The gold standard for diagnosing the Cystic fibrosis is

In most cases, the diagnosis of cystic fibrosis is based on persistently elevated sweat electrolyte concentrations (often the mother makes the diagnosis by recognizing her infant's abnormally salty sweat), characteristic clinical findings (sinopulmonary disease and gastrointestinal manifestations), an abnormal newborn screening test, or a family history. A minority of patients with cystic fibrosis, especially those with at least one “mild” CFTR mutation, may have a normal or near-normal sweat test (<60 mM/L). Measurement of nasal transepithelial potential difference in vivo can be a useful adjunct test under these circumstances; individuals with cystic fibrosis demonstrate a significantly more negative baseline nasal potential difference than controls. Sequencing the CFTR gene is, of course, the “gold standard” for diagnosis of cystic fibrosis. Therefore, in patients with suggestive clinical findings or family history (or both), genetic analysis may be warranted.