Erosions in inflammatory polyarthritis are symmetrical regardless of rheumatoid factor status: results from a primary care‐based inception cohort of patients

Background and aims. Symmetry is considered an important criterion for the differentiation of rheumatoid arthritis (RA) from other forms of inflammatory polyarthritis (IP), particularly those that are seronegative. Because of the inclusion of symmetry in the diagnostic and classification process, however, its true occurrence in RA cannot be assessed. As a surrogate, peripheral inflammatory arthropathies associated with rheumatoid factor production may be more likely to be symmetrical. We examined the degree of symmetry of erosions in an unselected cohort of patients with IP and tested the hypothesis that the presence of rheumatoid factor (RF) is associated with greater symmetry. Methods. All patients registered with The Norfolk Arthritis Register (NOAR; a UK primary-care based cohort of patients with IP with annual follow-up) and who had radiographs performed at the fifth anniversary from notification were included in the analysis. Radiographs of the hands and feet were read using the Larsen method; a score of 2 or more in any particular joint indicated an erosion. Log-linear modelling was used to determine the symmetry of erosions between right and left for the following joint groups: wrists, metacarpophalangeal joints, proximal interphalangeal joints and metatarsophalangeal joints. Log-linear modelling was also used to determine the influence of RF on symmetry. Results. Five hundred and thirty-seven patients contributed to the analysis. The median time to performing radiographs was 69 months (interquartile range 65.5 ‐74.8) from the onset of symptoms. A total of 212 (39%) patients had erosive disease. Overall, IP was found to be a symmetrical disease. Despite there being more erosions in RF-positive patients, there was no greater excess of symmetry in RF-positive compared with RF-negative patients. Conclusion. Radiographically, IP is a symmetrical disease irrespective of RF status. The use of symmetry as an important feature in identifying subgroups of patients with IP, such as RA, is challenged.