A new strategy of neonatal screening for cystic fibrosis. The association of immunoreactive trypsin and molecular biology in dried blood

Cystic fibrosis (CF) screening by means of immunoreactive trypsin (IRT) lacks specificity: only 1 out of 12 hypertrypsinemic neonates has cystic fibrosis. We propose here to analyse the KM.19 polymorphic site in the dried blood spots as an additional test in hypertrypsinemic neonates. A blind retrospective study of 114 hypertrypsinemic samples has been performed after polymerase chain reaction. Twenty-seven of 37 CF (74%) were homozygous for allele 2 (2-2) and could have been diagnosed on the 15th day of life. Fifty-five percent of the infants tested were homozygous for allele 1 (1-1), a very rare feature in CF, conferring them a probability of being normal of 99.8%. At the moment, this test could be of great help in the CF screening, even better than the search for the delta F508 mutation for which 45.9% of CF patients are homozygous.