Van Hoovels L, et al. - Experts assessed the technical and diagnostic performance of different rheumatoid factor (RF) and anti-cyclic citrullinated protein/peptide antibodies (ACPA) assays and estimated if differences in performance affected rheumatoid arthritis (RA) classification. They noted differences in analytical performance between assays. A poor numerical agreement between the different RF and ACPA assays was found. The likelihood ratio for RA increased with increasing antibody levels, for all assays. Findings suggested that poor harmonisation of RF and ACPA assays and test result interpretation could lead to varied RA classification according to 2010 American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) criteria when different assays were used.