Reference values for sweat testing

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Even though there have been studies on the effects of age, mainly to establish adult reference values, the reference values for sweat chloride testing have hardly changed over the last 40-50 years. In this issue of The Journal, Mishra et al from Australia have investigated the effect of age and calculated reference intervals by age group. In 282 healthy subjects and 40 subjects with cystic fibrosis (CF) there was no overlap of sweat chloride values between the CF and non-CF groups, although there was overlap of sweat sodium. Sweat chloride concentrations did increase progressively with age, until around 19 years of age. Ninety-five percent confidence intervals for sweat chloride were calculated from the age of 5 years to ≥20 years. These reference values should be extremely useful in clinical practice.

Additionally, Soultan et al from the State University of New York in Syracuse have presented their results on sweat chloride testing in infants identified by newborn screening as heterozygote carriers. They have observed that a small number of infants referred to their center for evaluation of their carrier status had sweat chloride values in the range of 24-29 mM/L, whereas the new standards for infants are using 30-59 mM/L as the borderline range.

The two papers are discussed in an editorial by O'Sullivan and Zwerdling, who point out that these two papers add some clarity, but also some confusion, regarding the interpretation of sweat chloride values. They argue against the lowering of the borderline range based on a relatively small number of observations and expresses concern about the possible effect of raising the spectre of CF in children who may never develop significant symptoms.

Clearly, interpretation of sweat chlorides is becoming more complicated in this age of newborn screening for CF, and we look forward to more research to clarify the outstanding issues.