LAUSR

We thank Chi et al 1 for their interest in our diagnostic score for macrophage activation syndrome (MAS) in systemic juvenile idiopathic arthritis (sJIA).2 Chi et al argue that the collection of MAS patient data at the onset of the syndrome and of sJIA patient data not only at the onset of the illness but also during a flare could have caused a selection bias. However, the primary purpose of our analysis was to scrutinise the ability of clinical and laboratory features to discriminate between MAS and active sJIA without MAS. Because the main sJIA manifestations at disease onset or at the time of a flare with ongoing systemic features are clinically similar, we thought that both time points were equally suitable as controls. Note that only 22% of MAS episodes observed in our series developed …