Letter: Rates and calcium concentrations of sweat in relation to cystic fibrosis.

The authors of the recent article which failed to show a significant 
difference between sweat calcium concentrations of CF patients and control subjects, 
suggest that the findings refute our hyperpermeability hypothesis. They found sweat 
Ca of 0.32 to 1.67 mEq/liter in 26 control subjects and 
0.42 to 2.03 mEq/liter in 16 CF patients. With these tremendous 
ranges many more subjects would be required to prove or disprove 
a difference. A difference undetectable in a moderate number of subjects 
is probably not of pathophysiologic significance if the Ca really varies 
unpredictably. Sweat Ca, however, is generally believed to vary with the 
rate of sweating, being much higher at lower rates, and measurements 
made at comparable rates might well show significant differences. The authors 
did attempt to control rate, but in a very inappropriate manner. 
They measured no constant rate, but took the average rate during 
the 30 minutes immediately after iontophoresis. This gives little information concerning 
the rate at which the sweat was actually produced. 