheumatoid arthritis (RA). The use of the term anti-CCP to introduce this topic reminds us of the important and dominant role that this term has taken on in our rheumatology lexicon. The term anti-CCP is often used to refer to not only antibodies against CCP epitopes but also antibodies directed to noncyclic citrullinated protein (or peptide) epitopes (ACPAs). For most practicing rheumatologists, placing an order for an anti-CCP test usually means that results will be generated using a so-called second-generation antiCCP test (anti–CCP-2) that employs a proprietary citrullinated cyclic peptide. The patent for the anti–CCP-2 test is held by the Netherlands Technology Foundation, and exclusive global rights to the proprietary CCP or peptides used in all anti–CCP-2 tests are held by AxisShield (Dundee, UK) and Euro-Diagnostica (Arnhem, The Netherlands). The anti–CCP-2 test is somewhat unusual among laboratory tests, because its derivation was based on empiricism and molding of the assay to achieve an RA diagnostic test with the highest sensitivity and specificity. The development of the anti-CCP test has been somewhat contrary to our usual reductionist tendencies for the establishment of other clinical tests, in which specificity typically requires pathophysiologic knowledge of the disease process and a focus on an explicit antigen or other marker of the disease process. To understand how the anti-CCP test was developed, it is valuable to understand its origins. The antiCCP test had its origins in studies performed more than 40 years ago, when serum autoantibodies that bound to perinuclear structures and to keratin were identified in RA patients (2,3). In 1998, Schellekens and colleagues identified citrulline as an important component of epitopes for RA autoantibodies that react with tissue fillagrin (4). This same group of investigators developed a first-generation anti-CCP test (anti–CCP-1) using a cyclic derivative of a citrullinated fillagrin peptide (5). In contrast to linear peptides, cyclic peptides are more conformationally constrained, and this was the rationale for using cyclic derivatives of citrullinated peptides in the original anti–CCP-1 assay to improve the sensitivity of the assay. The first-generation anti–CCP-1 test had a sensitivity for RA that was lower than the sensitivity of the rheumatoid factor (RF) test but had greater specificity than the RF test (5,6). In 2002, a second-generation anti-CCP test (anti– CCP-2) was developed by screening CCPs using RA patient sera and selecting the cyclic peptides with the highest sensitivity and specificity for a diagnosis of RA. The anti–CCP-2 test retains the high specificity (95%) of the anti–CCP-1 test and has a higher sensitivity than the anti–CCP-1 test, with a sensitivity for RA that is