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Subject: Quantitative Pilocarpine Iontophoresis Sweat Test
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The sweat test consists of the quantitative analyses of sweat chloride with or without sodium. This procedure, often referred to as the quantitative pilocarpine iontophoresis test, involves collection and quantification of sweat after pilocarpine iontophoresis with the use of gauze, filter paper, or Macroduct coils and quantitative analyses of sweat chloride. The sweat test entails three consecutive procedures: sweat stimulation, sweat collection, and sweat analysis.
Normal range (sweat chloride):
Less than 40 mmol/L (>3 months)
Greater than 30 mmol/L (<3 months)
Borderline results: sweat chloride 40–60 mmol/L
A positive test is defined by sweat chloride >60 mmol/L in sweat from both arms, provided that a minimum of 15 μL of sweat is obtained from each site.
Standard test for the diagnosis of CF
CF (see Table 16.71)
Endocrine disorders (e.g., untreated adrenal insufficiency, hypothyroidism, vasopressin-resistant diabetes insipidus, familial hypoparathyroidism, pseudohypoaldosteronism)
Metabolic disorders (e.g., malnutrition, glycogen storage disease type I, MPS I H [Hurler syndrome], MPS I S [Scheie syndrome], fucosidosis)
GU disorders (e.g., Klinefelter syndrome, nephrosis)
Allergic/immunologic disorders (e.g., hypogammaglobulinemia, prolonged infusion with prostaglandin E1, atopic dermatitis)
Neuropsychologic disorders (e.g., anorexia nervosa)
Others (e.g., ectodermal dysplasia, G6PD deficiency)
False-negative results if patient is edematous or if an inadequate quantity of sweat is collected and analyzed
Methodologic and technical errors
Values may be increased to CF range in healthy persons when sweat rate is rapid (e.g., exercise, high temperature), but pilocarpine test does not increase sweating rate.
Mineralocorticoids decrease sodium concentration in sweat by approximately 50% in normal subjects and 10–20% in CF patients whose final sodium concentration remains abnormally high.
Confirmation of a diagnosis of CF requires two positive sweat tests done on different days. Borderline results should be reported with the suggestion that the test be repeated if clinically indicated.
The preferred patient age for testing is after 48 hours. During the first 24 hours after birth, sweat electrolytes are transiently elevated and rapidly decline on the 2nd day. Therefore, sweat testing should not be performed within 48 hours after birth.