Diagnostic value of anti-cyclic citrullinated peptide antibodies in Greek patients with rheumatoid arthritis

In recent years it has become clear that early aggressive treatment in rheumatoid arthritis (RA) reduces joint damage and improves function [1]. To use a potentially toxic therapy as early as possible we require an accurate diagnosis of RA and also information about prognosis in an individual patient. Today, the diagnosis of RA depends mainly on clinical criteria that may take years to fulfill. Apart from clinical features, autoantibodies contribute to the diagnosis of various autoimmune diseases. In RA, rheumatoid factor (RF) has a fair sensitivity but a low specificity, since it is present in other rheumatic diseases, in infections, and in healthy people, especially the elderly [2].

In recent years, the introduction of serum antibodies against citrulline-containing molecules showed some promise as diagnostic tools in RA. Citrulline can be formed by posttranslational enzymatic conversion of arginine residues, catalyzed by peptidylarginine deiminase enzymes. Citrullinated molecules, the targets of these antibodies, include filaggrin, keratin, fibrin, and vimentin [3]. These antibodies are detected by ELISA, where a synthetic cyclic citrullinated peptide (CCP) is used as substrate. After the first generation of anti-CCP test (CCP1) [4, 5] a second generation of anti-CCP test (CCP2) has been introduced. The sensitivity of anti-CCP2 test in various populations ranges between 64% and 74% whereas the specificity ranges between 90% and 99% [6‐11]. A correlation of anti-CCP antibodies with radiographic joint damage has also been reported [12‐15].

In Northern European Caucasian populations, RA has been found to be associated with the HLA-DRB1 shared epitope. In these populations, anti-CCP antibody production is associated with the HLA-DRB1 shared epitope [14]. However, the shared epitope has been detected in a minority of Greek patients with RA [16]. Therefore, we studied the diagnostic and prognostic value of anti-CCP antibodies in Greek patients with RA.

The first study on anti-CCP antibodies (CCP1 test) reported a sensitivity of 68% and specificity of 98% for RA [4], whereas the same investigators when analyzed patients from other centers found sensitivity 45–80% and specificity 96–100% [4]. Other studies utilized CCP2 kits found sensitivity 64–74% and confirmed the high specificity of the test (90–99%) [6‐12]. Anti-CCP antibodies (CCP1 test) have been used successfully to diagnose persisted arthritis as compared to self-limited arthritis [5], or to differentiate RA (CCP2 test) from early undifferentiated arthritis [21]. Our findings of sensitivity of 63.2% and specificity of 95% for anti-CCP antibodies in RA are in agreement with these studies. The different frequencies of anti-CCP antibodies in various RA patient cohorts can be explained as follows: anti-CCP antibodies are directed against different epitopes in citrulline-containing molecules and sera from individual patients may contain different subsets of anti-CCP antibodies [22, 23]. As observed with other autoantibodies in autoimmune diseases, the production of anti-CCP antibodies in RA is influenced by HLA alleles. Serum anti-CCP antibody levels were higher in RA patients with the shared HLA-DRB1 (SE) epitope than in RA patients lacking the SE epitope [14]. Finally, RA treatment may decrease serum anti-CCP antibody levels. Anti-TNFα treatment has been found to decrease serum anti-CCP antibody levels [24]. The high specificity of anti-CCP antibodies is particularly useful in RF-negative RA patients. In our study, the frequency of anti-CCP antibodies in RF-negative RA patients was 34.9%, but this has been reported up to 40% [9].

Nearly a third of anti-CCP-negative RA patients (28.1%) were RF-positive. The relatively low percentage of RF in our RA group may be explained by the fact that our RA patients encompass the full range of RA severity/activity because of the Health system in Greece: Our Hospital functions as a primary as well as secondary and tertiary center. In other countries, where University Hospitals may only accept referrals from general practitioners, RA patients may include the more severe cases.

In agreement with another study [9], our study found an association of anti-CCP antibodies with disease activity as defined by swollen joint count and increased CRP concentration. Our study is a cross-sectional study and has not followed RA patients over years to address the significance of anti-CCP antibodies in joint damage. Nevertheless, our study demonstrated a correlation between anti-CCP antibodies and radiological joint score in patients with RA. Similarly, anti-CCP antibodies alone or together with the HLA-DRB1 shared epitope have been found to be associated with radiological joint damage [12]. In longitudinal studies, anti-CCP antibodies alone [15] or together with the HLA-DRB1 shared epitope [14] have been associated with faster radiological joint progression. In multiple regression analysis, anti-CCP1-positivity [25] and anti-CCP2-positivity [13] have been found to be predictor of radiological joint outcome. Taken together these findings suggest that anti-CCP antibodies are useful in predicting a severe disease course in RA.

The moderate sensitivity and high specificity of anti-CCP antibodies for RA, along with the appearance of anti-CCP antibodies before disease onset [26, 27], suggest that anti-CCP antibodies be included in the classification criteria for RA. Furthermore, the correlation of anti-CCP antibodies with radiographic joint damage in RA [13‐15, 25] provides additional aid for clinicians in deciding early aggressive treatment for a RA patient who is likely to have a severe disease course.

Anti-CCP2 antibodies had a better diagnostic value than RF for RA in Greeks. A considerable proportion (34.9%) of RF-negative RA patients were anti-CCP-positive. Furthermore, RA patients with anti-CCP antibodies were more likely to have more radiographic joint damage than patients without anti-CCP antibodies. These results confirm results in other populations and suggest that anti-CCP2 test is useful in everyday rheumatology practice.