A reference interval for sweat chloride in infants aged between five and six weeks of age

Ann Clin Biochem 2009;46:73-78
© 2009 Association for Clinical Biochemistry


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Original Articles

Rajamanickam Jayaraj1,
Paul V Barton2,
Paul Newland2,
Roger Mountford3,
Nigel J Shaw4,
Emma Mccarthy3,
David M Isherwood2 and
Kevin W Southern1

1 Institute of Child Health, University of Liverpool;
2 Department of Clinical Biochemistry, Royal Liverpool Children’s Hospital;
3 Department of Clinical Genetics;
4 Department of Neonatal Medicine, Liverpool Women’s Hospital, Liverpool, UK

Corresponding author: Dr Kevin W Southern, University of Liverpool, Royal Liverpool Children’s Hospital, Eaton Road, Alder Hey, Liverpool L12 2AP, UK. Email: kwsouth{at}liv.ac.uk

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 (MacroductTM [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 MacroductTM). The order of testing was randomly assigned. Filter paper samples (QPIT) were analysed using flame photometry and coulometry. MacroductTMsamples were analysed using ion-selective electrodes (ISE, AbbottArchitect c8000, UK).

Results: Insufficient sweat was collected on 28 occasions with the QPIT (coulometry) method and on 31 with the MacroductTM (ISE) capillarysystem. We achieved a 92% success rate in undertaking two sweatcollections consecutively (n = 177). Sweat chloride concentrationswere normally distributed with excellent limits of agreementbetween 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.

Conclusion: The MacroductTM (ISE) capillary sweat collection system is validin this age group. Sweat chloride concentrations above 30 mmol/Lshould prompt assessment in a specialist CF centre.

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