II. Indications

  1. Cystic Fibrosis Testing
  2. Newborn Screening positive for 2 CF mutation (confirmation)
  3. Newborn Screening positive for 1 CF mutation
  4. Ultra-high immunoreactive trypsinogen positive
  5. Presentations possibly consistent with CF (despite negative Newborn Screening)
    1. Poor weight gain, Failure to Thrive or Malnutrition
    2. Growth Delay
    3. Digital Clubbing
    4. Chronic respiratory symptoms
      1. Cough, Wheezing or Shortness of Breath
      2. Recurrent respiratory or sinus infections
      3. Nasal Polyposis in children
    5. Chronic gastrointestinal symptoms
      1. Steatorrhea or frequent bulky stools

III. Technique

  1. Obtain Sweat Chloride test at Cystic Fibrosis Foundation accredited lab
  2. Minimum sweat amount required: 50 mg (200 mg better)
  3. Quantitative Pilocarpine Iontophoresis
    1. Stimulates local sweat production (parasympathetic)
    2. Collect sweat on gauze pads
    3. Analyze Sodium and chloride content
  4. Validity
    1. Difference between Sodium and chloride >30 meq/L suggests an invalid test

IV. Labs: Interpretation

  1. Sweat Chloride >= 60 mmol/L (or meq/L)
    1. Consistent with Cystic Fibrosis (refer as if 2 CF mutations)
  2. Sweat Chloride <= 29 mmol/L (or meq/L)
    1. Cystic Fibrosis is unlikely
  3. Sweat Chloride 30 to 59 mmol/L (or meq/L)
    1. Intermediate for CF
    2. Consult with local CF center
    3. Consider repeat Sweat Chloride, Genetic Testing or evaluation at CF center

VI. Reference

  1. Bakerman (1984) ABC's of Lab Data, ILD, Greenville, NC
  2. Collins (2024) Am Fam Physician 109(5): 388-90 [PubMed]

Images: Related links to external sites (from Bing)

Related Studies