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Scenario 3: Family history positive for CF plus carrier screening negative for CF
- Mary had cystic fibrosis carrier screening and the results were negative (no CFTR variant found). The Mayo Clinic GeneGuide™ carrier screening for cystic fibrosis finds 90 out of 100 people who are carriers.
- The chance of Mary being a carrier is now reduced to approximately 3 percent (1 in 40) because after carrier screening a 1 in 10 chance remains that there is a CFTR variant that we cannot detect. Therefore, 1/4 x 1/10 = 1/40.