Neonatal Screening for Cystic Fibrosis in the Trent Region (UK): Two-Stage Immunoreactive Trypsin Screening Compared with a Three-Stage Protocol with DNA Analysis as an Intermediate Step

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Abstract
Objectives—: To assess neonatal screening for cystic fibrosis using immunoreactive trypsin, either alone or in conjunction with DNA analysis for the AF508 mutation. A novel three-stage screening protocol was compared with the previously introduced two-stage immunoreactive trypsin-DNA protocol. Design—: (a) Collection of data from a 41/2 year period (phase 1) of two-stage immunoreactive trypsin screening. The initial dried blood samples were obtained at 6 days of age and repeat samples at 27 days of age from babies with results above the 99.5th centile. Babies with persistent hypertrypsinaemia were referred for a diagnostic sweat test. (b) Retrospective DNA analysis: Patients with cystic fibrosis diagnosed in phase 1 were genotyped and most samples from babies with increased initial immunoreactive trypsin but normal results in the second sample were analysed for the δF508 mutation, (c) Phase 2, a prospective study of a three-stage neonatal screening protocol, in which only babies heterozygous for the δF508 cystic fibrosis mutation progressed to the second immunoreactive trypsin test. Setting—: The Trent neonatal screening programme. Subjects—: 437 859 babies born between August 1989 and March 1996. Main outcome measures—: Proportions of unaffected babies requiring a second blood sample or a sweat test. Overall sensitivity for the detection of cystic fibrosis. Results—: The two-stage screen failed to identify six out of 94 cases of cystic fibrosis (without meconium ileus). The introduction of the DNA analysis step would have resulted in one additional case being missed. With the three-stage screen there was a 92% reduction in babies requiring a second blood sample and an 80% reduction in negative sweat tests, results close to the predictions of the retrospective study. Conclusions—: The three-stage screening protocol is a marked improvement on the two-stage immunoreactive trypsin strategy and on the two-stage immunoreactive trypsin-DNA strategy recently introduced in some other screening programmes.