Makindo Medical Notes

Related Subjects: |Cystic Fibrosis |Sweat Test |Encopresis in Children |Enuresis/Bedwetting in Children |Acute Glomerulonephritis in Children |Nephrotic Syndrome in Children |Acute Appendicitis in Children |Gastro-oesophageal reflux in Children |Intussusception in Children |Panayiotopoulos Syndrome in Children |Reflex anoxic attacks in Children |Paediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections

💧 The Sweat Chloride Test is the gold-standard diagnostic test for Cystic Fibrosis (CF). 👉 A chloride concentration of <40 mmol/L is normal (low risk of CF). 👉 A concentration of >60 mmol/L strongly supports CF diagnosis. 👉 Intermediate (40–60 mmol/L) is indeterminate → requires repeat or genetic testing.

🎯 Purpose of the Test

• Most reliable diagnostic test for cystic fibrosis in both children and adults.
• CF patients have defective CFTR chloride channels → reduced chloride reabsorption in sweat ducts → abnormally salty sweat.
• Used alongside newborn screening and genetic testing for confirmatory diagnosis.

⚙️ Procedure

• Induction of Sweat: Pilocarpine iontophoresis (small electrical current drives pilocarpine into skin).
• Collection: Sweat absorbed onto filter paper or microtubes for 30–60 minutes.
• Measurement: Chloride concentration analysed in a laboratory sweat analyser.
• Caution: Test technically demanding → must be performed in specialist CF centres.
• False negatives: Most commonly due to oedema, insufficient sweat collection, or poor technique.

❌ False-Positive Results (Mimics of CF)

• Atopic eczema (skin barrier dysfunction).
• Adrenal insufficiency (altered salt balance).
• Ectodermal dysplasia.
• Glycogen storage diseases.
• Hypothyroidism.
• Severe dehydration.
• Malnutrition or chronic illness.

📊 Interpretation of Results

🧪 Indications for Testing

• Persistent respiratory infections, chronic productive cough, or bronchiectasis.
• Failure to thrive or steatorrhoea (due to pancreatic insufficiency).
• Positive family history or carrier status of CF gene.
• Abnormal newborn blood spot screening (immunoreactive trypsinogen).

⚠️ Limitations

• Young infants may produce insufficient sweat volume.
• Rare genetic variants → CF with normal sweat chloride → need CFTR genetic testing.
• Environmental and technical factors (e.g., dehydration, skin conditions) can affect results.

🔄 Follow-up

• Borderline/equivocal results → repeat sweat test in a reference lab, plus genetic testing.
• Confirmed CF → early referral to a specialist CF centre for enzyme replacement, airway clearance, nutrition, and multidisciplinary care.
• Ongoing monitoring of lung function, growth, and infection prevention is essential.

🇬🇧 UK Context

• The sweat test is available in specialist cystic fibrosis centres, paediatric respiratory clinics, and major NHS hospitals.
• Included in the UK newborn screening programme (blood spot test), allowing early diagnosis.
• Management follows NICE CF guidelines and the Cystic Fibrosis Trust standards of care.

📌 Key Clinical Pearls

• Always interpret sweat chloride results in the context of clinical features and genetic testing.
• False negatives are rare but dangerous → don’t exclude CF if strong phenotype present.
• High sweat chloride is pathognomonic for CF in the right clinical setting.