Adult‐onset Still's disease (AOSD) is a clinical entity with a 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 2 clinical subsets of AOSD and investigated the clinically significant characteristics of the 2 subtypes.

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 of 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.

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 interleukin‐18 (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–cyclic citrullinated peptide antibodies and 1 patient with the non‐RA subtype had rheumatoid factor. These findings distinguish these patients from patients with true RA.

There were 2 subsets of patients with AOSD in the examined population. Patients with high levels of IL‐18 or ferritin presented with severe systemic inflammatory disorders (non‐RA subtype) and patients with low levels of IL‐18 or ferritin developed severe arthritis (RA subtype).