💧 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

Result (mmol/L)	Interpretation	Next Steps
<30–40	Normal	CF very unlikely unless strong clinical suspicion → consider repeat/genetics if atypical.
40–60	Intermediate	Borderline → repeat sweat test, genetic analysis, consider nasal potential difference studies.
>60	Abnormal	Strongly supports CF diagnosis (with symptoms/family history).

🧪 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.

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Sweat Test