Synonym

• Iontophoretic sweat test
• Pilocarpine iontophoresis sweat test
• Sweat sodium test
• Chloride (Sweat)
• Sodium (Sweat)

Tubes

• Minimum 30-50 mg of sweat
• Gauze pad/filter paper or microbore tubing

Additional information

• Sweat collection procedure:
• Wash the test area; usually the flexor surface of forearm or thigh (in infants), preferably on the right side with distilled water and dry it
• Local stimulation of sweat with pilocarpine iontophoresis:
• Place a gauze pad saturated with USP pilocarpine solution on positive electrode and another pad saturated with normal saline solution on negative electrode over the test area
• A low-level electric current (4–5 mAmp) is delivered at intervals (15-20 seconds) for a total of 5 minutes
• Disconnect the leads, remove the gauze pads, cleanse the area and completely dry it
• Sweat collection:
• Place a pre-weighed gauze pad/filter paper or microbore tubing over the positive electrode site (red spot) and secure it with a paraffin wax film and/or tape (airtight seal)
• Allow the sweat to collect for 30 minutes
• Gently blot the paraffin wax film against the gauze to collect any condensate, then remove the paraffin film and gauze pad with forceps
• Transfer the gauze pad without the paraffin film immediately in a pre-weighed vial, close, and reweigh the vial
• At least 0.075 gm (75 mg) of sweat should be collected using 2x2 inch gauze pad
• Sweat collection in infants is not done before 48 hours, and collection is preferred after 3-4 weeks for accurate results
• Sample collection preferred when patient is clinically stable, well hydrated, free of acute illnesses, and after discontinuing mineralocorticoids
• Do not collect sweat from head, chest, trunk, or areas with diffuse inflammation, rash, erythema, eczema, serous, or bloody discharge
• The test area must be completely dry, free from contamination by powder or antiseptic
• In cold weather, or if the testing room is cold, a warm covering should be placed over the arm or other site of sweat collection
• It may be necessary to stimulate sweating in 2 places to obtain sufficient sweat for testing, especially in infants
• Use battery-powered equipment to prevent electric shock if possible
• Send the sample collected to lab immediately

Info

• Sweat chloride test estimates the concentration of the electrolyte, chloride, in the sweat
• Abnormally high concentrations of chloride and sodium appear in the secretions of eccrine sweat glands in persons with cystic fibrosis (2-5 times the normal), from birth onward

Clinical

• The clinical utility of the sweat chloride test includes:
• Aids in the diagnosis and confirmation of cystic fibrosis (CF)
• As a screening test in individuals with a family history of CF or in siblings of diagnosed patients
• Screening for CF in individuals with respiratory conditions such as:
• Recurrent and persistent pneumonia (Staphylococcus, pseudomonas etc.)
• Recurrent bronchiolitis
• Pansinusitis or nasal polyposis
• Bronchiectasis or chronic atelectasis
• Screen for CF in individuals with gastrointestinal conditions such as:
• Malabsorption stools
• Failure to thrive in infancy and childhood
• Intestinal obstruction in children and young adults
• Intussusception in children and adolescents
• Unexplained hepatic cirrhosis, cholelithiasis, or pancreatitis <age 30 years
• Symptoms of fat-soluble vitamin (A, D, E, K) deficiency
• Screen for CF in the context of:
• Salty-tasting skin
• Hyponatremic dehydration or metabolic alkalosis in infancy
• Recurrent hyponatremic heat prostration in adults
• Azoospermia or infertility
• Hypoproteinemia or anasarca in infancy
• Hyperprothrombinemia
• Cystic fibrosis is an autosomal recessive genetic disorder characterized by abnormal exocrine secretions involving multiple organs, resulting in chronic and recurrent endobronchial infections, pancreatic enzyme insufficiency, intestinal malabsorption, and associated complications
• The clinical presentation of CF may vary from person to person, and will typically include:
• Respiratory problems including:
• Persistent cough/sputum production
• Pneumonia (recurrent)
• Shortness of breath
• Sinusitis
• Wheezing
• Clubbing of fingertips and toes in children and young adults
• Delayed growth/Failure to thrive
• Diabetes related to chronic pancreatitis
• Difficulty in bowel movements or diarrhea
• Fatigue
• Frequent greasy, bulky stools
• Hepatosplenomegaly
• Hypoproteinemia and edema
• Male infertility
• Meconium ileus (no bowel movements in first 24-48 hours of life)
• Rectal prolapse
• Salty-tasting skin
• Weight loss
• The classic clinical triad consists of malabsorption due to pancreatic insufficiency, chronic suppurative lung disease, and failure to thrive

