Immunoreactive trypsin screening for cystic fibrosis.

Immunoreactive cationic trypsin (irCT) was measured in 22 cystic fibrosis (CF) and 132 control infants. IrCT was analysed with radioimmunoassay of dried blood samples collected for PKU screening around the 5th day of life and stored on filter paper. The mean +/- 1 SD level of irCT for the control infants was 42 +/- 19 micrograms/1. Sixteen of the 22 CF children had an irCT level above 100 micrograms/1 (mean + 3 SD) while 6 had a level at or below this cut-off limit. A specificity of 99%, which gives a sensitivity of 73%, and an approximative noise: signal ratio of 30:1, suggests that the irCT test may be unsatisfactory as a neonatal screening method for CF.