Review Question - QID 100806

The clinical picture and family history is suspicious for a diagnosis of cystic fibrosis (CF). When a sweat test is negative, but clinical suspicion for CF is high, a useful diagnostic adjunct is a nasal transepithelial potential difference.

The nasal transespithelial potential difference measures the voltage difference across the nasal epithelium, comparing the difference between charges on the respiratory epithelial surface and the interstitial fluid. In individuals affected with CF, this measurement is significantly more negative than normal, due to increased luminal sodium absorption.

Holmes et al. review an algorithm for the evaluation of chronic cough and emphasize the need to eliminate congenital conditions, cystic fibrosis, and immune disorders as possible diagnoses in children with chronic cough and recurrent infection.

Taylor et al. discuss the physiologic measurement of the sweat test and transespithelial potential difference. The authors explain how the nasal transepithelial potential difference is useful in the diagnosis of cystic fibrosis patients with mild presentation and a normal sweat test.

Illustration A shows the transepithelial potential difference (Vt) in normal (A) and cystic fibrosis (CF; B) nasal epithelia.

Incorrect Answers:
Answer 2: Chest radiograph in patients with CF may show persistent abnormalities such as bronchiectasis, atelectasis, infiltrates, and hyperinflation. However, these findings are all non-specific and may not be present early in the disease or in mild cases.
Answer 3-5: Skeletal surveys, CBCs, and urine studies are not useful in the diagnosis of CF.