Pathology tests database

Sample and container requirements

Sweat

Minimum volume required

A minimum sweat volume of 15 µL must be collected over a 25 minute collection period to meet the minimum sweat rate of 1.0 g/m2/min

Special requirements

None

Patient preparation

Sweat tests can be performed after 2 weeks of age in infants greater than 3kg who are normally hydrated and without significant systemic illness. Sweat tests can be attempted in term infants after 7 days of age if clinically important. However, it should be noted that the failure rate of sweat collections is higher than that of sweat tests carried out after 6 weeks of age. (Post term gestation 40/40) and will need repeating if an insufficient quantity of sweat is collected. Sweat tests should be delayed in subjects who are dehydrated, systemically unwell or who have eczema affecting the potential stimulation sites. Sweat tests should be delayed in subjects who are oedematous and/or on systemic corticosteroids. Sweat tests SHOULD NOT be performed in subjects who are on oxygen by an open delivery system. This would not apply to an infant on head box or nasal prong oxygen.

Sample viability

Turnaround time

1 day

Reference range

<50 mmol/L: Cystic fibrosis unlikely >90 mmol/L: Supports the diagnosis of CF (Source: Guidelines for the performance of the Sweat Test for the investigation of Cystic Fibrosis in the UK)

Clinical guidance

Please refer to LabTestsOnlineUK to learn more about sweat sodium. https://labtestsonline.org.uk/tests/sweat-chloride-test

Notes