New lab tests have great promise in the care of children with juvenile idiopathic arthritis (JIA). This includes S100 proteins, IL-18, and CXCL9. However, results of these tests can differ due to the local procedures used at different centers. It is currently not known whether test results from different labs are comparable. This study will determine how levels of new JIA lab tests compare across different international rheumatology centers. We will first use samples “spiked” with known amounts of these lab markers to build conversion factors. We will then use samples from children with systemic JIA to see if these conversion factors work in the real world. These findings will be very significant to allow wider use of new lab tests in daily practice. These findings will also benefit future international studies on JIA.