The Sweat Chloride Test and Cystic Fibrosis

Why sweat?

The story starts with a New York heat wave in 1948, in which physicians noticed that children with CF became dangerously salt-depleted. Di Sant'Agnese and colleagues then showed sweat chloride was strikingly elevated in these children. That observation, and the standardized test that Gibson and Cooke published in 1959, converted CF from a clinical suspicion into a laboratory diagnosis. In sweat-duct epithelium, chloride is normally reabsorbed on its way out through the duct; in CF, the mutant CFTR protein fails to reabsorb it, so chloride (and sodium, which follows) remains in the sweat at high concentrations. The sweat test is essentially a bedside functional assay for CFTR.

The Cystic Fibrosis Foundation and the CLSI have codified the procedure. In outline:

Under the current CFF Consensus (Farrell 2017), a single set of chloride reference ranges applies to all ages, replacing the older infant-specific breakpoints. The three ranges below are what the guideline endorses.

Because sweat volume is often the limiting factor (especially in newborns), the guidelines specify minimum sample sizes and QNS (quantity-not-sufficient) rate targets.

The test in the newborn screening pathway

Most CF diagnoses in high-income countries now start with a newborn screen based on immunoreactive trypsinogen (IRT), often combined with CFTR mutation panels. A positive screen prompts a confirmatory sweat chloride test at a CF-accredited center. In an infant with a positive newborn screen, a sweat chloride ≥ 60 mmol/L confirms CF; < 30 mmol/L indicates CF is unlikely; and 30-59 mmol/L on two separate occasions is intermediate, at which point extended CFTR gene analysis and/or CFTR functional testing (e.g., nasal potential difference, intestinal current measurement) may be needed.

A specific pathway exists for infants with a positive newborn screen who have an intermediate sweat chloride or fewer than two CF-causing mutations: the label CRMS/CFSPID (cystic fibrosis screen-positive, inconclusive diagnosis) is applied, and the child is followed clinically at intervals in case the picture evolves.

Limitations and known pitfalls

The test is exquisitely dependent on technique. Insufficient sweat volume is the most common preanalytical problem: in one Turkish laboratory audit, the QNS rate was 13.8% for infants ≤ 3 months and 6% for older patients, exceeding CFF benchmarks. Falsely elevated sweat chloride has been reported in a small number of non-CF conditions (adrenal insufficiency, ectodermal dysplasia, malnutrition, atopic dermatitis, hypothyroidism), which is why any positive test must be confirmed and interpreted at a CF center rather than in isolation.

Sweat conductivity, skin-precipitation patches, direct skin chloride electrodes, and sodium or osmolality measurements are explicitly listed as unacceptable for diagnosis. Duplicate samples must be analyzed separately, not pooled. Sweat chloride is also increasingly used as a biomarker in trials of CFTR modulators, where changes in chloride reflect drug effect.

Methodology and limitations

This page summarizes the current Cystic Fibrosis Foundation Consensus Guidelines (Farrell PM et al., J Pediatr 2017), the CFF Sweat Test Clinical Care Guidelines page, the CLSI sweat testing standard (2009), the StatPearls chapter on Sweat Testing (NBK547728), the Gibson-Cooke original method (Pediatrics 1959;23:545-549), the review by Collie et al. on sixty-five years of sweat testing (Pediatr Pulmonol 2014), a Turkish audit of QNS rates (Karahan Kilic 2020), and GeneReviews Cystic Fibrosis. Numeric claims (60, 30-59, < 30 mmol/L thresholds; 75 mg / 15 µL minimum samples; ≤ 10% / ≤ 5% QNS targets; 30-minute maximum collection time; ≤ 10 mmol/L detection limit; ≤ 160 mmol/L reporting limit) were traced directly to the CFF guidelines and secondary confirmations.

Limitations: guideline thresholds evolved over time (older CFF cut-offs used ≤ 39 mmol/L as the upper bound of normal for patients > 6 months; the 2017 harmonized cut-off is < 30 mmol/L for all ages). This page uses the current 2017 CFF values. Sweat chloride is a functional CFTR assay; a diagnosis of CF also requires clinical context (symptoms, newborn screen, or family history) and often genetic confirmation. Nothing on this page is medical advice; sweat testing should be interpreted only by a CF-accredited center.

