Sweat Chloride Test

Introduction

Cystic Fibrosis is one of the most common, life limiting, inherited, multi - organ disorders of childhood. It was a well known fact that the children who suffered from Cystic Fibrosis (CF), tasted salty when kissed. As per an old European saying, “‘Woe to that child which when kissed on the forehead tastes salty. They are bewitched and soon will die’. The disease was soon found to be correlated with excess of sodium & hence chloride the sweat. Sweat chloride testing emerged as one of the gold standard tests for diagnosing CF. It is a reliable test for the diagnosis of CF in approximately 98% of patients with CF.

But why are we testing for sweat chloride in an era of Genomics?

We are now empowered with a new set of advanced and accurate diagnostic modality. CF has been found to be associated with abnormality in a gene called as the CFTR gene or the cystic fibrosis trans membrane regulator gene. The CFTR gene encodes a trans-membrane glycoprotein. This acts as an electrolyte transporter at the apical membrane of epithelial cells. Since the discovery of the gene, over 1200 disease-associated mutations have been identified. However, in view if the large number of mutations available, routine screening tests are not able to detect all CFTR gene mutations, and a negative screening test does not ensure a normal CFTR genotype. Genotyping has also revealed that some CF mutations are associated with milder disease phenotypes and normal or borderline abnormal concentration of sweat electrolytes.