Assay of serum immunoreactive trypsin in dried blood spots and the early detection of cystic fibrosis.

Immunoreactive trypsin (IRT) content in blood spots from 17 cystic fibrosis (CF) children born between 1975-80 were assayed with reagents from two commercial radioimmunoassay kits. The manufacturers' procedures were modified to allow assay of blood spot IRT. Half of the CF samples had a significantly raised IRT content compared to normal blood spots. Of the blood spots from five children whose meconium screening gave negative results, three or five (depending on the kit used) had a raised IRT content. On the other hand, less than 50% of children initially detected by meconium screening had raised blood spot IRT values. In our hands, analysis of blood spots for IRT does not consistently identify the newborn with CF and offers no advantage over other screening methods.