Newborn infants are supposed to be screened for cystic fibrosis based on testing of a dried blood spot.
The sensitivity of the screening test is not 100% (reported as 94-97%).
The screening test may be negative because of:
(1) laboratory error
(2) blood trypsinogen level (immunoreactive trypsinogen, or IRT) below the threshold for further testing
(3) testing only for the most common CFTR mutations
Reasons to consider further testing:
(1) history of cystic fibrosis in close family members
(2) failure to thrive, abnormal stools, salt loss syndrome, excessive appetite, or other findings in early infancy
(3) meconium ileus
Further steps to evaluate the patient:
(1) perform a sweat test
(2) refer to a CF specialist
(3) consider CFTR gene sequencing or expanded mutation analysis (especially if negative for common CFTR mutations)
