Laboratory Diagnosis of Cystic Fibrosis: An Update

PATHCHAT Edition No. 56
June 2019
Please contact your local Ampath pathologist for more information.

Introduction

✅ Cystic Fibrosis (CF) Overview:

• One of the most common life-limiting autosomal recessive disorders in South Africa.
• Affects individuals across all South African population groups.
• Advances in treatment have increased median survival to >36 years.
• Early and accurate diagnosis improves outcomes.

✅ Cause of CF:

• Pathogenic variants in the CFTR gene on chromosome 7.
• >2,000 CFTR variants identified, but not all are disease-causing.
• CFTR functions as a cAMP-regulated chloride channel, affecting sodium and bicarbonate transport.

📌 Defective CFTR leads to thick, sticky secretions in multiple organs, including lungs, pancreas, liver, intestines, and reproductive tract.

Clinical Presentation of CF

✅ Typical Symptoms Based on Age:

✔ Neonates & Infants:

• Meconium ileus (bowel obstruction at birth).
• Failure to thrive.
• Recurrent respiratory infections.

✔ Children & Adolescents:

• Chronic cough with thick mucus.
• Sinusitis and nasal polyps.
• Gastrointestinal symptoms (malabsorption, steatorrhoea).
• Recurrent pancreatitis.

✔ Adults:

• Recurrent respiratory infections and bronchiectasis.
• Male infertility (due to congenital bilateral absence of the vas deferens).

📌 CF is increasingly diagnosed in adults due to milder mutations or late-onset presentations.

Diagnostic Criteria for CF (Cystic Fibrosis Foundation Consensus Report)

✅ Diagnosis Requires:

1. One or More of the Following:
   • Typical CF symptoms.
   • Positive family history (sibling with CF).
   • Positive newborn screening test.

AND

2. Evidence of CFTR Dysfunction:
   • Sweat chloride concentration ≥60 mmol/L (on two separate occasions).
   • Identification of two pathogenic CFTR gene variants.
   • Abnormal nasal epithelial ion transport test.

📌 Sweat chloride remains the gold standard for CF diagnosis.

Laboratory Testing for CF

✅ 1. Sweat Testing (Gold Standard for Diagnosis):

• Sweat chloride testing should be performed at an accredited CF centre.
• Ampath uses the Chlorochek® chloridometer for highly accurate chloride measurement.

🔹 Sweat Collection Methods:

• Macroduct® coil (preferred method, requires only 15 μL sweat).
• Nanoduct® analyser (used for screening, but not diagnostic).
• Sweat conductivity testing can be used for screening but must be confirmed by sweat chloride measurement.

🔹 Interpretation of Sweat Chloride Levels:

✔ Neonates (0–6 months):

• ≤29 mmol/L: Normal (CF very unlikely).
• 30–59 mmol/L: Intermediate (Possible CF).
• ≥60 mmol/L: Abnormal (Diagnostic of CF).

✔ Individuals >6 months:

• ≤39 mmol/L: Normal (CF very unlikely).
• 40–59 mmol/L: Intermediate (Possible CF).
• ≥60 mmol/L: Abnormal (Diagnostic of CF).

📌 Sweat chloride >60 mmol/L on two separate occasions confirms CF.

✅ 2. Genetic Testing for CF (CFTR Gene Analysis):

• Performed using Next-Generation Sequencing (NGS).
• Detects all known CFTR sequence variants (both benign and pathogenic).
• MLPA (Multiplex Ligation-dependent Probe Amplification) is performed to detect large deletions or duplications.

🔹 Indications for Genetic Testing:

• Confirming CF diagnosis in equivocal cases.
• Carrier screening for at-risk family members.
• Prenatal testing in known carrier couples.
• Pre-implantation genetic diagnosis (PGD) for assisted reproduction.

📌 Genetic counselling is recommended before and after testing to support patients and families.

✅ 3. Immunoreactive Trypsinogen (IRT) – Newborn Screening

• IRT is elevated in neonates with CF due to impaired pancreatic enzyme secretion.
• Heel prick blood collection on Guthrie cards (dried blood spot).
• Screening can be performed within the first 8 weeks of life.
• Positive results require confirmation by sweat testing or CFTR gene analysis.

📌 IRT levels decline after infancy, limiting its use beyond the neonatal period.

✅ 4. Faecal Elastase – Pancreatic Function Testing

• Assesses pancreatic exocrine function in suspected CF.
• Low faecal elastase (<200 μg/g) indicates pancreatic insufficiency.
• Helpful in monitoring CF progression and guiding enzyme replacement therapy.

📌 Faecal elastase should be used alongside sweat testing and genetic analysis for comprehensive CF assessment.

Approach to Testing for CF (South African CF Association Guidelines, 2017)

1. Suspect CF Based on Clinical Features.
2. Perform Sweat Chloride or Conductivity Testing.

✔ If Sweat Chloride <29 mmol/L or Conductivity <40 mmol/L:

• CF unlikely.

✔ If Sweat Chloride 30–59 mmol/L or Conductivity 41–79 mmol/L:

• Intermediate range, perform CFTR gene testing.
• If two CFTR mutations are detected → CF confirmed.
• If 0–1 mutations detected → Consider atypical CF or carrier status.

✔ If Sweat Chloride >60 mmol/L or Conductivity >80 mmol/L:

• CF highly likely.
• Confirm with CFTR gene testing and faecal elastase.

📌 Patients with atypical CF should be closely followed up at a CF centre.

Key Takeaways for Clinicians

✅ Cystic fibrosis is a common genetic disorder in South Africa and should be considered in any patient with suggestive symptoms.
✅ Sweat chloride testing remains the gold standard for diagnosis, but genetic testing provides confirmation and treatment guidance.
✅ Newborn screening with immunoreactive trypsinogen (IRT) allows early detection but requires confirmatory testing.
✅ Faecal elastase testing is useful in identifying pancreatic insufficiency in CF patients.
✅ Genetic counselling is essential for patients undergoing CFTR mutation testing, especially for carrier screening and prenatal diagnosis.

📌 Early diagnosis and intervention improve outcomes for CF patients through better respiratory and nutritional management.