Newborns with CF tend to have elevated levels of blood IRT. Although these levels fall within a few weeks after birth, elevated IRT is a reliable biochemical marker to identify newborns at increased risk for CF. NBS is performed in all states and many foreign countries using IRT alone or in combination with molecular testing. If IRT alone is used, a second sample is typically collected and those newborns with two elevated IRT results are called screen positive and are referred for further testing (IRT-IRT model). In a model where CFTR mutation analysis is performed on the original sample for those with an elevated IRT (IRT-DNA model), infants with one or two CFTR mutations identified are called screen positive, and those with two mutations have a presumptive diagnosis (for review, please see the CLSI Approved Guidelines for Newborn Screening for Cystic Fibrosis [3]). Diagnosis of infants with presymptomatic CF leads to better management and long-term outcomes, a fact underscored by a recommendation from the ACMG to include CF as one of their 29 recommended “core conditions” for uniform NBS in the USA [4].

In a symptomatic individual, the identification of two classic CFTR mutations is sufficient to establish a diagnosis for CF; however, sweat testing remains the diagnostic gold standard as recommended by the CF Foundation [5]. In the majority of classic CF cases, both mutations can be identified by a panel of common mutations. For symptomatic individuals in whom two of the common mutations are not identified, full CFTR gene sequencing and, if necessary, CFTR gene deletion/duplication analysis are useful to determine the likelihood that the individual’s clinical features are part of a CFTR-related disorder. The availability of both biochemical and molecular diagnostic tests is particularly useful for individuals with non-classic clinical presentations and normal or borderline sweat tests, at-risk newborns from whom it is difficult to collect sufficient quantities of sweat, and individuals in whom one classic CFTR mutation and one CFTR variant of uncertain or unknown clinical significance have been identified. Identification of familial mutations is useful for testing at-risk relatives.

In 1997, a National Institutes of Health Consensus Development Conference recommended CF carrier testing for all couples planning a pregnancy, regardless of family history. However, at that time, there was no standardization of CF testing among laboratories and no commercially available reagents or kits. The largest challenges to standardize mutation panels were the ethnic diversity and admixture of the US population, which complicated the selection of mutations for a standardized screening panel. The ACMG and the ACOG recommended a panethnic panel of 23 mutations (which was revised from an original 25 mutations) that occur at a frequency of >0.1 % in any of the major US ethnic groups, plus reflex testing for four additional sequence variants under specified conditions (Table 13.1) ([6–8], revised 2011). These organizations developed and distributed an educational document entitled “Preconceptual and Prenatal Carrier Screening for Cystic Fibrosis” to the members of ACMG and ACOG, and established CF carrier screening as a standard of care in the USA.

The ACMG/ACOG panel is not designed for diagnostic testing for individuals with clinical features of classic or non-classic CF or men with CBAVD, although the panel is often used as an initial screen in these clinical settings. A larger mutation panel provides increased detection for symptomatic individuals.