Background Anti-citrullinated peptide antibodies (ACPA) are an important serological marker in the diagnosis of rheumatoid arthritis (RA). A variety of different test systems are available for ACPA detection.
Objectives This study compares two ACPA assays in samples sent for routine testing in a clinic specialized in rheumatology. Special objective is to compare the two assays in Rheumatoid factor (RF) negative and early RA.
Methods 295 sera were collected from patients for whom a CCP test was ordered. Samples were tested for ACPA by QUANTA Lite® CCP 3 (INOVA Diagnostics, Inc., San Diego, CA, USA) and EliA® CCP (Phadia, Germany). RF was determined using turbidimetry and RF(II) (Biokit, Spain).
Results Good qualitative (96.6%, kappa= 0.93) and quantitative agreements (Spearman rho=0.77; p<0.0001) were observed between the two tests. Seven samples were CCP3+/CCP2- and one sample was CCP2+/CCP3-. Of the 9 CCP3+/CCP2- patients, 6 (66.7%) had RA, one patient had ankylosing spondylitis, one osteoarthritis and one psoriasis arthritis. The CCP3-/CCP2+ patient had juvenile RA. At the manufacturer’s cut-offs, the sensitivities and specificities were 77.3/98.1% (CCP2), 81.6%/96.8% (CCP3) and 65.2%/89.6% (RF), respectively. At a cut-off which yields 99% specificity, the sensitivities were 59.6% (CCP2) and 69.5% (CCP3). 49/141 (34.8%) RA patients were RF negative. The mean age and the average disease duration were 62.3 years (standard deviation, SD=12.0) and 8.5 years (SD=8.8), respectively. The sensitivities were 49.0% (CCP2) and 57.1% (CCP3) in the RF negative group and 38.7% (CCP2) and 51.6% (CCP3) in the RF negative and early RA (disease duration <5 years; n=31).
Conclusions A good overall agreement between CCP2 and CCP3 was observed. Discrimination between RA and non-RA cases was better using CCP3, especially in early and RF negative RA. Both ACPA assays outperformed RF. Further studies, preferrentially in a uniform multi-center design, are needed to confirm the higher sensitivity of CCP3 in early and RF negative RA.
Disclosure of Interest A. Swart: None Declared, G. Lakos Employee of: INOVA, J. Wu Employee of: INOVA, I. Gürtler: None Declared, M. Mahler Employee of: INOVA