Frequently asked questions

What is the sweat chloride test?

A diagnostic laboratory test that measures the concentration of chloride ions in sweat stimulated by pilocarpine iontophoresis. It is the gold-standard functional test for cystic fibrosis, because the underlying CFTR channel defect causes chloride reabsorption to fail in sweat-duct epithelium.

What sweat chloride level indicates cystic fibrosis?

Per the 2017 Cystic Fibrosis Foundation guidelines, a sweat chloride of ≥ 60 mmol/L supports a diagnosis of CF at any age. Values < 30 mmol/L make CF unlikely, and 30-59 mmol/L is intermediate and requires further evaluation, including repeat testing and often CFTR genetic analysis.

Who developed the sweat chloride test?

Lewis Gibson and Robert Cooke, at Johns Hopkins, standardized pilocarpine iontophoresis for sweat stimulation and quantitative chloride analysis in a 1959 paper in Pediatrics. Their protocol, known as the Gibson-Cooke method or quantitative pilocarpine iontophoresis test (QPIT), remains the reference standard.

How much sweat is needed?

At least 75 mg using the Gibson-Cooke gauze or filter-paper method, or at least 15 microliters using a Macroduct coil, collected over no more than 30 minutes. Samples smaller than these are quantity-not-sufficient (QNS) and should not be analyzed.

When can a newborn have a sweat test?

Sweat testing is generally deferred until the infant is more than 48 hours old, weighs at least 2 kg, and is clinically stable. Guidelines target a QNS rate of no more than 10% for infants ≤ 3 months at accredited centers.

Are there other conditions that can raise sweat chloride?

Yes, but rarely. Adrenal insufficiency, some ectodermal dysplasias, malnutrition, hypothyroidism, and atopic dermatitis have all been reported as causes of falsely elevated sweat chloride. That is why any positive test must be confirmed and interpreted at a CF center in the context of the newborn screen, symptoms, and CFTR genotype.

Sources

Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).

1. Gibson LE, Cooke RE. A test for concentration of electrolytes in sweat in cystic fibrosis of the pancreas utilizing pilocarpine by iontophoresis. Pediatrics. 1959;23(3):545-549. DOI
2. Farrell PM, White TB, Ren CL, et al. Diagnosis of cystic fibrosis: consensus guidelines from the Cystic Fibrosis Foundation. J Pediatr. 2017;181S:S4-S15.e1. PubMed
3. Cystic Fibrosis Foundation. Sweat Test Clinical Care Guidelines. CFF clinical care resource, current guideline summary. CFF
4. Farrell PM, Rosenstein BJ, White TB, et al. Guidelines for diagnosis of cystic fibrosis in newborns through older adults: Cystic Fibrosis Foundation Consensus Report. J Pediatr. 2008;153(2):S4-S14 (historical CFF consensus). (secondary) Full text
5. Collie JT, Massie RJ, Jones OA, LeGrys VA, Greaves RF. Sixty-five years since the New York heat wave: advances in sweat testing for cystic fibrosis. Pediatr Pulmonol. 2014;49(2):106-117. (secondary) DOI
6. Karahan Kilic O, Karahan ZC. The most common preanalytic problem of sweat testing: insufficient sweat volume. Int J Med Biochem. 2020;3(3):137-140. (secondary) Full text
7. Vaidyanathan S, Warren J. Sweat testing (chapter). StatPearls, NCBI Bookshelf, updated 2023. (secondary) StatPearls
8. Katkin JP. Cystic fibrosis. GeneReviews, University of Washington, updated 2023. (secondary) GeneReviews

How to cite this page

Related reading on this site

• How doctors measure sweat: gravimetry and sudorometry
• What is sweat made of?
• Sweat sodium and electrolyte losses in exercise
• Red flags for secondary hyperhidrosis
• When should I see a doctor about sweating?