β‐adrenergic sweat test in children with inconclusive cystic fibrosis diagnosis: Do we need new reference ranges?

BACKGROUND: Investigating inconclusive cystic fibrosis (CF) diagnosis in children is difficult without advanced cystic fibrosis transmembrane conductance regulator (CFTR) function tests. This study investigated the utility of beta (β)‐adrenergic sweat test to exclude CF in participants with inconclusive diagnosis (CF suspects) in South Africa. METHODS: β‐adrenergic sweat test and sweat chloride tests (SCT) were performed simultaneously in CF suspects and adult controls (healthy, CFTR heterozygotes and CF). Cholinergic and β‐adrenergic induced sweat rate was measured by evaporimetry (transepithelial water loss [TEWL]: g H(2)O/m(2)/h) following intradermal injections. Next‐generation sequencing of CFTR was performed in CF suspects. CF diagnosis was defined by genotype. RESULTS: Thirty‐seven controls (10 healthy, 14 CF, 13 CFTR heterozygotes) and 32 CF suspects (26 children; 6 adults) were enrolled. Six were excluded from formal analyses due to β‐adrenergic sweat test failure. In adults, evaporimetry was superior to SCT for diagnosis of CF with β‐adrenergic:cholinergic ratio TEWL ≤ 0.05 achieving 100% sensitivity and specificity. Twenty‐two CF suspect children (age range: 3.4–15.6 years) completed β‐adrenergic sweat testing of which none had CF confirmed by genotyping: β‐adrenergic:cholinergic ratio > 0.05 successfully excluded CF in all but one child who was CFTR heterozygous. Median peak β‐adrenergic TEWL and β‐adrenergic:cholinergic ratio in CFTR negative and CFTR heterozygous children was significantly lower than adult controls. CONCLUSION: β‐adrenergic sweat test is more accurate than SCT for excluding CF in children with inconclusive diagnosis. Established reference ranges for β‐adrenergic sweat test may not be suitable for children due to lower β‐adrenergic sweat secretion compared to adults.