Clinical use
Sweat testing quantitatively measures electrolytes in a sweat sample and is the main test used to support a clinical diagnosis of Cystic Fibrosis (CF). The chloride concentration of the sweat sample is considered to be the most important electrolyte and must be measured. Sweat chloride concentration is related to the defective chloride channel (the defining feature of CF), due to the mutation in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene.
Background
CF is the most frequently observed life limiting autosomal recessive disease in North European populations, with an incidence of 1:2500 live births. In the Caucasian population, the heterozygote carrier incidence is suspected to be around 1:20.
CF is caused by mutations in the CFTR gene, located on the long arm of chromosome 7 with over 1000 mutations identified. The most common mutation (ï„F508) accounts for over 70% of all cases. CF is a major cause of malabsorption and chronic pulmonary disease in children. All cases will eventually develop chronic pulmonary disease; this is responsible for most of the morbidity and nearly all the mortality associated with CF. The disease is systemic, affecting many types of exocrine gland throughout the body. Abnormal electrolyte secretion is a characteristic finding, resulting in an elevation of the sodium and chloride content in the sweat. Pancreatic endocrine and exocrine dysfunctions are common, with 85% of cases presenting with pancreatic insufficiency.
Reference ranges
CF is the most frequently observed life limiting autosomal recessive disease in North European populations, with an incidence of 1:2500 live births. In the Caucasian population, the heterozygote carrier incidence is suspected to be around 1:20.
CF is caused by mutations in the CFTR gene, located on the long arm of chromosome 7 with over 1000 mutations identified. The most common mutation (ï„F508) accounts for over 70% of all cases. CF is a major cause of malabsorption and chronic pulmonary disease in children. All cases will eventually develop chronic pulmonary disease; this is responsible for most of the morbidity and nearly all the mortality associated with CF. The disease is systemic, affecting many types of exocrine gland throughout the body. Abnormal electrolyte secretion is a characteristic finding, resulting in an elevation of the sodium and chloride content in the sweat. Pancreatic endocrine and exocrine dysfunctions are common, with 85% of cases presenting with pancreatic insufficiency.
Specimen requirements
10uL of sweat is the minimum volume required for analysis. Sweat is obtained by an iontophoresis procedure performed by biomedical scientists at the sweat test clinic held in Paediatric Outpatients.
Sweat tests can be performed on infants who are at least two weeks old, greater than 3 kilos in weight, normally hydrated and who have no significant systemic illness. Sweat tests can be attempted in infants greater than 7 days old if deemed clinically important; however the test may need to be repeated if insufficient sweat is obtained. Sweat test should be delayed for patients who are: dehydrated, systemically unwell, have eczema affecting all major collection sites, are oedematous or are on systemic corticosteroids. Sweat tests should not be performed upon patients who are on oxygen via an open delivery system.
Turnaround time
Sweat chloride analysis is usually performed within 1 working day of sweat collection.