To identify measures distinguishing macrophage activation syndrome (MAS) in systemic juvenile idiopathic arthritis (sJIA) from familial hemophagocytic lymphohistiocytosis (FHL) and virus-associated hemophagocytic lymphohistiocytosis (VA-HLH) and to define appropriate cutoff values. To evaluate suggested dynamic measures differentiating MAS in patients with sJIA from sJIA flares.

In a cohort of patients referred for evaluation of hemophagocytic lymphohistiocytosis, we identified 27 patients with sJIA and MAS (MAS/sJIA) fulfilling the criteria of the proposed preliminary diagnostic guideline for the diagnosis of MAS in sJIA. Ten measures at diagnosis were compared between the MAS/sJIA group and 90 patients with FHL and 42 patients with VA-HLH, and cutoff values were determined. In addition, 5 measures were analyzed for significant change from before MAS until MAS diagnosis.

Neutrophil count and C-reactive protein were significantly higher in patients with MAS/sJIA compared with patients with FHL and patients with VA-HLH, with 1.8 × 10⁹/L neutrophils (sensitivity 85%, specificity 83%) and 90 mg/L C-reactive protein (74%, 89%) as cutoff values. Soluble CD25 <7900 U/L (79%, 76%) indicated MAS/sJIA rather than FHL/VA-HLH. Platelet (−59%) and white blood cell count (−46%) displayed a significant decrease, and neutrophil count (−35%) and fibrinogen (−28%) showed a trend during the development of MAS. However, a substantial portion of patients had values at diagnosis of MAS within or above the normal range for white blood cells (84%), neutrophils (77%), platelets (26%), and fibrinogen (71%).

Readily available measures can rapidly differentiate between MAS/sJIA and FHL/VA-HLH. The findings substantiate that a decline of measures may facilitate the distinction of MAS from flares of sJIA.