Clinical Manifestations of Adult‐Onset Still's Disease Presenting With Erosive Arthritis: Association With Low Levels of Ferritin and Interleukin‐18

Objective: Adult-onset Still's disease (AOSD) is a clinical entity with heterogeneous etiology. We have encountered patients with AOSD who had severe polyarthritis and who fulfilled the classification criteria for rheumatoid arthritis (RA); however, most patients with AOSD typically exhibit mild arthritis. In this study, we proposed two clinical subsets of AOSD and investigated the clinically significant characteristics of the two subtypes. Methods: We retrospectively analyzed 71 consecutive patients with AOSD. We reviewed the medical records of all patients who were followed up for more than 2 years. We classified all the patients with AOSD into the following 2 subsets: an RA subtype for patients who met the criteria for RA according to the American College of Rheumatology and a non-RA subtype for patients who did not meet the criteria for RA. Results: Our results indicated that the non-RA subtype was accompanied by severe inflammatory complications, including pleuritis and hemophagocytic syndrome. In addition, the serum ferritin and serum IL-18 levels were significantly higher in patients with the non-RA subtype than in those with the RA subtype. Interestingly, only 1 patient with the RA subtype had anti-CCP antibodies, and 1 non-RA subtype patient had rheumatoid factor. These findings distinguish these patients from patients with true RA. Conclusions: There were two subsets of patients with AOSD in the examined population. Patients with high levels of IL-18 or ferritin presented with severe systemic inflammatory disorders (the non-RA subtype), and patients with low levels of IL-18 or ferritin developed severe arthritis (RA subtype).