Indirect measurements of sweat electrolyte concentration in the laboratory diagnosis of cystic fibrosis.

Aim: To investigate whether analytical methods based on the colligative physical chemical properties of ions or solutes in sweat are less effective than the specific measurement of electrolytes in the diagnosis of cystic fibrosis (CF).

Methods: A single sweat sample was collected (Macroduct) from each of 211 infants and children, of whom 57 had CF, for the measurement of sodium, chloride, osmolality, and conductivity.

Results: The ranges within which CF and non-CF individual values overlapped (equivocal ranges), were wider for sodium and osmolality measurement than for chloride or conductivity. Neither of the latter two measurements provided a discriminatory advantage over the other. The utilisation of broadly based age related ranges for non-CF control subjects served to improve the discriminatory power of all four measurements to an extent that, in this cohort, both chloride and conductivity provided complete discrimination.

Conclusion: Sweat conductivity is as effective as chloride measurement in the laboratory diagnosis of CF.