Macrophage activation syndrome is characterized by an overwhelming inflammatory reaction driven by excessive expansion of T cells and hemophagocytic macrophages. Levels of soluble interleukin‐2 receptor α (sIL‐2Rα) and soluble CD163 (sCD163) may reflect the degree of activation and expansion of T cells and macrophages, respectively. This study was undertaken to assess the value of serum sIL‐2Rα and sCD163 in diagnosing acute macrophage activation syndrome complicating systemic juvenile idiopathic arthritis (JIA).

Enzyme‐linked immunosorbent assay was used to assess sIL‐2Rα and sCD163 levels in sera from 7 patients with acute macrophage activation syndrome complicating systemic JIA and 16 patients with untreated new‐onset systemic JIA. The results were correlated with clinical features of established macrophage activation syndrome, including ferritin levels.

The median level of sIL‐2Rα in the patients with macrophage activation syndrome was 19,646 pg/ml (interquartile range [IQR] 18,128), compared with 3,787 pg/ml (IQR 3,762) in patients with systemic JIA (P = 0.003). Similarly, the median level of sCD163 in patients with macrophage activation syndrome was 23,000 ng/ml (IQR 14,191), compared with 5,480 ng/ml (IQR 2,635) in patients with systemic JIA (P = 0.017). In 5 of 16 patients with systemic JIA, serum levels of sIL‐2Rα or sCD163 were comparable with those in patients with acute macrophage activation syndrome. These patients had high inflammatory activity associated with a trend toward lower hemoglobin levels (P = 0.11), lower platelet counts, and significantly higher ferritin levels (P = 0.02). Two of these 5 patients developed overt macrophage activation syndrome several months later.

Levels of sIL‐2Rα and sCD163 are promising diagnostic markers for macrophage activation syndrome. They may also help identify patients with subclinical macrophage activation syndrome.