In cystic fibrosis there is a defect in the cystic
fibrosis transmembrane conductance regulator (CFTR) gene on
chromosome 7. This codes a transmembrane protein
affecting chloride transport and giving rise to excessively viscous
secretions. The disease shows an autosomal recessive
pattern of inheritance, with a relatively high incidence of 1:2500
live births in white Northern European populations. Over
1000 mutations at this locus have been associated with cystic
fibrosis, although some are very rare. The
∆F508 mutation accounts for about 70% of UK cases.
The sodium and chloride levels in the sweat from affected
individuals are high and the chloride concentration usually exceeds
the sodium concentration. Since the 1950s measurement of
sweat electrolytes has been used as a diagnostic test for cystic
Sweat is collected from the flexor surface of the forearm,
or occasionally the inner thigh, after stimulation of the area with
pilocarpine, which is delivered to the sweat glands by a small
electric current - iontophoresis. Sweat chloride and
sodium are then measured.
Request on ICE
The test is performed by appointment only: contact the
The test takes about an hour
The test is usually performed on young children with
symptoms typical of cystic fibrosis, such as repeated respiratory
infections, and/or exocrine pancreatic insufficiency. It
is also undertaken if there is a family history of the disease,
although if the genotype is known DNA studies may be
It is not normally possible to test children under two
months old, because of the physical difficulty of sweat
collection. In children up to six weeks old, a
immuno-reactive trypsin assay may be used to screen for
Sweat tests should not be performed in subjects who are
dehydrated, oedematous, systemically ill, on corticosteroids, have
significant eczema or are being ventilated.
Chloride is measured on the day of collection
Causes for Rejection
Occasionally it is not possible to collect sufficient
sample. A repeat test will be offered at once, or at a
later date. If the sample is still insufficient, and
clinical suspicion is high, thenDNA studies may be
Cystic fibrosis is unlikely with a sweat chloride level of
A chloride level of >60 mmol/L is consistent with a
clinical diagnosis of cystic fibrosis.
Abnormal results will be telephoned to the requesting
Clinician. A confirmatory sweat test will be done if
requested. A blood sample should be sent for
DNA studies, and family studies may be required.
Equivocal results (sweat chloride 40 - 60 mmol/L)
should have a further sweat test performed, and if this is also
equivocal DNA studies should be considered.
Equivocal results may be seen in unaffected children, or
those with atypical cystic fibrosis - i.e. caused by gene defects
other than homogenous ∆F508 mutation.