Please note: While some information will still be current in a year, other information may already be out of date in three months time. If you are in any doubt, please feel free to ask.

Multiple sweat tests

My son just had two sweat test done. Both were in the high normal range. 34 and 34. They are concerned because they don't see this happen frequently and want me Full bore with more testing. Any advice?
Dear Parent thank you for your question regarding your son’s sweat test result.

In general, to be able to interprete the values you mention, it is important that the sweat test has been performed by an experienced center using the so-called "pilocarpine ionotophoresis" to measure the chloride content of the sweat.

The majority of people with CF will have a sweat chloride result above 60mmol/l. This is the standard cut off point for a diagnosis of CF both in the UK and USA. However diagnosis of CF may not be clear in up to 10% of cases and certain gene mutations are associated with equivocal or normal sweat test for example R117H.
For infants up to and including 6 months of age a sweat chloride equal or less than 29mmol/l CF is very unlikely, between 30-59mmol/l is equivocal and the infant needs further follow-up. If the sweat chloride is greater or equal to 60mmol/l then CF is likely diagnosis.
In those over 6 months of age a sweat chloride of equal or less than 39mmol/l CF is very unlikely, between 40-59mmol/l is equivocal and further follow-up is required. If sweat chloride is equal or greater than 60mmol/l CF is likely.
In 2005 Cystic Fibrosis Foundation Patient Registry reported a sweat chloride of less than 60mmol/l in only 3.5% of CF patients and only 1.2% of patients had a sweat chloride of less than 40mmol/l.
Those infants that have a sweat chloride greater than 30mmol/l and are less than 6 months of age should have further evaluation. This should include extended CF gene analysis and repeat sweat chloride at 6 month to 12 month intervals until such times as a clear diagnosis is reached. In a small number of people the sweat chloride may remain inconclusive. Patients over 6 months, if there are ongoing concerns, should be kept under review and have further investigations and follow-up as deemed necessary. I hope you have found this information useful and you should discuss this further with your local CF team.
Full details of these consensus guidelines for USA and Europe are available through the internet. See references below.

Dr Laura Jenkins
CF Paediatric Associate Specialist

A European consensus for evaluation and management of infants with an equivocal diagnosis following newborn screening for CF 1. J. Cyst Fibros 2009 Jan8(1):71-8
Guidelines for diagnosis of CF in newborns through older adults CF Foundation consensus report J Pediatr 2008 August 153(2):S4-S14

The answer is edited by: Laura Jenkins