MPO and juvenile idiopathic arthritis: The serum concentration of HNL in the whole group of 69 children with JIA was significantly higher compared with that in healthy children, p < 0.001 but the corresponding analysis of HNL in plasma did not show any difference compared with healthy controls, Fig. 1A and B. The serum concentration of MPO in the whole group of children with JIA was significantly higher compared with that in healthy children, p = 0.002 (data not shown).