Sweating the small stuff: adequacy and accuracy in sweat chloride determination

Area of Science:

• Clinical diagnostics
• Biochemistry
• Pediatric medicine

Background:

• Sweat chloride testing is the gold standard for diagnosing cystic fibrosis (CF).
• Understanding factors influencing sweat chloride concentration and test reliability is crucial for accurate diagnosis and monitoring treatment efficacy.

Purpose of the Study:

• To evaluate variables affecting sweat rate.
• To determine the analytical and diagnostic capability of sweat chloride analysis across varying sweat rates.
• To assess the biological variability of sweat chloride concentration.

Main Methods:

• Retrospective analysis of 1397 sweat chloride tests from 904 individuals over 21 months.
• Comparison of sweat collection from forearms versus legs.
• Analysis of the correlation between sweat weight and chloride concentration.
• Calculation of biologic coefficient of variation (CV) and reference change value (RCV).

Main Results:

• Forearm sweat collection yielded greater weight than leg collection.
• Negligible correlation (r=-0.06) was observed between sweat weight and chloride concentration.
• The mean individual biologic CV was 13.1%, with a reference change value of 36%.
• 100% of CF cases were detected using a 60 mmol/L cutoff, irrespective of sweat weight (≥75, 40, or 20 mg).

Conclusions:

• Forearm sweat collection is recommended, especially for infants.
• Sweat chloride concentration is largely independent of sweat rate.
• A change exceeding 36% in sweat chloride concentration signifies a clinically relevant response to CFTR-targeted therapy.
• Sweat collections below 75 mg are diagnostically accurate.