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Fragile X DNA Testing: by Annette K. Taylor, M.S., Ph.D. The fragile X DNA test has revolutionized fragile X syndrome diagnosis and accompanying genetic counseling. It has been available since 1991 and provides definitive diagnosis of fragile X syndrome and extremely accurate carrier detection. Reliable for people of any age, it can also be performed prenatally. It has superseded the fragile X cytogenetic test due to its greater reliability and accuracy in diagnosis and its ability to identify unaffected carriers.
Many asymptomatic carriers of fragile X syndrome are unaware they are carriers because there may not be a family history of fragile X syndrome, or because a relative with fragile X syndrome may not have been diagnosed. The carrier rate in females is quite high, at approximately 1/300. It is recommended that fragile X carrier testing be offered to all women of reproductive age who have a relative with mental retardation of unknown cause. This article offers guidance in the following topics related to fragile X testing: Indications for testing Arrangements to make before testing Collecting and shipping the sample The fragile X DNA test From sample to result: A glimpse into the laboratory The report: What do the results mean? The benefits of fragile X testing Resources on testing Printer friendly text-only version for entire diagnosis section Annette K. Taylor, M.S., Ph.D. |
Although fragile X syndrome is the most common cause of inherited intellectual impairment, it is
underdiagnosed. Awareness of fragile X syndrome and the utility of the fragile X DNA test is growing among non-geneticist physicians but is not yet widespread. Knowledge of this disorder is particularly
important in pediatrics, neurology, obstetrics/gynecology and general practice. Wide variability in the clinical presentation of the disorder is one reason the diagnosis may be missed. Although certain
physical and behavioral features are often associated with fragile X syndrome, they are not always present. In at least 10% of cases in males, intellectual impairment is the only presenting sign. The
classic triad of long face, prominent ears and macroorchidism is present in just 60% of cases. Mental retardation is not a constant, either. Approximately 15% of males with fragile X syndrome have an IQ
above 70 (Hagerman et al., 1994) . In cases such as these the possibility of fragile X syndrome may not be considered. Similarly, females with fragile X syndrome may not be correctly diagnosed because symptoms can be subtle. |
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