Methods based on in vivo measurements of NPD (10, 25), combined ion channel measurements (11, 26), rectal CFTR-mediated Cl− secretion (intestinal combined ion channel measurements) (26), and the evaluation of β-adrenergic secretory response by evaporometry (27) as CF disease parameters may be useful for the diagnosis of patients with questionable QPIT results, CF diagnosis, and to predict patients prognosis. The gene discussed is CFTR; the disease is cystic fibrosis.