What is involved in testing for cystic fibrosis?

Testing for Cystic Fibrosis

Cystic fibrosis diagnosis requires a multistep approach beginning with sweat chloride testing (the gold standard), followed by CFTR genetic analysis, and clinical evaluation—with a sweat chloride ≥60 mmol/L being diagnostic when accompanied by clinical features, positive family history, or positive newborn screening. 1

Newborn Screening (Initial Detection)

• All newborns in screening programs undergo immunoreactive trypsinogen (IRT) testing via heel prick in the first days after birth, measuring a pancreatic enzyme in dried blood spots 2
• Elevated IRT is NOT diagnostic—it can be elevated due to prematurity, stressful delivery, or carrier status, and must be followed by confirmatory testing 2, 1
• Screening algorithms vary by state/country, using either:
  • IRT/repeat IRT protocols: Second blood spot at ~2 weeks of age, with referral for sweat testing if both elevated 2
  • IRT/DNA protocols: Single elevated IRT followed by DNA mutation analysis on the same blood spot 2
• Newborn screening detects 90-95% of CF cases without meconium ileus, with IRT/DNA protocols achieving >98% sensitivity 2

Sweat Chloride Testing (Diagnostic Gold Standard)

Technical Requirements:

• Must be performed on infants >2 kg body weight, ≥36 weeks corrected gestational age, ideally after 10 days of age but by 4 weeks 1
• Sweat induction uses pilocarpine iontophoresis on the forearm or thigh with mild electrical stimulation 2
• Analysis must occur within hours of collection to ensure accuracy 1
• Bilateral testing is recommended to confirm results 1

Diagnostic Interpretation:

• ≥60 mmol/L = Diagnostic for CF when clinical features, positive family history, or positive newborn screening are present 2, 1
• 30-59 mmol/L = Intermediate/ambiguous, requiring repeat testing as infants with CF often initially present in this range 1
• <30 mmol/L = Normal, but does NOT exclude CF in atypical cases 3, 4

Critical Pitfall: Normal sweat chloride values can occur in patients with mild CF mutations (e.g., compound heterozygotes with 3849+10kb C→T), who may still develop bronchiectasis and progressive lung disease 3, 4. Maintain high suspicion despite normal results if clinical features suggest CF.

CFTR Genetic Testing (Confirmatory)

• Blood test or cheek swab to identify CFTR gene mutations 2, 1
• Identification of two disease-causing mutations confirms CF diagnosis regardless of sweat test results 1, 5
• Initial testing typically uses commercially available panels detecting common mutations (e.g., ΔF508, most frequent Class II mutation) 2
• For screen-positive infants with <2 disease-causing variants, perform CFTR sequencing including intronic regions 2, 1
• Approximately 2000 different CFTR mutations have been identified across six functional classes 2

Genetic Counseling:

• Parents of screen-positive infants should undergo CFTR genetic evaluation when phasing of variants (cis vs. trans) would inform diagnostic status 2
• Licensed/certified genetic counselors with CF expertise should be accessible to families 2

Clinical Evaluation Requirements

Diagnosis requires BOTH:

1. Clinical presentations: Chronic respiratory symptoms, pancreatic insufficiency, chronic sinusitis, or other CF features 2, 1
2. Evidence of CFTR dysfunction: Elevated sweat chloride OR two CF-causing mutations 1

Advanced/Alternative Testing (When Standard Tests Are Inconclusive)

• Nasal potential difference measurement: In vivo demonstration of abnormal ion transport across nasal epithelium 1, 6
• Intestinal current measurement: Bioassay of CFTR function in expert centers 6
• Extended CFTR sequencing: Full-gene sequencing of coding/flanking regions and deletion/duplication analysis 2

These advanced tests are reserved for patients with:

• Intermediate sweat chloride values (30-59 mmol/L)
• Only one identified CF mutation
• Strong clinical suspicion despite normal/borderline sweat chloride 6, 4

Follow-Up for Inconclusive Cases (CRMS/CFSPID)

For screen-positive infants with inconclusive diagnosis:

• Repeat sweat chloride testing at 6 months and annually until at least age 8 years 2
• Annual follow-up by CF clinician and nurse 2
• Respiratory cultures at each visit (at least until age 8) and as clinically indicated 2
• Avoid routine laboratory evaluations (fat-soluble vitamins, liver function, glucose monitoring) unless clinically indicated 2

Diagnostic Algorithm Summary

1. Newborn screening (IRT) → If elevated:
2. Sweat chloride test (≥60 mmol/L diagnostic; 30-59 repeat; <30 consider if high suspicion)
3. CFTR genetic testing (two mutations = diagnostic)
4. If inconclusive: Advanced testing (nasal potential difference, extended sequencing) at expert centers
5. Clinical correlation required throughout

The false-positive to true-positive ratio for newborn screening ranges from 5:1 to 25:1 depending on algorithm used, emphasizing the necessity of confirmatory sweat testing 2.