II. Indications
- Cystic Fibrosis Testing based on clinical suspicion
- Newborns (after age 48 hours)
- Newborn Screening positive for 2 CF mutation (confirmation)
- Newborn Screening positive for 1 CF mutation
- Ultra-high immunoreactive trypsinogen positive
- Presentations possibly consistent with CF (despite negative Newborn Screening)
- Poor weight gain, Failure to Thrive or Malnutrition
- Growth Delay
- Digital Clubbing
- Chronic respiratory symptoms
- Cough, Wheezing or Shortness of Breath
- Recurrent respiratory or sinus infections
- Nasal Polyposis in children
- Chronic gastrointestinal symptoms
- Steatorrhea or frequent bulky stools
III. Mechanism
- Sweat Glands in Cystic Fibrosis secrete a salty fluid with high chloride concentrations
IV. Technique
- Obtain Sweat Chloride test at Cystic Fibrosis Foundation accredited lab
- Minimum sweat amount required: 50 mg (200 mg better)
- Quantitative Pilocarpine Iontophoresis
- Stimulates local sweat production (parasympathetic)
- Collect sweat on gauze pads
- Analyze Sodium and chloride content
- Validity
- Difference between Sodium and chloride >30 meq/L suggests an invalid test
V. Labs: Interpretation
- Sweat Chloride >= 60 mmol/L (or meq/L)
- Consistent with Cystic Fibrosis (refer as if 2 CF mutations)
- Sweat Chloride <= 29 mmol/L (or meq/L)
- Cystic Fibrosis is unlikely
- Sweat Chloride 30 to 59 mmol/L (or meq/L)
- Intermediate for CF
- Consult with local CF center
- Consider repeat Sweat Chloride, Genetic Testing or evaluation at CF center
VI. Causes: False Positive Tests
- Glucose 6 Phosphate Dehydrogenase deficiency (G6PD)
- Hypothyroidism
- Glycogen Storage Disease
- Untreated Adrenal Insufficiency
- Malnutrition
VII. Reference
- Bakerman (1984) ABC's of Lab Data, ILD, Greenville, NC
- Collins (2024) Am Fam Physician 109(5): 388-90 [PubMed]