Chloride, Sweat


Chloride, Sweat

Synonym/acronym: Sweat test, pilocarpine iontophoresis sweat test, sweat chloride.

Common use

To assist in diagnosing cystic fibrosis.

Specimen

Sweat (0.1 mL minimum) collected by pilocarpine iontophoresis.

Normal findings

(Method: Ion-specific electrode or titration)
Conventional & SI Units
Normal0–40 mEq/L or mmol/L
Borderline41–60 mEq/L or mmol/L
Consistent with the diagnosis of CFGreater than 60 mEq/L or mmol/L

Description

Cystic fibrosis (CF) is a genetic disease that affects normal functioning of the exocrine glands, causing them to excrete large amounts of electrolytes. Patients with CF have sweat electrolyte levels two to five times normal. Sweat test values, with family history and signs and symptoms, are required to establish a diagnosis of CF. CF is transmitted as an autosomal recessive trait and is characterized by abnormal exocrine secretions within the lungs, pancreas, small intestine, bile ducts, and skin. Clinical presentation may include chronic problems of the gastrointestinal and/or respiratory system. CF is more common in Caucasians. Sweat conductivity is a screening method that estimates chloride levels. Sweat conductivity values greater than or equal to 50 mmol/L should be referred for quantitative analysis of sweat chloride. Testing of stool samples for decreased trypsin activity has been used as a screen for CF in infants and children, but this is a much less reliable method than the sweat test. The American College of Obstetricians and Gynecologists (ACOG) suggests that carrier screening be discussed as an option to patients (and couples) who are pregnant or are considering pregnancy. Laboratories generally offer a panel of the current and most common cystic fibrosis mutations recommended by ACOG and the American College of Medical Genetics. Some states include CF in the neonatal screening performed at birth. Genetic testing can also be reliably performed on DNA material harvested from whole blood, amniotic fluid (submitted with maternal blood sample), chorionic villus samples (submitted with maternal blood sample), or buccal swabs to screen for genetic mutations associated with CF and can assist in confirming a diagnosis of CF, but the sweat electrolyte test is still considered the gold standard diagnostic for CF.

The sweat test is a noninvasive study done to assist in the diagnosis of CF when considered with other test results and physical assessments. This test is usually performed on children, although adults may also be tested; it is not usually ordered on adults because results can be highly variable and should be interpreted with caution. Sweat for specimen collection is induced by a small electrical current carrying the drug pilocarpine. The test measures the concentration of chloride produced by the sweat glands of the skin. A high concentration of chloride in the specimen indicates the presence of CF. The sweat test is used less commonly to measure the concentration of sodium ions for the same purpose.

This procedure is contraindicated for

  • high alertPatients with skin disorders (e.g., rash, erythema, eczema).

Indications

  • Assist in the diagnosis of CF
  • Screen for CF in individuals with a family history of the disease
  • Screen for suspected CF in children with recurring respiratory infections
  • Screen for suspected CF in infants with failure to thrive and infants who pass meconium late
  • Screen for suspected CF in individuals with malabsorption syndrome

Potential diagnosis

Increased in

  • Conditions that affect electrolyte distribution and excretion may produce false-positive sweat test results.

  • Addison’s disease
  • Alcoholic pancreatitis (dysfunction of CF gene is linked to pancreatic disease susceptibility)
  • CF
  • Chronic pulmonary infections (related to undiagnosed CF)
  • Congenital adrenal hyperplasia
  • Diabetes insipidus
  • Familial cholestasis
  • Familial hypoparathyroidism
  • Fucosidosis
  • Glucose-6-phosphate dehydrogenase deficiency
  • Hypothyroidism
  • Mucopolysaccharidosis
  • Nephrogenic diabetes insipidus
  • Renal failure

Decreased in

    Conditions that affect electrolyte distribution and retention may produce false-negative sweat test results.

    Edema Hypoaldosteronism Hypoproteinemia Sodium depletion

Critical findings

  • 20 yr or younger: greater than 60 mEq/L or mmol/L (SI greater than 60 mEq/L) considered diagnostic of CF
  • Older than 20 years: greater than 70 mEq/L or mmol/L (SI greater than 70 mEq/L) considered diagnostic of CF

  • Note and immediately report to the health-care provider (HCP) any critically increased values and related symptoms. Values should be interpreted with consideration of family history and clinical signs and symptoms.

  • It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.

  • Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

  • The validity of the test result is affected tremendously by proper specimen collection and handling. Before proceeding with appropriate patient education and counseling, it is important to perform duplicate testing on patients whose results are in the diagnostic or intermediate ranges. A negative test should be repeated if test results do not support the clinical picture.

Interfering factors

  • An inadequate amount of sweat may produce inaccurate results.
  • Improper cleaning of the skin or improper application of gauze pad or filter paper for collection affects test results.
  • Hot environmental temperatures may reduce the sodium chloride concentration in sweat; cool environmental temperatures may reduce the amount of sweat collected.
  • 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.

