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Cystic Fibrosis (CF)

The test used to screen for cystic fibrosis is Immunoreactive trypsinogen (IRT), which is measured by immunoassay (AutoDELFIA®).

Elevated results (above the 99.5th centile) undergo mutation analysis (panel of 4 common mutations). If no mutations are identified yet the initial IRT is above the 99.9th centile, a second sample is collected on day 21, for repeat IRT analysis. If one mutation is identified, a larger panel of mutations is tested. Babies with one mutation detected require a second sample on day 21, for repeat IRT analysis.

Any baby with two mutations or two raised IRT results is referred as 'CF suspected'. Clinical follow up of positive CF cases is usually undertaken by the regional CF team at RMCH, however, there are a few hospitals within the region that carry out their own clinical follow up in collaboration with the regional CF centre (shared care centres). Sweat testing is the diagnostic test for cystic fibrosis. Further genetic testing may also be undertaken. Sweat testing for screen positive cases takes place on Thursdays at RMCH. Families have an appointment with the CF team the same day as the sweat test.

False negative screening results have been reported in some premature babies and may also be associated with viral infection leading to acute gastroenteritis or respiratory illness. Babies with CF and meconium ileus may have a low IRT in the first week. False positive results have been reported in conjunction with chromosomal abnormalities, renal failure, bowel atresia, congenital infections and extreme prematurity. Elevated IRT concentrations can also occur in children who are sick on PICU.

Cut-offs for IRT testing vary with kit lot. The 99.5th centile is determined from local data and we also take part in a national collaboration at each kit lot change (pilot use of new kit lot and data combined from all labs using kit lot). Together these processes help ensure that appropriate cut-offs are in use.