Second, there are legitimate reasons to withhold parenteral prostanoids from patients who meet guidelines for treatment with a parenteral prostanoid. Hay et al reported that parenteral prostanoids were not offered to patients with significant comorbid illnesses including metastatic cancer, concomitant mild or moderate parenchymal lung disease, end-stage renal disease, severe debility, complex congenital heart disease, and hypertrophic cardiomyopathy. Since patients with PAH often have comorbidities, clinicians and patients with PAH may benefit from studies that examine the efficacy and safety of parenteral prostanoids in patients with some of these comorbidities, to better inform the decision to withhold potentially life-saving therapy. Previous inconsistent compliance with medications or follow-up was another reason for withholding parenteral prostanoids, as was the inability to manage a parenteral prostanoid. For example, Tonelli et al withheld parenteral prostanoids from a patient with dementia and from a patient with frequent catheter-related infections. Because variation in practice is common, experienced clinicians who care for patients with PAH probably withhold parenteral prostanoids differentially. Thus, another opportunity exists to examine variations in the way that clinicians use parenteral prostanoids.