IL18 and adult-onset Still disease: Although serum levels of free IL-18 have been shown to be elevated in AOSD and correlate with clinical and biological markers of disease activity [7, 31], we have not been able to accurately assess IL-18 activity because the assay used in this study cannot distinguish free IL-18 that is not bound to IL-18 BP (active) and IL-18 complexed with IL-18 BP (inactive).