Interpretation of sweat chloride test results for diagnosis of cystic fibrosis

Sweat chloride concentration	Result	Interpretation
≤29 mmol/L	Normal	CF very unlikely
30 to 59 mmol/L	Intermediate	Possible CF, additional testing recommended*
≥60 mmol/L	Abnormal	Diagnosis of CF, if confirmed by a second test and if clinical symptoms consistent with CF are present^¶

Asymptomatic infants with equivocal results of the diagnostic process (sweat chloride test and DNA analysis) are given a provisional diagnosis of CRMS, also known as CFSPID.^[1]

CF: cystic fibrosis; CRMS: cystic fibrosis-related metabolic syndrome; CFSPID: cystic fibrosis screen positive, inconclusive diagnosis.
* Additional testing usually consists of molecular diagnosis (DNA analysis) and a second sweat test.
¶ Clinical symptoms of CF are not required among newborns identified through a screening program, or among siblings of patients with CF who are diagnosed by shared genotype.

Reference:

Cystic Fibrosis Foundation, Borowitz D, Parad RB, et al. Cystic Fibrosis Foundation practice guidelines for the management of infants with cystic fibrosis transmembrane conductance regulator-related metabolic syndrome during the first two years of life and beyond. J Pediatr 2009; 155:S106.

Data from: Farrell PM, White TB, Ren CL, et al. Diagnosis of Cystic Fibrosis: Consensus Guidelines from the Cystic Fibrosis Foundation. J Pediatr 2017; 181S:S4.

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Interpretation of sweat chloride test results for diagnosis of cystic fibrosis