The sensitivity of molecular testing for cystic fibrosis in this population is approximately 50%.
The sensitivity of molecular testing for cystic fibrosis in this population is actually 75% to 90%. Although it is not absolutely necessary in this patient, it could be useful because this child has nephrotic syndrome and the sweat chloride test result is falsely low. Furthermore, there seems to be a family history of cystic fibrosis. The mother can refuse the test, and an elevated sweat chloride test result should be followed after the nephrotic syndrome resolves. Thus, the main role of the physician in this case is to accurately educate the patient’s family.
With the most current 25-allele–screening panel, the sensitivity of molecular testing for cystic fibrosis is 75% to 90% in non-Ashkenazi white North Americans, 97% in Ashkenazi Jews, 60% in Hispanic Americans, 50% in African Americans, and less than 10% in Asians.
Deletion of the three-nucleotide codon for Phe-508 (ΔF508) accounts for 70% of CFTR mutations in cystic fibrosis. However, currently, there are more than 1300 known additional mutations. Most are rare, and screening for all of them would be very expensive. In the current 25-allele screen, only 7 mutations in addition to the ΔF508 mutation account for at least 1% of cystic fibrosis mutations.
Although molecular testing for cystic fibrosis confirms the diagnosis, many mutations are rare, still unknown, or not available for testing. Thus, a negative molecular test does not exclude the diagnosis, particularly if the sweat chloride test result is positive and the patient has signs and symptoms of cystic fibrosis.