Additional information

• In the United States, CF has a birth prevalence of 1 in 3,500 live births and a carrier rate of 1 in 28 Caucasians. Incidence rates are lower in African Americans (1 in 15,000) and Asian Americans (1 in 31,000)
• Sweat electrolyte levels fluctuate cyclically in women with chloride levels peaking 5–10 days before onset of menses
• Interpretation of test results requires consideration of other clinical findings, past medical history and further laboratory testing
• Sweat chloride values of >60 mEq/L (>60 mmol/L) in children: Positive, diagnostic of CF
• Sweat chloride values of >70 mEq/L (>70 mmol/L) in adolescence and adults: Positive, diagnostic of CF
• Sweat chloride values of 40–60 mEq/L (40–60 mmol/L): Borderline or equivocal results, repeat test
• A gap of >30 mEq/L (>30 mmol/L) between sodium and chloride values indicates calculation or analysis error or contamination of the sample
• Diagnostic accuracy is improved in borderline cases by a suppression test using fludrocortisone
• A wide discrepancy between the results from each arm is indicative of a problem with technique, repeat test
• Advantages of this test:
• Assesses functional deficit, therefore capable of detecting patients who have rare variants of CF
• Noninvasive
• Less expensive than genetic tests
• Factors interfering with test results include:
• In infants of <1 month as inadequate amount of sweat
• Levels vary widely during puberty
• If the specimen container that stores the gauze or filter paper is handled without gloves, the test results may show a false increase in the final weight of the collection container
• Presence of pure salt depletion, common during hot weather (possible false normal) or cool environmental temperatures may reduce the amount of sweat collected
• False positives (15%)
• Improper cleaning of the skin or improper application of gauze pad or filter paper for collection
• Evaporation of sweat secondary to inadequate sealing during collection
• Untreated adrenal insufficiency
• AIDS (some reports of abnormal sweat electrolytes)
• False negatives (12%)
• Inexperience of operator
• Inadequate sweat collected
• Poor skin preparation
• Poor iontophoretic contact with skin
• Dehydration or edema of areas where sweat is collected
• Related laboratory tests include:
• ₁-antitrypsin/phenotype
• Amylase
• Anion gap
• Electrolytes
• Fecal analysis
• Genetic studies
• Nasal potential difference
• Osmolality
• Pancreatic function tests (3-day fecal collection)
• Phosphorus
• Sweat potassium
• Trypsin test
• Trypsinogen test

Nl Result

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.

High Result

Conditions associated with elevated sweat chloride levels include:

• Metabolic causes (true positives)
• Addison’s disease
• Alcoholic pancreatitis
• Chronic pulmonary infections
• Congenital adrenal hyperplasia
• Cystic fibrosis
• Familial cholestasis
• Familial hypoparathyroidism
• Fucosidosis
• Glucose-6-phosphatase deficiency (G6PD)
• Glycogen storage disease type I
• Hypothyroidism
• Mucopolysaccharidosis
• Nephrogenic diabetes insipidus
• Renal failure
• Vasopressin - resistant diabetes insipidus

• Skin and sweat gland disease
• Atopic dermatitis
• Ectodermal dysplasia
• Malnutrition
• Meconium ileus

Low Result

Conditions associated with decreased sweat chloride levels include:

• Edema
• Hypoaldosteronism
• Hypoproteinemia
• Sodium depletion

References

• Cystic Fibrosis Foundation®. The Sweat Test. [Homepage on the Internet]©2007. Last reviewed on June 5, 2007. Last accessed on December 31, 2007. Available at URL: http://www.cff.org/AboutCF/Testing/SweatTest/
• Desax MC et al. Nanoduct(R) sweat testing for rapid diagnosis in newborns, infants and children with cystic fibrosis. Eur J Pediatr. 2007 Apr 14; [Epub ahead of print].
• eMedicine from WebMD®. Cystic Fibrosis. [Homepage on the Internet]©1996-2007. Last updated on March 2, 2007. Last accessed on December 31, 2007. Available at URL: http://www.emedicine.com/ent/topic515.htm
• Green A et al. Guidelines for the performance of the sweat test for the diagnosis of cystic fibrosis. Ann Clin Biochem. 2007 Jan;44(Pt 1):25-34.
• LabTestsOnline®. Sweat Test. [Homepage on the Internet]©2001-2007. Last reviewed on January 20, 2006. Last accessed on December 31, 2007. Available at URL: http://www.labtestsonline.org/understanding/analytes/sweat_chloride/glance.html
• Mishra A et al. The Limitations of Sweat Electrolyte Reference Intervals for the Diagnosis of Cystic Fibrosis: A Systematic Review. Clin Biochem Rev. 2007 May;28(2):60-76.