Letter: Bronchial lability in cystic fibrosis.

It has always seemed to me that the use of the Exercise 
Lability Index (ELI) distorts results and may be misleading because a 
falsely high value for lability is obtained when the PEF at 
rest is low. It is incorrect to compare percentage values from 
such patients with percentage values from those with figures at rest 
which are near to normal. The Jones Liability Index avoids this 
distortion. I do not suggest that a proportion of CF patients 
do not have abnormal lability but would maintain that the ELI 
introduces inaccuracy which may blur the distinction between CF and other 
respiratory conditions and may lead to the postulation of problems concerning 
lability in CF which do not necessarily exist. Failure to use 
a bronchodilator agent to reinforce brief exercise may also cause distortion 
but in the opposite direction. 