Sweat Chloride Test

The sweat chloride test measures the concentration of chloride in sweat stimulated by pilocarpine iontophoresis. In cystic fibrosis, mutations in the CFTR chloride channel prevent reabsorption of chloride from sweat in the duct, so the sweat is salty. A raised sweat chloride is the confirmatory test for cystic fibrosis. Quantitative analysis by chloridometer, ion selective electrode or colorimetry is the diagnostic method. Conductivity measurements are screening tests only.

The test follows a positive newborn blood spot screen (raised immunoreactive trypsinogen), a clinical presentation suggestive of cystic fibrosis, or a family history. It is also used for CFTR-related disorders and to assess response to CFTR modulator therapy.

Cystic fibrosis affects about 1 in 2500 to 3500 live births in people of European descent, 1 in 15 000 in African Americans, and 1 in 30 000 or fewer in Asian populations. About 1 in 25 white Europeans carries a CFTR variant. The diagnosis is increasingly made by newborn screening, and later in life in atypical disease.

Indications. Positive newborn screen, meconium ileus, prolonged neonatal jaundice with cholestasis, recurrent or chronic respiratory infection, bronchiectasis, chronic Pseudomonas aeruginosa or Burkholderia cepacia infection, nasal polyps in a child, failure to thrive with steatorrhoea, recurrent pancreatitis, rectal prolapse, congenital absence of the vas deferens (male infertility), salt depletion in hot weather, family history, and monitoring of CFTR modulator therapy.

Specimen. Sweat collected from the forearm or thigh after pilocarpine iontophoresis, with a minimum of 75 mg of sweat on gauze or filter paper, or 15 microL in a microduct collector. The infant should be at least 48 hours old (ideally 2 weeks or more), with a weight of at least 2 kg and without acute illness or dehydration. Collection time is 30 minutes. A quality assured laboratory performs the test.

Sweat chloride reference ranges.

• Under 30 mmol/L: normal, and cystic fibrosis is unlikely.
• 30 to 59 mmol/L: intermediate, and cystic fibrosis is possible. Further testing is needed.
• 60 mmol/L or more: consistent with cystic fibrosis in a patient with compatible features.
• In infants up to 6 months, intermediate range is 30 to 59 mmol/L, and 60 mmol/L or more is diagnostic.

Interpretation.

• A result of 60 mmol/L or more on two occasions in a patient with a compatible clinical picture or a positive newborn screen confirms cystic fibrosis.
• A result under 30 mmol/L in a symptomatic patient makes cystic fibrosis very unlikely, though a few patients with residual CFTR function have lower values.
• An intermediate result needs CFTR gene analysis (extended panel or full sequencing), repeat testing, and review for CFTR-related disorders (pancreatitis, bronchiectasis, congenital absence of vas deferens) or CFTR-related metabolic syndrome in an infant.

Causes of false positive results.

• Untreated adrenal insufficiency, hypothyroidism, nephrogenic diabetes insipidus, hypoparathyroidism, glycogen storage disease type I, fucosidosis, mucopolysaccharidoses, ectodermal dysplasia, malnutrition, familial cholestasis, pseudohypoaldosteronism, atopic dermatitis, G6PD deficiency, and Klinefelter syndrome.
• Technical: sample evaporation, contamination with salt, contamination with skin products, inadequate cleaning, and an assay error.

Causes of false negative results. Oedema, hypoproteinaemia, dilution of the sweat with water, insufficient sweat volume (short collection, wrong technique), skin dehydration, and cystic fibrosis genotypes with partial CFTR function.

Other tests in the same setting.

• Sweat sodium is less specific and not used for diagnosis.
• Conductivity measurement is a screen only, and a raised value is confirmed by a chloride measurement.
• Immunoreactive trypsinogen on a newborn blood spot is a screening test.
• CFTR variant testing confirms the genotype, and 2 disease causing variants in trans plus a raised sweat chloride confirm the diagnosis.
• Nasal potential difference and intestinal current measurement are research or reference laboratory tests in unclear cases.

Monitoring of CFTR modulators. A fall of sweat chloride by 10 mmol/L or more, or into the intermediate range, is a biomarker of CFTR function recovery, though it is not used to adjust dose.

Critical values and findings needing immediate action.

• Positive result in an infant: urgent referral to a specialist cystic fibrosis centre for pancreatic status, nutrition, respiratory culture and treatment.
• Hyponatraemic, hypochloraemic metabolic alkalosis with dehydration in an infant or in hot weather in a known patient (pseudo-Bartter syndrome), which needs urgent salt and fluid replacement.
• Meconium ileus or distal intestinal obstruction.
• Respiratory distress, haemoptysis, or pneumothorax in a patient with confirmed disease.

Interferences and pitfalls.

• Insufficient sweat is the commonest technical failure and is reported as such. It is not interpreted, and the test is repeated.
• The test should not be done in the first 48 hours of life, in sick or dehydrated infants, or in infants under 2 kg, because of poor sweat production.
• Fludrocortisone and some other drugs alter sweat electrolytes.
• Contamination with creams, salt on gauze or sample evaporation gives false results.
• Cystic fibrosis cannot be excluded by a normal test in a patient with a strong clinical picture. Genetic testing and repeat testing are needed.
• In adults, sweat chloride rises slightly with age, and a borderline result may be less specific.
• Carriers have normal or slightly raised sweat chloride.

Special groups.

• Neonates and infants: the commonest group, tested after a positive blood spot screen, and repeated when sweat is inadequate.
• Children: tested for failure to thrive, recurrent infection and nasal polyps.
• Adults: late diagnosis occurs in mild disease with pancreatitis, bronchiectasis, sinus disease and male infertility.
• Pregnancy: carrier screening and partner testing are offered in some settings.
• Regions: cystic fibrosis occurs in all populations but is less often diagnosed in Africa and Asia, with different variant distributions.

Follow up tests. CFTR gene analysis (panel of common variants, then full sequencing and deletion analysis), immunoreactive trypsinogen, faecal elastase, sputum or cough swab culture, chest radiograph or CT, spirometry, fat soluble vitamin levels (A, D, E) and prothrombin time, glucose tolerance test, liver function tests and ultrasound, sinus imaging, semen analysis and scrotal ultrasound, and family screening.