Dried blood spot screening for cystic fibrosis [letter].

We have measured immunoreactive trypsin IRT activity in dried blood spot samples 
and have found abnormally high levels in infants with cystic fibrosis 
(CF). We agree with Crossley et al. that this test deserves 
further investigation as a screen for CF in the newborn. The 
low value for the 94 week infant is consistent with their 
finding that, in children with low pancreatic enzyme output, the increased 
IRT results are restricted to the first few months of life. 
It remains to be proved whether older CF patients with residual 
pancreatic enzyme secretion do indeed continue to have increased IRT levels. 
If they do, the IRT test would also deserve further investigation 
as a tool, to be used in conjunction with stool pancreatic 
enzyme activity, in smaller centres who have had problems with inaccurate 
sweat tests. 