XDH and hydrops fetalis: Although several studies and meta-analyses have identified elevations of serum UA as an independent marker of poor prognosis in HF patients (Jankowska et al., 2007; Tamariz et al., 2011) and pre-clinical (Stull et al., 2004) and case-control studies (Gotsman et al., 2012), as well as the later OPT-HF study (Hare et al., 2008), showed that XO inhibition was associated with improved survival in HF, the EXACT-HF trial failed to demonstrate a clinical benefit of high-dose allopurinol in HFrEF patients with increased UA concentrations.