What is the sensitivity of a positive sweat chloride test (> 60 mEq/L) ?

• 96 - 99% (Up to date Review)
• False negatives: Edema, malnutrition, dilution of sample/lab error
• Several papers show that atypical cases may have a negative sweat test
• Table 1 (Goodchild et al., in Cystic Fibrosis by Hodson and Geddes)

What is the specificity of a negative sweat chloride test (< 60 mEq/L) ?

Sweat chloride concentration in adults with pulmonary diseases.
Davis PB; Del Rio S; Muntz JA; Dieckman L
Am Rev Respir Dis  1983 Jul;128(1):34-7

Results:

• 187 subjects (None had CF: 166 had another pulmonary disease)

• 96% had a sweat chloride concentration less than 60 mEq/L

• 4% of non-CF patients had a positive sweat test > 60 mEq/L

• False Positives: evaporation, contamination, other diseases

What other diseases might have a sweat test > 60 mEq/L ?

What do you do if a sweat test is borderline ?

• Genetic testing for mutant CF transmembrane regulator (CFTR)
• Nasal potential difference measurement

What are the diagnostic criteria for CF ?