Carrier risk as a function of the proportion of detectable mutations

The degree to which the risk of being a carrier is reduced after a negative test is determined by the number of mutations screened and the proportion of cystic fibrosis (CF) attributable to those mutations. As an example, if a non-Hispanic White person of Northern European descent does not carry the F508del-CFTR mutation (which accounts for 75 percent of all CF cases in this population), their risk of being a CF carrier is reduced from 1 in 25 (the a priori risk based solely on ethnic origin) to 1 in 99. If they do not carry any of the most common mutations causing 90 percent of CF cases in their ethnic group, their risk is reduced to 1 in 246.