OBJECTIVES: Retrospective database studies of PAH using US payer claims data have limitations due to lack of specific ICD-9 codes for PAH and ability to identify patient severity.  A previous study validated an algorithm including patients with non-specific PH codes and a claim for an advanced PAH drug therapy. This study used provider-reported disease severity – FC – to examine the impact of FC on healthcare resource utilization and cost to determine whether functional decline is associated with increased healthcare resource utilization and costs.  METHODS: Medicare and commercial patients who received an endothelin-receptor antagonist (ERA), phosphodiesterase type 5 inhibitor (PDE5) or prostacyclin (PG12) and reported a medical claim with an ICD-9-CM of 416.0, 416.8 or 416.9 or a medical claim indicating right heart catheterization (RHC) were identified from pharmacy and medical claims data from 2009 through 2013. The date of initial therapy served as the index date. Using provider-reported data from prior authorization forms required for prescribing advanced PAH therapies listed above, the WHO-FC was found for 437 patients (FCII=99; FCIII=282; FCIV=56). RESULTS: Linear models indicated that the FCIV cohort averaged significantly more inpatient admissions (Ps<0.001), longer lengths of stay (Ps<0.01) and more ER visits (Ps<0.01) than the FCII or FCIII cohorts. Costs analyses indicated that FCIV had significantly higher medical costs than FCII ($130,572/year vs $120,588/year) (P=0.002) and significantly higher total costs (pharmacy + medical) ($180,828/year vs $162,072/year) (P=0.004). CONCLUSIONS: PAH patients demonstrated a high level of utilization and costs. Disease severity, indicated by WHO-FC, is associated with higher utilization and costs for FCIV patients. Only small differences were found between FCII and FCIII. The FCIII cohort was twice as large as the FCII cohort. This suggests difficulty differentiating between these two FCs and potential bias in the sampling when relying on the prior authorization form for this clinical information.