We recently developed a sensitive XOR assay for the measurement of human plasma XOR activity by utilizing [13C2,15N2]xanthine coupled with LC–TQMS.[18,19] Via this novel method, Otaki et al. recently showed that patients with congestive heart failure who had high plasma XOR activity had significantly lower LVEF, and that those with low, or high XOR activity had increased cardiovascular events and reduced survival rate [26]. This evidence concerns the gene XDH and congestive heart failure.