A major strength of the article is the authors’ analyses of their own university hospital discharge records. They conducted a smaller, nested study to address their hypothesis that changes in billing codes may be the reason for the observed trends. They performed a sensitivity analysis of discharge records from the years 1999 and 2002, years when coding changes were introduced. In 1999, > 80% of patients (25/30) coded 416.0 (“primary or idiopathic”) were misclassified (12 with left-sided heart disease, nine with PAH, three with lung disease, and one with pulmonary emboli). Only two of the remaining five patients were confirmed to have IPAH. In 2002, the 416.8 code was more commonly used with patients (22/27). Among the three patients coded as 416.0, only one was properly classified. Some IPAH cases were misclassified as 416.8, and it appears that if the coder was in doubt of the diagnosis, 416.8 served as the default code. Changes in coding altered such apparent trends, confirming the authors’ assumptions.