i) clinical features and tests to support a diagnosis of PAH: the diagnosis of PAH requires right heart catheterization (RHC) to confirm a hemodynamic profile compatible with PAH. This includes a mean pulmonary artery pressure (PAP) ≥ than 25 mmHg at rest, with a pulmonary capillary wedge pressure < 15 mmHg. (If slightly elevated, will confirm with LVEDP measure as is our usual standard of care) and pulmonary vascular resistance (PVR) of > 3 Wood units. In addition, there should be no features to suggest other associations for PAH (also included in Group 1) or evidence to suggest PAH owing to left heart disease (Group 2), PH due to lung diseases (Group 3), Chronic thromboembolic pulmonary hypertension (Group 4) or miscellaneous disorders of unclear mechanism ii) clinical features and tests to support a specific designation of each subset of PAH: