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With interest we read the article by Hecht et al 1 who studied the associations of anticitrullinated peptide antibodies (ACPA) and rheumatoid factor (RF) with bone erosions in rheumatoid arthritis (RA). The background of this study was that ACPAs directed against citrullinated vimentin induce differentiation of osteoclasts, indicating that ACPA has a direct effect on bone resorption. 2 No biological data are available on whether RF also affects bone cells and, given the overlap between ACPA-positivity and RF-positivity, the authors wanted to distinguish the association between RF and bone erosions from the association between ACPA and bone erosions. In a crosssectional study, high-resolution peripheral quantitative CT (HR-pQCT) images of metacarpophalangeal (MCP) 2–4 joints of 238 patients with RA were made and groups of patients with different combinations of autoantibodies were compared. The authors …