Background Adult-onset Still’s disease (AOSD) is an acute inflammatory disorder of unknown origin that characterized by high spiking fever, polyarthralgia, a salmon-pink skin rash, liver dysfunction and lymphoadenopathy. It is well known that a chronic and erosive arthritis is observed in a number of patients with AOSD, which sometimes may be resistant to a conventional steroid therapy.

Objectives In the present study, we aimed to determine clinical features of chronic and erosive arthritis in patients with AOSD, and to establish a biomarker distinguishing them from all of patients with AOSD.

Methods Seventy-three patients with AOSD who were treated in Institute of Rheumatology, Tokyo Women’s Medical University enrolled in this study. The patients group consisted of 26 men and 47 women. We classified the patients with AOSD into 2 groups; rheumatoid arthritis (RA)-subtype (n = 16) who met the revised criteria of American College of Rheumatology clinical diagnostic criteria for RA and nonRA-subtype (n = 57) who didn’t met it.

Results Our results indicated that serum levels of ferritin and IL-18 were extremely high in patients with AOSD. In particular, serum levels of them were significantly higher in nonRA-subtype than those in RA-subtype, suggesting these biomarkers could be useful for predicting severe arthritis. To define the optimal cut-off point with the highest diagnostic accuracy, we performed ROC for distinct serum ferritin and IL-18 concentrations. The two subtypes were classified by serum ferritin of 1500ng/ml and serum IL-18 of 44 ng/ml. The autoantibodies including ANA, RF and anti-CCP antibody were not detected in most patients with either nonRA-subtype and RA-subtype. Wrist, knee, MCP, MTP joint arthralgia were more frequently observed and cervical spine, hip, DIP joint arthralgia and radiographic findings were more frequently observed than RA. Ankylosing carpal arthritis was observed in 8 patients of RA-subtype. Patients with ankylosing carpal arthritis were accompanied with organized pneumonia, and thrombotic-thrombocytopenic-purpura and two patients in them received the operation of joints.

Conclusions Our observations suggest that ferritin and IL-18 may be the useful biomarkers for predicting severe arthritis such like RA. The patterns of arthritis in patients with RA-subtype AOSD are different from those of true RA, and few patients with RA-subtype AOSD have rheumatoid factor or anti-CCP antibody. These findings strongly suppose that the etiology of AOSD with severe arthritis is distinct from true RA.

Disclosure of Interest None Declared