Clinical Indication

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 fibrosis.  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 Form

Combined Pathology Blood form (Yellow/Black or Blue for GP's)

Availability

The test is performed on Thursday morning, in the Pathology department at Basildon hospital, by appointment only (telephone ext. 3018).  Patients/carers will be in the department for about an hour. A patient information leaflet is available

Specific Criteria

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 appropriate.

Patient Preparation

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 blood immuno-reactive trypsin assay may be used to screen for cystic fibrosis.
Sweat tests should not be performed in subjects who are dehydrated, oedematous, systemically ill, on corticosteroids, have significant eczema, or are being ventilated.

Turnaround Time

Sodium and chloride will be measured on the day of collection.

Specimen

Sweat 

Volume

Minimum 15ul

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, then DNA studies may be useful.

Interpretation

Cystic fibrosis is unlikely with a sweat chloride level of <40 mmol/L.
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.
If the results do not seem appropriate for the clinical condition of the patient, please discuss with the laboratory (ext. 3018).