Sweat chloride test for cystic fibrosis
A sweat chloride of 60 mmol/L or higher supports a diagnosis of cystic fibrosis, because faulty chloride transport leaves the sweat unusually salty.
Remember
In a person with symptoms suggestive of cystic fibrosis, a sweat chloride of 60 mmol/L or higher supports the diagnosis, but the result should be confirmed on a separate occasion or by another method and read with the clinical picture.
Cystic fibrosis (CF) is caused by mutations in the CFTR gene, which makes a channel that moves chloride across cell surfaces. The faulty channel leaves too much chloride in sweat, so the sweat is unusually salty. A sweat chloride test measures this directly.
A sweat chloride test is a key diagnostic investigation in a person with symptoms suggestive of CF. A concentration of 60 mmol/L or higher supports the diagnosis. A result of 30 to 59 mmol/L is intermediate, which means it needs further assessment, and a lower result does not exclude every case.
The result should be confirmed with a repeat test or with CFTR gene testing, and read alongside the clinical picture.
The sweat test is used for older children and adults who were not picked up at birth, although NICE advises a CF gene test for adults being assessed for cystic fibrosis, so in adults the gene test may be used first. In the UK, newborn screening starts with a heel-prick blood-spot test, and babies with a suspected positive result are referred for diagnostic assessment, including sweat testing. A positive screen is not itself a diagnosis, and a normal screen does not exclude CF, because screening can miss cases.
Typical clues are repeated chest infections and poor weight gain despite a good appetite. In the lungs, thick mucus is hard to clear and invites infection. Exocrine pancreatic insufficiency (the pancreas not delivering enough digestive enzymes) is common in CF: thick secretions obstruct the pancreatic ducts, which reduces the delivery of enzymes to the intestine and can cause malabsorption (poor absorption of fat and protein). Because some people with CF retain pancreatic function, malabsorption should be assessed rather than assumed. When fat is not digested, it makes stools bulky, greasy and hard to flush away.
Key facts
- Test
- a sweat chloride test is a key diagnostic investigation in a person with symptoms suggestive of cystic fibrosis.
- Supports CF
- a sweat chloride of 60 mmol/L or higher.
- Intermediate range
- 30 to 59 mmol/L needs further assessment, and a lower result does not exclude every case.
- Why the sweat is salty
- faulty CFTR chloride transport leaves too much chloride in sweat.
- Confirming
- a repeat test or CFTR gene testing, read with the clinical picture.
- Newborn screen
- a positive result is not itself a diagnosis, and babies with a suspected positive result are referred for diagnostic assessment, including sweat testing.
Exam traps
- A reduced sweat chloride does not support cystic fibrosis, because faulty chloride transport leaves too much chloride in sweat, and it is a raised sweat chloride that supports the diagnosis.
- A normal newborn screening result does not exclude cystic fibrosis, because screening can miss cases, and it cannot support the diagnosis either.
- A reduced FEV1, the volume of air that can be blown out in the first second, shows airflow limitation in many lung diseases, so it does not identify cystic fibrosis.
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