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Editor,—As I understand the Scientific Method, a statement purporting to be factual, either in a scientific article or in a discussion with peers, must be supported by cited evidence that may be publicly examined for its scientific veracity.
The paper by Heeley et al 1provides data to illustrate the equivalence of conductivity and chloride in cystic fibrosis (CF) diagnosis, and therefore corroborates the findings of an earlier clinical trial by Hammondet al.2 Further, a statistical comparison of the extensive published sweat chloride data of Shwachmanet al with the conductivity data of Hammond shows that the two are of equal discriminatory power in CF diagnosis.3
Despite this evidence, Dr LeGrys has authored a document4that contains a number of assertions on this subject and on other aspects of sweat testing, that are not supported by any published results of original work of which I am aware. No clinical trial data exist which show that conductivity should only be used as a screen, that it is in any way inferior to chloride as a reliable diagnostic discriminator, or that conductivity readings of 50 mmol/l are positive for CF. Dr LeGrys' call for more studies on this matter may be seen as an evasion of the true issue. I suggest that the time has come, albeit belatedly, for her to substantiate her case, not with opinions, but by providing proper citations for relevant experimentally obtained data to support her contentions in the said document.
In a separate article5 Dr LeGrys refers to conductivity as a “qualitative” assay, appearing to infer that it is less reliable than chloride analysis. The term “quantitative”, used in the pad-absorption method merely indicates that gravimetric means are used to measure the obtained sweat. It is obvious that this must be done to allow measurement of chloride concentration since elution of collecting pads is involved. The conductivity method is unequivocally quantitative because it measures a solution property in a micro cell of defined geometry. The inference is therefore absurd and irrelevant.
LeGrys, in her letter makes the incredible statement that since sodium is not as reliable as chloride as a discriminator it would seem “logical” (sic), that because conductivity measures both, the discriminatory advantage of chloride would be cancelled out. The logic of this is difficult to comprehend. Increase in sweat chloride due to functional aberration of the chloride channel must be compensated by increase of one or another of the available cation species—for example, potassium, sodium, or ammonium, in order to satisfy the Law of Electrical Equivalence. Such an increase in chloride will therefore be reflected by a proportionate increase in the total electrolyte concentration, which is the basis of analysis by electrical conductivity.
It is regrettable that lack of proper attention to basic scientific principles has persisted in the NCCLS guidelines for sweat testing for a considerable time without correction and has produced increasing confusion among medical technologists, particularly in the United States. It is sincerely hoped that the author of this document will see fit to amend it appropriately by substituting scientific accuracy for prejudice.
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