Cystic fibrosis (CF), a rare genetic disorder, characterized by multisystem involvement including progressive, potentially fatal pulmonary disease, has been described as being the most prevalent inherited fatal disorder in the White population of Northern European origin. However, this autosomal recessive disorder also presents in other populations, including African-Americans, Hispanics, and Asians. In patients with CF,  Researchers first noted sweat chloride abnormality in CF patients in 1953; this spawned the development of the sweat test in 1959. Since the discovery of the CF gene in 1989, which codes for the protein cystic fibrosis transmembrane regulator (CFTR), there are reports of more than 2000 mutations.  The cystic fibrosis transmembrane regulator is at the apical surface of the epithelial cells in the airways, gastrointestinal tract, pancreas, genitourinary system, and the sweat glands in the skin. The defective, deficient or absent CFTR function results in abnormal chloride transport across the chloride channels and abnormal sodium transport along with the secondary effect on water movement across the cell membrane. Decreased chloride secretion along with increased sodium reabsorption (along with water as a secondary effect) across the apical surface of the epithelial cells results in increased viscosity of secretions in the organs involved and in the case of skin, elevated levels of chloride in the sweat. Detection of elevated values of sweat chloride, in a suspected patient, by quantitative pilocarpine iontophoresis test (QPIT) is considered to be the gold standard for the diagnosis of cystic fibrosis.[1]

The Cystic Fibrosis Foundation (CFF) recommends that a sweat test be done at a CFF-accredited care center by a trained technician. The sweat test is typically performed on the patient’s arm or leg. The test involves iontophoresis of pilocarpine to stimulate sweat production by sweat glands. Collection modalities include gauze, filter paper, or macroduct coil.[2] Since the sweat chloride can present as transiently elevated in the first 24 hours of life, the CFF clinical care guidelines recommend that sweat testing should be done after 48 hours of birth or later. Two samples are collected due to the underlying variability of the test and the potential for an insufficient sample. Pilocarpine iontophoresis is done for 5 minutes, followed by sweat collection for not more than 30 min. For the gauze or filter paper method, the stimulated area must be 2 x 2 inches. The minimum quantity of sweat collected for the gauze method is 75 mg, and a minimum sample volume of 15 microliters is necessary for the macroduct method.[2]

The clinician conducts iontophoresis of pilocarpine on the patient's skin. In the original Gibson-Cooke method, two electrodes, one covered in gauze soaked with pilocarpine and the other with deionized water, are placed on the patient's arm or leg. A small, painless, electrical current is then applied for 5 minutes; referred to as iontophoresis. Afterward, a collecting source, such as gauze, filter paper, or a Macroduct coil is used to collect the sweat. The allotted time for sweat collection should not exceed 30 minutes. Using the Gibson-Cooke method for QPIT, a minimum of 75mg of sweat is required adequate testing. However, most centers in the US utilize collection via macroduct coil, which requires only 15 microliters of sweat for analysis. Sweat should not be collected from multiple sites for the same test as the rate of sweating determines the chloride content. The collected source then goes to the lab for quantification of chloride.[2][3]

Indications for the sweat test include individuals suspected of having cystic fibrosis, either through a positive newborn screening test or if clinical features suggestive of CF are present. The sweat test is considered to be the gold standard as a confirmatory test. CF genotyping is recommended to help decide CF mutation-specific therapy, in case the sweat test results are borderline or sweat test is not technically possible (e.g., severe eczema). The CFF recommends CF genotyping in all patients. In the United States, the newborn screen tests for an elevated level of IRT (immunoreactive trypsinogen) and if positive may reflex to a DNA test for known mutations of the CFTR gene, depending on the each state’s protocol. A child demonstrating a positive newborn screen will subsequently require confirmatory sweat testing at a CF center.[3]

In children clinically suspected to have cystic fibrosis, a referral to a CF center for sweat testing is indicated. Patients with cystic fibrosis may present with a myriad of different presentations. Neonates may present with meconium ileus. Smaller children may present with pulmonary complications such as recurrent pneumonia, upper respiratory infections, wheezing, and coughing. Gastrointestinal manifestations include failure to thrive with malabsorptive stools, recurrent abdominal pain. Given the clinical suspicion, children obtain a referral for testing.[4] Table 1 describes some of the common indications for sweat testing. Sweat testing may also serve as a surrogate biomarker for evaluating CFTR function. The result of a sweat test informs clinicians regarding the effects of CFTR-modulating drugs such as ivacaftor.[4]

An elevated level of chloride (over 60 mmol/L) in the sweat is diagnostic for cystic fibrosis, sweat chloride levels in less than 29mmol/L is normal. A level between 30 to 60 mmol/L is considered borderline, and repeat sweat test or further testing is required. This range may reflect a heterozygous carrier which cannot be accurately detected with sweat testing. The result of a positive newborn screen needs to be interpreted together with the sweat to test to reliably diagnose CF. Specific known mutations of CF do not cause elevated sweat chloride, and therefore sweat testing does not provide clarity.[1]

Patients with a positive newborn screen and a negative sweat test sometimes receive diagnoses other than CF, such as CF-related metabolic syndrome (CRMS) and CF screen-positive, inconclusive diagnosis (CFSPID). A normal sweat chloride test does not exclude the diagnosis, in which case the clinician should pursue genotyping or other alternative diagnostic studies (such as nasal membrane potential difference, intestinal current measurement, semen analysis, or assessment of pancreatic function), especially if there is a high index of clinical suspicion for cystic fibrosis.[1]