Cystic fibrosis other diagnostic studies
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Other Diagnostic Studies
Pulmonary function tests measure how well the lungs are functioning, and are used to measure the need for and response to antibiotic therapy. DEXA scans can screen for osteoporosis and testing for fecal elastase can help diagnose insufficient digestive enzymes.
Prenatal diagnosis
Couples who are pregnant or who are planning a pregnancy can themselves be tested for CFTR gene mutations to determine the likelihood that their child will be born with cystic fibrosis. Testing is typically performed first on one or both parents and, if the risk of CF is found to be high, testing on the fetus can then be performed. Cystic fibrosis testing is offered to many couples in the US.[1] The American College of Obstetricians and Gynecologists (ACOG) recommends testing for couples who have a personal or close family history. Additionally, ACOG recommends that carrier testing be offered to all Caucasian couples and be made available to couples of other ethnic backgrounds.[2]
Because development of CF in the fetus requires each parent to pass on a mutated copy of the CFTR gene and because CF testing is expensive, testing is often performed on just one parent initially. If that parent is found to be a carrier of a CFTR gene mutation, the other parent is then tested to calculate the risk that their children will have CF. CF can result from more than a thousand different mutations and, as of 2006, it is not possible to test for each one. Testing analyzes the blood for the most common mutations such as ΔF508 — most commercially available tests look for 32 or fewer different mutations. If a family has a known uncommon mutation, specific screening for that mutation can be performed. Because not all known mutations are found on current tests, a negative screen does not guarantee that a child will not have CF.[3] In addition, because the mutations tested are necessarily those most common in the highest risk groups, testing in lower risk ethnicities is less successful because the mutations commonly seen in these groups are less common in the general population. These couples may therefore consider testing through labs that offer CF screens with a high number of mutations tested.
Couples who are at high risk for having a child with CF will often opt to perform further testing before or during pregnancy. In vitro fertilization with preimplantation genetic diagnosis offers the possibility to examine the embryo prior to its placement into the uterus. The test, performed 3 days after fertilization, looks for the presence of abnormal CF genes. If two mutated CFTR genes are identified, the embryo is not used for embryo transfer and an embryo with at least one normal gene is implanted.
During pregnancy, testing can be performed on the placenta (chorionic villus sampling) or the fluid around the fetus (amniocentesis). However, chorionic villus sampling has a risk of fetal death of 1 in 100 and amniocentesis of 1 in 200,[4] so the benefits must be determined to outweigh these risks prior to going forward with testing. Alternatively, some couples choose to undergo third party reproduction with egg or sperm donors.
References
- ↑ ACOG Committee Opinion #325: Update on Carrier Screening for Cystic Fibrosis. Obstet Gynecol 2005; 106:1465.
- ↑ American College of Obstetricians and Gynecologists and American College of Medical Genetics. Preconception and prenatal carrier screening for cystic fibrosis. Clinical and laboratory guidelines.American College of Obstetricians and Gynecologists, Washington, DC, October 2001.
- ↑ Elias, S, Annas, GJ, Simpson, JL. Carrier screening for cystic fibrosis: Implications for obstetric and gynecologic practice. Am J Obstet Gynecol 1991; 164:1077. PMID 2014829
- ↑ Tabor A, Philip J, Madsen M, Bang J, Obel EB, Norgaard-Pedersen B. Randomised controlled trial of genetic amniocentesis in 4606 low-risk women. Lancet. 1986 Jun 7;1(8493):1287–93. PMID 2423826