Re-evaluating the first-tier status of fragile X testing in neurodevelopmental disorders

Purpose Evaluate whether fragile X syndrome (FXS) testing should be transitioned to a second-tier test in global developmental delay, intellectual disability, and autism spectrum disorder in the absence of family history and suggestive clinical features. Methods Determine the diagnostic yield of FXS testing performed by the Alberta Children's Hospital (ACH) Molecular Diagnostic Laboratory between 2012 and 2017. Retrospective chart review of FXS-positive patients to determine presence or absence of suggestive clinical features and family history. Results Of the 2486 pediatric patients with neurodevelopmental disorders tested for FXS, 25 males and 5 females were positive. This corresponds to a 1.2% diagnostic yield of FXS testing at our center. Retrospective chart review of the FXS-positive cases revealed that 96% of FXS patients had either, if not both, clinical features or family history suggestive of FXS present at the time of testing. Only one patient had neither family history nor clinical features suggestive of FXS. Conclusion In 96% of FXS-positive cases, there was sufficient clinical suspicion raised on the basis of clinical features and/or family history to perform targeted FXS testing. We thus propose that in the absence of suggestive clinical features or family history, FXS testing should be transitioned to a second-tier test in neurodevelopmental disorders.