Sweat testing for cystic fibrosis [letter].

Bray et al. conclude that the mass screening of newborn babies with 
chloride ion-selective electrodes is a feasible method for the diagnosis of 
cystic fibrosis; they also suggest that this procedure might be used 
in conjunction with meconium analysis to reduce the incidence of false 
positive results. In 1972 we reported the details of a child 
with proved cystic fibrosis in whom raised sweat sodium but normal 
sweat chloride levels were obtained during treatment with cloxacillin. Attention was 
drawn to the implication of the finding with regard to screening 
procedures, and the suggestion was made that, in at least some 
children with cystic fibrosis, the cloxacillin radical might replace the chloride 
ion in the sweat. When mass screening for cystic fibrosis is 
to be undertaken using methods based on the estimation of sweat 
chloride levels, it is therefore prudent to inquire whether the infant 
is receiving medication from any source and, if so, to interpret 
the results with caution. - These are instances of false-negative sweat 
chloride readings, but there is no evidence yet from our 1205 
tests of any negative cystic fibrosis sweat chloride; the statistical chance 
of this is in any case small. These several cases suggest 
a need for a systematic investigation of the effect of medication 
on sweat ion levels in general as well as in cystic 
fibrosis cases. 