GDF15 and idiopathic pulmonary arterial hypertension: An inverse correlation was observed between age-adjusted GDF15 and 6-minute walk distance (6MWD) in the combined PVOD, IPAH, and HPAH cohort, indicating an association with disease severity (Figure 1G); however, no such correlation was observed in the PVOD-alone cohort (Supplemental Figure 2B).