The sweat test is the gold standard for diagnosing CF. This study uses sweat-inducing techniques (e.g., pilocarpine iontophoresis) followed by chemical analysis to determine the chloride level in collected sweat. Abnormally high concentrations of chloride appear in the secretions of eccrine sweat glands in persons with CF. This condition is present at birth and persists throughout life. It has been recommended that the sweat test be performed in a laboratory accredited by the Cystic Fibrosis Foundation.

1. The forearm is the preferred site for stimulation of sweating, but in thin or small babies, the thigh, back, or leg may be used. It may be necessary to stimulate sweating in two places to obtain sufficient sweat for testing, especially in young infants. At least 100 μL of sweat is necessary. 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.

2. Stimulate sweat production by applying gauze pads or filter paper saturated with a measured amount of pilocarpine and attachment of electrodes through which a current of 4–5 mA is delivered at intervals for a total of 5 minutes (a total of 5–12 minutes, according to the National Institutes of Health).

3. Remove the electrodes and pad, and thoroughly wash the area with distilled water; dry carefully.

4. Remember that successful iontophoresis is indicated by a red area about 2.5 cm in diameter that appears where the electrode was placed.

5. Scrub the skin thoroughly with distilled water and dry carefully. The area for sweat collection must be completely dry, free from contamination by powder or antiseptic, and free of any area that might ooze.

6. Collection of sweat occurs by applying preweighed filter paper or sweat collection cups that are taped securely over the red spot. The inside surfaces of the collecting device should never be touched.

7. Leave the paper on for at least 1 hour before removing and then place in a preweighed flask to avoid evaporation. Weigh the flask again. The desired volume of sweat is 200 mg; the minimum volume necessary is 100 mg.

8. If a cup is used, leave in place for 1 hour and then carefully remove by scraping it across the iontophoresed area. This “puddles” the sweat in the cup to reduce evaporation and to redissolve any salts left by the evaporation. Use suction capillary tubes to remove sweat from the collection cups.

1. Children with CF have chloride values >60 mEq/L >60 mmol/L).

2. Borderline or gray-zone cases are those with values of 40–60 mEq/L (50–60 mmol/L) for chloride. These persons require retesting. Potassium values do not assist in differentiating borderline cases.

3. In adolescence and adulthood, chloride levels >80 mEq/L (>80 mmol/L) usually indicate CF.

4. Elevated sweat electrolytes also can be associated with the following conditions:

   1. Addison disease

   2. Congenital adrenal hyperplasia

   3. Vasopressin-resistant DI

   4. Glucose-6-phosphate dehydrogenase deficiency

   5. Hypothyroidism

   6. Familial hypoparathyroidism

   7. Alcoholic pancreatitis

Pretest Patient Care

1. Explain test purpose and procedure. The sweat test is indicated for the following persons:

   1. Infants who pass initial meconium late; who have intestinal obstruction, failure to thrive, steatorrhea, chronic diarrhea, rapid respiration and retraction with chronic cough, asthma, hypoproteinemia (especially on soybean formula), atelectasis, hyperaeration on x-ray, hyperprothrombinemia, or rectal prolapse; who taste salty; or who are offspring of a parent with CF (i.e., the obligate heterozygote)

   2. Persons suspected of having CF or celiac disease, all siblings of patients with CF, or persons with disaccharide intolerance, recurrent pneumonia, chronic atelectasis, chronic pulmonary disease, bronchiolectasis, chronic cough, nasal polyposis, cirrhosis of the liver, and hypertension

   3. Any parents who request a sweat test on their child

2. Inform the patient that a slight stinging sensation is usually experienced, especially in fair-skinned persons.

3. Follow guidelines in Chapter 1 regarding safe, effective, informed pretest care.

1. The sweat test is not valuable after puberty because levels may vary over a very wide range among individuals.

2. Dehydration and edema, particularly of areas where sweat is collected, may interfere with test results.

3. Sweat testing is not considered accurate until the third or fourth week of life because infants younger than 3 weeks may not sweat enough to provide a sufficient sample.

4. Test may be falsely normal in patients with salt depletion, as in periods of hot weather.

Sweat Chloride

• Normal: <40 mEq/L (<40 mmol/L)

• Borderline: 40–60 mEq/L (40–60 mmol/L)

• CF: >60 mEq/L (>60 mmol/L)