Sweat Chloride Testing in Pediatrics
Sweat Chloride Testing in Pediatrics
Overview
The sweat chloride test is the gold standard diagnostic test for Cystic Fibrosis (CF). It measures the concentration of chloride in sweat, which is abnormally high in patients with CF due to CFTR protein dysfunction.
Key Points
- Gold standard for CF diagnosis
- Requires specialized equipment and trained personnel
- Minimum sweat weight requirements must be met
- Should be performed at accredited CF centers
- Results available within 24 hours
Physiological Basis
- CFTR protein dysfunction leads to abnormal chloride transport
- Results in increased chloride concentration in sweat
- Test stimulates localized sweating using pilocarpine
- Measures chloride concentration in collected sweat
Clinical Indications
Primary Indications:
- Newborn Screening:
- Positive newborn screen for CF
- Follow-up of borderline results
- Confirmation of genetic testing
- Clinical Suspicion:
- Chronic respiratory symptoms
- Failure to thrive
- Chronic diarrhea
- Recurrent pancreatitis
- Male infertility evaluation
- Family History:
- Siblings of CF patients
- Parents who are known carriers
- Extended family screening when indicated
Procedure & Technique
Pre-test Requirements:
- Patient Preparation:
- No moisturizers or lotions on test area
- Patient should be well-hydrated
- No acute illness
- Minimum age: typically >48 hours of life
- Minimum weight: typically >2 kg
Step-by-Step Procedure:
- Iontophoresis Phase:
- Clean test site (forearm)
- Apply pilocarpine gel electrodes
- Pass mild electric current (2-4 mA)
- Duration: 5 minutes
- Sweat Collection:
- Remove electrodes
- Clean area thoroughly
- Apply collection device (Macroduct or filter paper)
- Collection time: 20-30 minutes
- Minimum sweat weight: 75 mg (Macroduct: 15 μL)
- Sample Analysis:
- Immediate transport to laboratory
- Analysis using chloridometer
- Quality control measures
Result Interpretation
Reference Values by Age:
- Infants up to 6 months:
- Normal: ≤29 mmol/L
- Intermediate: 30-59 mmol/L
- Abnormal: ≥60 mmol/L
- Over 6 months of age:
- Normal: ≤39 mmol/L
- Intermediate: 40-59 mmol/L
- Abnormal: ≥60 mmol/L
Clinical Correlation:
- Positive Results (≥60 mmol/L):
- Highly suggestive of CF
- Requires confirmation with repeat testing
- Genetic testing recommended
- Referral to CF specialist
- Intermediate Results (30-59 mmol/L):
- Repeat testing needed
- Consider extended CFTR mutation analysis
- Clinical correlation essential
- Follow-up monitoring required
Limitations & Precautions
Technical Limitations:
- Sample Collection Issues:
- Insufficient sweat quantity
- Evaporation during collection
- Contamination risks
- Equipment malfunction
- Patient Factors:
- Age limitations
- Skin conditions
- Dehydration
- Edema
False Results:
- False Positives:
- Malnutrition
- Adrenal insufficiency
- Nephrogenic diabetes insipidus
- Glycogen storage disease type 1
- False Negatives:
- Edema
- Systemic steroids
- Some CFTR mutations
- Technical errors
