I went looking just recently and found this article from the CDC. <a target=_blank class=ftalternatingbarlinklarge href="http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5313a1.htm
">http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5313a1.htm
</a>
I think basically the IRT is a level that is identified in the screen. If it seems high, then they do the DF508 test, if you get one there they continue looking. So my guess is your docs will want to check for the other mutation for your dd.
Here's the section I thought was most relevant:
Multiple protocols and algorithms are used to screen newborns for CF (Table 1) (Box 1). All protocols begin with a first-tier phenotypic test that measures IRT in dried blood spots. Infants who have an elevated IRT measurement are then referred for further testing. The specific value used to decide whether IRT is sufficiently elevated to warrant further testing varies. Different lab kits for IRT produce varying distributions of IRT measures, and screening programs set cutoffs on the basis of evaluations of specimens from their own populations and the screening protocols and algorithms used (Table 2). Screening programs in five states (Colorado, Connecticut, Montana, New Jersey, and Wyoming) set absolute cutoffs for a normal IRT value on the first newborn blood spot (range: 90--105 ng/mL). Programs in four other states (Connecticut, Massachusetts, New York, and Wisconsin) classify as abnormal specimens in the top 4%--5% of IRT values for a given day or month, and programs in two states (Mississippi and Pennsylvania) use the top 10% of daily IRT values (55).
Programs in five states (Colorado, Connecticut, Montana, South Carolina, and Wyoming) use an IRT-repeat IRT protocol. This protocol requires obtaining a second dried blood spot specimen at approximately age 2 weeks. At this age, elevated IRT values are more specific for CF because IRT values decrease with age in infants without CF. In Colorado, a routine second specimen is collected from all newborns, whereas in the other states, an elevated IRT on the first specimen necessitates collection of a second specimen from those infants. If the repeat IRT is elevated, with cutoffs varying among states, the child is referred for sweat testing. A sweat chloride of >60 mEq/L, confirmed by a second elevated sweat test result, is considered diagnostic for CF (37), although infants with CF might have lower sweat test results (38,56).
In eight states (Connecticut, Massachusetts, Mississippi, New Jersey, New York, Oklahoma, Pennsylvania, and Wisconsin), screening programs use an IRT/DNA algorithm in which a single elevated IRT test is followed by DNA analysis on the same blood spot specimen to detect >1 specific CFTR mutation (55). Those newborns with a positive screening test (i.e., elevated IRT test and >1 detected mutation) are usually referred to a diagnostic center for a sweat chloride test to make a definitive diagnosis, although children with two disease-causing mutations are considered to have CF, regardless of the results of the sweat test (37). The program in one state (New Jersey) tests for only the common DF508 mutation although second specimens are requested for a repeat IRT test if no mutation is detected. Programs in three other states (Connecticut, Mississippi, and Pennsylvania) test first for the DF508 mutation and, if one DF508 mutant allele is detected, apply a multiple-mutation panel to screen for other mutations. Programs in the remaining four states (Massachusetts, New York, Oklahoma, and Wisconsin) use or plan to use a multiple-mutation panel to test all specimens with an elevated IRT.
Programs in all seven states (Connecticut, Massachusetts, Mississippi, New Jersey, New York, Pennsylvania, and Wisconsin) that have implemented IRT-DNA algorithms also include a protocol that identifies infants without an identified mutation but with a highly elevated IRT value as being at risk for CF, because a child might have mutations for which a test is not administered. The cutoff of a highly elevated IRT is variable; cutoffs representing an IRT value of 99.8% or 99.9% of screened newborns or absolute values of 130, 170, or 200 ng/mL are used in different programs (55). Programs in three states (Massachusetts, New Jersey, and New York) treat such infants as positive screens and request that they be referred for sweat testing unless a repeat specimen with a normal IRT is first received. Programs in the remaining four states (Connecticut, Mississippi, Pennsylvania, and Wisconsin) report the results but do not request referrals for sweat testing (55). Screening programs that use mutation testing as part of their screening algorithm can choose from commercially available reagents for detection of multiple mutations (17,57), all of which have been developed to address the prenatal testing market's response to recommendations from the American College of Medical Genetics (ACMG) (58). The ACMG panel was chosen on the basis of the relative frequency of CFTR mutations among U.S. residents with CF (58) but did not take into account genotype-phenotype associations among persons with varying levels of severity of CF (25) or mutations detected with high frequency among certain racial/ethnic populations (59). Inclusion of a greater number of CFTR mutations in newborn screening panels will increase the number of persons identified as carriers and the numbers receiving either an ambiguous diagnosis of CF or a diagnosis of classic CF (17). Determination of which mutations should be included in panels for newborn screening should be developed by a workgroup of newborn screening specialists and persons with CF expertise.
to the best of my knowledge it is something that increases the chances of finding a certain gene, but they don't identify the gene itself until they do genetic testing.