A reference interval for sweat chloride in infants aged between five and six weeks of age.
BACKGROUND
This study was designed to establish a reference interval for sweat chloride for infants without evidence of cystic fibrosis (CF), aged between 5 wk and 6 wk, a time when sweat testing is an integral part of newborn screening for CF. In addition, we compared the gold standard method of sweat testing (quantitative pilocarpine iontophoresis [QPIT, coulometry]) with an emerging methodology (Macroduct [ISE]).


METHODS
This was a prospective study on healthy infants at 5-6 wk of age. Sweat collection was undertaken at home on both outer thigh areas using two methods (QPIT and Macroduct ). The order of testing was randomly assigned. Filter paper samples (QPIT) were analysed using flame photometry and coulometry. Macroduct samples were analysed using ion-selective electrodes (ISE, Abbott Architect c8000, UK).


RESULTS
Insufficient sweat was collected on 28 occasions with the QPIT (coulometry) method and on 31 with the Macroduct (ISE) capillary system. We achieved a 92% success rate in undertaking two sweat collections consecutively (n = 177). Sweat chloride concentrations were normally distributed with excellent limits of agreement between the two methods of sweat collection and analysis (n = 150). Median (IQR) sweat chloride was 11.2 mmol/L (8-13) with QPIT (coulometry) method with a 99.5th centile (n = 165) of 24 mmol/L.


CONCLUSIONS
The Macroduct (ISE) capillary sweat collection system is valid in this age group. Sweat chloride concentrations above 30 mmol/L should prompt assessment in a specialist CF centre.
Jayaraj
Rajamanickam
R
Institute of Child Health, University of Liverpool, Liverpool, UK.
Barton
Paul V
PV
Newland
Paul
P
Mountford
Roger
R
Shaw
Nigel J
NJ
McCarthy
Emma
E
Isherwood
David M
DM
Southern
Kevin W
KW
D002712:Chlorides / Q000032:analysis*; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D011446:Prospective Studies; D012016:Reference Values; D013542:Sweat / Q000737:chemistry*
D002712:Chlorides