NPPB and pulmonary arterial hypertension: <h4>Background</h4>Current guidelines recommend a four-strata model based on World Health Organization Functional Class (WHO FC), 6-min walk distance (6MWD) and serum levels of brain natriuretic peptide (BNP) or N-terminal pro-BNP (NT-proBNP) for risk stratification in patients with pulmonary arterial hypertension (PAH) during follow-up.