Sweat chloride concentration in cystic fibrosis patients with cystic fibrosis trans-membrane conductance regulator 11234V mutation.

C fibrosis (CF) is one of the most common inherited diseases in Caucasian people. It is caused by mutations in both the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a transmembrane glycoprotein.1 One of the main consequences of mutations in the CFTR gene is a dysfunction of ion channels resulting in elevated sweat chloride concentrations, pancreatic insufficiency, and progressive lung disease.2 Sweat testing is a general term referring to the quantitative, or qualitative analysis of sweat to determine the electrolyte concentration, conductivity, or osmolarity for confirmation of a CF diagnosis, and despite the development of genetic testing, analysis of sweat chloride concentration remains the laboratory gold standard for the diagnosis of CF.3 The cut-off chloride value of 60 mEq/L is considered frankly abnormal. It is possible to make diagnosis of CF even with a chloride value <60 mEq/L in the presence of other signs.3 Since the inception of the sweat test 50 years ago, the pad-absorption technique using quantitative pilocarpine iontophoresis test (QPIT) is probably the one most commonly used, and has been accepted as the standard for sweat testing.4 Sweat conductivity represents a non-selective measurement of ions. A Wescor Sweat-Chek conductivity analyzer (Wescor Inc., Logan, Utah, USA) designed specifically for use with Wescor Macroduct sweat collector (WMSC [Wescor Inc., Logan, Utah, USA]), was approved by the Cystic Fibrosis Foundation as a screening method.3 The first objective of this study was to define the pathologic range of chloride level in a sweat test in CF patients with CFTR I1234V mutation, common in a large kindred Arab tribe in the Gulf region,5 utilizing the gold standard QPIT (Gibson-Cooke test), which has become available for the first time in our institution. The second objective was to compare the QPIT with the WMSC system for measuring chloride concentration and sweat conductivity. Forty-one patients (1.3-31 years old), known to have CF with homozygous CFTR mutation I1234V were compared with 18 healthy volunteers (2 children, and 16 adult students and laboratory staff). Cystic fibrosis was diagnosed earlier on the basis of positive sweat results in duplicate using Hitachi 911 (Sysmed Lab Inc., Tokyo, Japan) ion-selective electrodes (chloride concentration >60 mmol/L), and a positive result on gene mutation analysis, plus clinical symptoms, and a positive family history of CF. Informed consent was obtained for all study subjects, and ethical clearance was obtained from the research committee of the Medical Research Centre, Hamad Medical Corporation, Doha, Qatar before enrolling participants in the study. Each patient was tested by QPIT on the right forearm, and by the Macroduct sweat collection system, and chloride and sweat conductivity determination on the left forearm on the same day. All tests were carried out by experienced personnel by following the guidelines of the National Committee for Clinical Laboratory Standards at Hamad Medical Corporation, Qatar, from May 2007 to February 2008. The QPIT was performed with measurements of sweat weight, and chloride concentrations. Sweat specimens were collected concurrently from the right arm by first cleaning the skin with water, then applying a 2-inch-square salt-free gauze pad soaked with 0.5% pilocarpine solution, and covered with a 33-mm-diameter metal electrode. After a 4-minute pause, the skin was rinsed with water and dried thoroughly, reweighed gauze was then applied over the stimulated area, and covered with a Parafilm. After 30 minutes the Parafilm was lifted, the gauze removed with forceps, and then rapidly reweighed in an analytical balance before the electrolytes were eluted with water. A minimum sweat weight of 50 mg was required for analysis. Chloride concentrations were analyzed with a digital Jenway chloridometer (Jenway Ltd, Essex, UK), colorimetric titration method. On the left arm, sweat simulation was achieved by pilocarpine iontophoresis from disposable gel discs, 2.5 cm in diameter that were fitted into circular recessed stainless steel electrodes. Current was applied with a controlled rate-of-change to avoid patient discomfort, and collection was facilitated by a disposable plastic device that provided a shallow concave surface designed to cover precisely the circular skin area previously stimulated. A small amount of a water-soluble dye (10 μL) on the concave surface of the disk allowed easy visualization of the sweat collected. The amount of sweat was sufficient when the capillary was filled (approximately 30 minutes). Both sweat conductivity and chloride concentration was then measured using a Sweat-Chek conductivity analyzer, and a Sherwood colorimeter (Sherwood Scientific Ltd, Cambridge, UK)). Descriptive statistics (mean, median, and standard deviation) were calculated and the associations between