Cystic Fibrosis (CF) is one of the most common inherited disorders, primarily caused by mutations of a single gene, CF transmembrane conductance regulator (CFTR). An estimated 1.2 million individuals are tested for CF every year in the US (1). Over 127 mutations are associated with the pathogenesis of CF (1). The simultaneous identification of each mutation seems a daunting diagnostic task. Furthermore, the genotype and phenotype are not well correlated. Some patients have mutations but manifest no clinical symptoms, and vice versa. Molecular testing can add important value to the diagnosis of CF, but it alone is not sufficient.
Sweat chloride testing is essential in the work up of patients with suspected CF because it is a direct assessment of phenotypic function of CFTR. One clinical manifestation of mutations in the CFTR gene is lack of reabsorption of sodium chloride in the sweat glands, which results in significant elevations in sweat chloride concentrations. Because of its direct assessment of phenotype, sweat chloride testing has been a diagnostic gold standard for CF for the last several decades.
The original paper-based screening test for CF was used prior to the debut of the sweat chloride test (2). A piece of flexible paper impregnated with chromate salt was pressed against palms of patients. The elevated sweat chloride would generate a colorimetric chloride precipitation on paper. Although the assay is inexpensive, rapid and easy to perform, the detection is not quantitative. Even worse, the involved silver chromate is a carcinogen. The Cystic Fibrosis Foundation clearly regards the chloride precipitation reaction as “inappropriate” for applying on skin.
Many hospitals began to collect sweat using induction devices like the MacroductTM system, and analyze the chloride content on a chloridometer. These systems are superior to paper-based tests for quantitative detection. However, the cost and operation, not surprisingly, is high and complex. Additionally, the collection and analyss of sweat are segmented. It requires manual handling of sweat, more or less increasing the likelihood of condensation error due to the evaporation. Besides, the consequent inconvenience and diagnostic dilemma restrict the instrumental diagnosis to specialized CF centers.
Encouraged by recent vigorous progress of paper-based microfluidic analysis, we looked at the diagnostic utility of a modernized paper- based sweat chloride test (4). The test employs an inexpensive but reliable ion-exchange paper (0.012$/cm2) to exchange chloride (the most abundant anion in sweat) with hydroxide ion to increase the pH of sweat and change the color of an adjacent pH test paper. The colorimetric results can be instantly captured and analyzed by a smart phone, which turns out to be quantitative (R2=0.994 from 0 to 100 mM).
The new paper-based test is advantageous for many reasons. First, the test is free of biohazards and can be directly attached to skin, just like a “Band Aid”, for an on-site detection. Because of the weak base ion-exchange group, the pH would not exceed 9.0. Second, the on-site detection and compact configuration minimize the possibility of evaporation error. Third, the detection is quantitative and specific, satisfying the needs of diagnosis. Other advantages include accessibility, a short testing time (< 5 min) and a tiny sweat volume requirement (1-2 µl). A clinical trial in China is currently ongoing.
For these reasons, paper-based CF testing is promising. These tests could be developed at the point of care and even reduce the need for specialized CF centers. Is it time to give paper based sweat chloride testing another chance?
1. Sosnay PR, Siklosi KR, Van Goor F, Kaniecki K, Yu H, Sharma N, et al. Defining the disease liability of variants in the cystic fibrosis transmembrane conductance regulator gene. Nature genetics 2013;45(10):1160-7.
2. Knights EM, Brush JS, Schroeder J. Simplified screening test for cystic fibrosis of the pancreas. JAMA: The Journal of the American Medical Association 1959;169(12):1279-80.
3. Warwick W, Hansen L, Werness M. Quantification of chloride in sweat with the cystic fibrosis indicator system. Clin Chem 1990;36(1):96-8.
4. Mu, X, Tian XL, Xu KF, Zheng Z. On-site colorimetric detection of sweat chloride ion for diagnosing cystic fibrosis. Clin Chem 2013;59(10S):A181.