Sweat chloride assay by inductively coupled plasma mass spectrometry: a confirmation test for cystic fibrosis diagnosis

Abstract

The current guidelines for sweat chloride analysis identify the procedures for sweat collection, but not for chloride assay, which is usually performed by methods originally not aiming at the low concentrations of chloride found in sweat. To overcome this limitation, we set up, characterized, and adopted an original inductively coupled plasma mass spectrometry (ICP-MS) method for sweat chloride determination, which was designed for its easy use in a clinical laboratory. The method was linear in the range 8.5E−3 to 272.0E−3 mM, precision exhibited a relative standard deviation < 6%, and accuracy was in the range 99.7–103.8%. Limit of blank, limit of detection, and limit of quantitation were 2.1 mM, 3.2 mM, and 7.0 mM, respectively, which correspond to real concentrations injected into the mass spectrometer of 3.9E−3 mM for LOD and 8.5E−3 mM for LOQ. At first, the method was tested on 50 healthy volunteers who exhibited a mean chloride concentration of 15.7 mM (25–75th percentile 10.1–19.3 mM, range 2.8–37.4 mM); then, it was used to investigate two patients with suspected cystic fibrosis, who exhibited sweat chloride values of 65.6 mM and 81.2 mM, respectively. Moreover, the method was cross-validated by assaying 50 samples with chloride concentration values in the range 10–131 mM, by both ICP-MS and coulometric titration, which is the technology officially used in Tuscany for cystic fibrosis newborn screening. The reference analytical performances and the relatively low cost of ICP-MS, accompanied by the advantageous cost of a single sweat chloride assay, make this technology the best candidate to provide a top reference method for the quantification of chloride in sweat. The method that we propose was optimized and validated for sweat samples ≥ 75 mg, which is the minimum amount requested by the international protocols. However, the method sensitivity and, in addition, the possibility to reduce the sample dilution factor, make possible the quantification of chloride even in samples weighting < 75 mg that are discarded according to the current guidelines.

Sweat Testing and Recent Advances

“…Although not involved in any of the current English language ST guidelines, ICP-MS is utilized by a number of ST laboratories, especially in Australia, the UK, and Italy. Authors confirm this method is as safe and accurate as the conventional coulometric method and suggest it be recognized as a candidate reference method for the monitoring and diagnosis of CF ( 17 , 39 , 40 ).…”

Microsampling of biological fluids for elemental and isotopic analysis by ICP-MS: strategies and applications for disease diagnosis

“…Although a non-metal, a few words must be reserved for the quantication of chloride in sweat of new-borns by ICP-MS to detect cystic brosis, the commonest, inherited life-shortening condition of childhood. 161,162 Coulometric titration is commonly used for such purposes; however ICP-MS has proved to offer sensitivity for the recently reduced sweat Cl reference interval accepted for infants. Other advantages include the low sample volume requirements and the addition of an internal standard.…”

Atomic spectrometry update: review of advances in the analysis of clinical and biological materials, foods and beverages

“…Sweat chloride remains the gold standard diagnostic marker for cystic fibrosis and the use of ICP-MS for sweat chloride analysis is no longer novel. However, Marvelli et al 147 are the first to publish the use of ICP-MS for analysis of chloride in sweat samples collected via the Gibson and Cooke technique . The method boasts an LOQ of 7.0 mM with linearity up to 224.5 mM and acceptable precision (<6% RSD).…”