What is the procedure for performing a sweat chloride test to diagnose cystic fibrosis (CF)?

How to Perform Sweat Chloride Test in Cystic Fibrosis

Sweat chloride testing should be performed according to standardized procedural guidelines using pilocarpine iontophoresis for sweat stimulation, followed by quantitative chloride measurement within a few hours of collection, with the test performed bilaterally in newborns weighing >2 kg and at least 36 weeks corrected gestational age. 1, 2

Patient Preparation and Timing

For Newborns with Positive CF Screening

• Perform testing bilaterally when the infant weighs >2 kg and is at least 36 weeks corrected gestational age 1, 2
• Schedule testing as soon as possible after 10 days of age, ideally by the end of the neonatal period (4 weeks of age) 1, 2
• Do not delay CF treatment while awaiting diagnostic confirmation in infants with presumptive CF identified through newborn screening 1, 3

For All Other Patients

• Testing can be performed at any age from newborn to adult when CF is suspected based on clinical features or family history 1, 2
• Ensure adequate hydration status, as dehydration can affect sweat production 2

Standardized Testing Procedure

Sweat Stimulation

• Use pilocarpine iontophoresis as the standard method for sweat stimulation 1, 4
• Follow CLSI 2009 Guidelines (or most current version) for procedural standardization 1

Sweat Collection

• Collect sweat bilaterally, particularly in newborns, to increase likelihood of obtaining adequate specimens 1, 2
• Ensure minimum sweat quantity is collected (typically >75 mg for Gibson-Cooke method) 4
• Critical timing: Perform sweat chloride analysis within a few hours of sweat collection to ensure accuracy 1, 3

Measurement Methods

• Quantitative chloride measurement is the gold standard, using either coulometric or colorimetric titration methods 5, 6, 4
• Sweat conductivity can be used as an alternative measurement, expressed in mmol/L NaCl equivalents, though it requires conversion for interpretation 5, 6
• Direct potentiometry using chloride-selective electrode is another acceptable method 6

Result Interpretation

Diagnostic Thresholds

• ≥60 mmol/L: Diagnostic for CF when patient has clinical features consistent with CF, positive family history, or positive newborn screening 1, 2
• 30-59 mmol/L: Intermediate/ambiguous range requiring repeat testing on two separate occasions; may indicate CF and warrants extended CFTR gene analysis 1, 2, 3
• <30 mmol/L: CF unlikely, though CF should still be considered if evolving clinical criteria and/or CFTR genotyping support CF diagnosis 1, 2

For Sweat Conductivity Results

• Conductivity values >90 mmol/L NaCl equivalents have 83.3% sensitivity and 99.7% specificity for CF diagnosis 5
• Conductivity <75 mmol/L NaCl equivalents effectively excludes CF 5
• Calculated equivalents: 30 mmol/L Cl⁻ = 57 mmol/L NaCl Eq; 40 mmol/L Cl⁻ = 66 mmol/L NaCl Eq; 60 mmol/L Cl⁻ = 84 mmol/L NaCl Eq 6

Quality Control Considerations

Technical Accuracy

• Intra-laboratory coefficients of variation should be <5% for values between 10-100 mmol/L 6
• Within-test variability (comparing right and left arm on same day) is low, with differences typically between -3.2 and +3.6 mmol/L 7
• Between-test variability is substantial, with differences ranging from -18.2 to +14.1 mmol/L on repeat testing, particularly affecting the intermediate range 7

Common Pitfalls to Avoid

• Insufficient sweat production: Not all infants produce adequate sweat quantities, particularly those <2 weeks old or <2 kg body weight 2
• Delayed analysis: Sweat must be analyzed within hours of collection to prevent evaporation and concentration changes 1, 3
• Single borderline result: Changes in diagnostic conclusion occur frequently with repeat testing, especially in the 30-59 mmol/L range (55/197 subjects in one study) 7
• Operator-dependent variability: Less automated methods have greater operator influence on result quality 6

Follow-Up Testing Strategy

When to Repeat Testing

• Repeat sweat chloride testing at 6 months of life and annually at least until age 8 years for patients with CRMS/CFSPID 1
• Repeat immediately if initial result is in intermediate range (30-59 mmol/L) 1, 2, 3
• Consider repeat testing if clinical suspicion remains high despite normal initial result 4, 8

Complementary Diagnostic Testing

• Perform CFTR genetic testing via blood test or cheek swab when sweat chloride is diagnostic or intermediate 2, 3
• For CRMS/CFSPID patients with <2 disease-causing variants, perform CFTR sequencing including intronic regions 1, 2
• Rare exception: Some confirmed CF cases present with normal sweat chloride concentrations, requiring genotyping or electrophysiologic measurements for diagnosis 8