Nursing Implications and Procedure

Pretest

  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this test can assist in diagnosing an inherited disease that affects the lungs.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s endocrine and respiratory systems, especially failure to thrive or CF in other family members, as well as results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient and caregiver. Encourage the caregiver to stay with and support the child during the test. The iontophoresis and specimen collection usually takes approximately 75 to 90 min. Address concerns about pain and explain that there is no pain associated with the test, but a stinging sensation may be experienced when the low electrical current is applied at the site.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.

Intratest

  • Potential complications: N/A
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection.
  • high alertThe test should not be performed if the patient is receiving oxygen by means of an open system related to the remote possibility of explosion from an electrical spark. If the patient can temporarily receive oxygen via a facemask or nasal cannula, then sweat testing can be done.
  • high alertThe patient is placed in a position that will allow exposure of the site on the forearm or thigh. To ensure collection of an adequate amount of sweat in a small infant, two sites (right forearm and right thigh) can be used. The patient should be covered to prevent cool environmental temperatures from affecting sweat production. The site selected for iontophoresis should never be the chest or left side because of the risk of cardiac arrest from the electrical current.
  • The site is washed with distilled water and dried. A positive electrode is attached to the site on the right forearm or right thigh and covered with a pad that is saturated with pilocarpine, a drug that stimulates sweating. A negative electrode is covered with a pad that is saturated with bicarbonate solution. Iontophoresis is achieved by supplying a low (4 to 5 mA) electrical current via the electrode for 12 to 15 min. Battery-powered equipment is preferred over an electrical outlet to supply the current.
  • The electrodes are removed, revealing a red area at the site, and the site is washed with distilled water and dried to remove any possible contaminants on the skin.
  • Preweighed disks made of filter paper are placed on the site with a forceps; to prevent evaporation of sweat collected at the site, the disks are covered with paraffin or plastic and sealed at the edges. The disks are left in place for about 1 hr. Distract the child with books or games to allay fears.
  • After 1 hr, the paraffin covering is removed, and disks are placed in a preweighed container with a forceps. Use gloves to handle the specimen container; do not directly handle the preweighed specimen container or filter paper. The container is sealed and sent immediately to the laboratory for weighing and analysis of chloride content. At least 100 mg of sweat is required for accurate results.
  • Terminate the test if the patient complains of burning at the electrode site. Reposition the electrode before the test is resumed.
  • Promptly transport the specimen to the laboratory for processing and analysis. Do not directly handle the preweighed specimen container or filter paper.

Post-Test

  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient/caregiver.
  • Observe/assess the site for unusual color, sensation, or discomfort.
  • Inform the patient and caregiver that redness at the site fades in 2 to 3 hr.
  • Instruct the patient to resume usual diet, fluids, medications, and activity, as directed by the HCP.
  • Nutritional Considerations: If appropriate, instruct the patient and caregiver that nutrition may be altered because of impaired digestive processes associated with CF. Increased viscosity of exocrine gland secretion may lead to poor absorption of digestive enzymes and fat-soluble vitamins, necessitating oral intake of digestive enzymes with each meal and calcium and vitamin (A, D, E, and K) supplementation. Malnutrition also is seen commonly in patients with chronic, severe respiratory disease for many reasons, including fatigue, lack of appetite, and gastrointestinal distress. Research has estimated that the daily caloric intake needed for children with CF between 4 and 7 yr may be 2,000 to 2,800 and for teens 3,000 to 5,000. Tube feeding may be necessary to supplement regular high-calorie meals. To prevent pulmonary infection and decrease the extent of lung tissue damage, adequate intake of vitamins A and C is also important. Excessive loss of sodium chloride through the sweat glands of a patient with CF may necessitate increased salt intake, especially in environments where increased sweating is induced. The importance of following the prescribed diet should be stressed to the patient and caregiver.
  • If appropriate, instruct the patient and caregiver that ineffective airway clearance related to excessive production of mucus and decreased ciliary action may result. Chest physical therapy and the use of aerosolized antibiotics and mucus-thinning drugs are an important part of the daily treatment regimen.
  • Recognize anxiety related to test results, and be supportive of impaired activity related to perceived loss of independence and fear of shortened life expectancy. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate. Educate the patient regarding access to counseling services. Help the patient and caregiver to cope with long-term implications. Recognize that anticipatory anxiety and grief related to potential lifestyle changes may be expressed when someone is faced with a chronic disorder. Provide information regarding genetic counseling and possible screening of other family members if appropriate. Provide contact information, if desired, for the Cystic Fibrosis Foundation (www.cff.org).
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Explain that a positive sweat test alone is not diagnostic of CF; repetition of borderline and positive tests is generally recommended. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include α1-antitrypsin/phenotype, amylase, anion gap, biopsy chorionic villus, blood gases, fecal analysis, fecal fat, newborn screening, osmolality, phosphorus, potassium, and sodium.
  • Refer to the Endocrine and Respiratory systems tables at the end of the book for related tests by body system.