XSense® may be more suitable for population-based screening.

Quest Diagnostics’ XSense®, Fragile X with Reflex, a test for fragile X syndrome, has been approved by New York State’s Department of Health. It is reportedly the first test for fragile X syndrome to be sanctioned by New York to employ a new laboratory analysis technique that bypasses the need to perform the Southern blot DNA analysis method in 99% of cases.

Southern blot takes several days to weeks, while XSense provides results in about a week for most patients, according to Quest Diagnostics. It employs a technique called triplet-primed polymerase chain reaction by capillary electrolysis (triplet-primed PCR-CE) to assess the number of CGG repeats.

With the clearance, Quest Diagnostics can offer the test to physicians in New York as well as in all other U.S. states. New York approves laboratory-developed tests that are not FDA cleared before allowing them to be performed on patients in the state. To gain approval, labs must validate that a test performs as it is intended, based on validation data collected according to CDC, Prevention’s Clinical Laboratory Improvement Amendments, and New York State requirements. New York is the only state that independently approves laboratory-developed tests, which are also regulated at the federal level.

 “New York’s approval is significant because it means a new, highly innovative genetic analysis technique for fragile X has fulfilled state-required quality standards that are widely regarded in the lab industry as highly rigorous,” remarks Charles Strom, M.D., Ph.D., medical director, Genetic Testing Center, Quest Diagnostics Nichols Institute. “While it is highly prevalent, fragile X is not widely tested for, due in part to technical limitations with conventional tests that our XSense technique largely surmounts.”

Physicians can use XSense to aid their identification of women who, as carriers, may be unaffected or slightly affected by fragile X syndrome but are at risk of passing it to offspring. An estimated 1 in 260 women are genetic carriers of fragile X syndrome, according to the National Fragile X Foundation. Quest Diagnostics’ scientists believe, however, that carrier prevalence may be higher.

Medical guidelines recommend fragile X testing for some patients such as women seeking reproductive counseling with a personal or family history of mental retardation. They do not, however, support population-based carrier or newborn testing, in part due to technical lab-testing hurdles, Quest Diagnostics points out.

Since the XSense technique bypasses the need for Southern blot in more than 99% of cases, Quest Diagnostics concluded that it would be suitable for high-volume population screening and diagnostic testing on a range of patients including women and newborns. The firm’s research was published in the March issue of Genetics in Medicine. It found that the XSense technique showed 100% agreement with the widely used lab-testing method, which requires Southern blot analysis in about 20% of cases.

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