Annals of Clinical Biochemistry > Volume 46, Number 1 > Pp. 73-78

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Original Articles

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

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 (Macroduct^TM [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^TM). The order of testing was randomly assigned. Filter paper samples (QPIT) were analysed using flame photometry and coulometry. Macroduct^TM 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^TM (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.

Conclusion: The Macroduct^TM (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.

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