Laboratory testing is not useful in diagnosing PAH, but is required to identify associated disorders like thyroid disease and iron deficiency as well as provide information on disease severity and end-organ damage (with circulating biomarkers such as brain-natriuretic peptide, N-terminal pro brain natriuretic peptide (NT-proBNP) [25,26] and cystatin C [27]). This evidence concerns the gene NPPB and pulmonary arterial hypertension.