title: Results from an external proficiency testing program: 11 years of molecular genetics testing for myotonic dystrophy type 1.
abstract: The aim of this study was to examine the performance of laboratories offering assessment for myotonic dystrophy type 1 (DM1) using external proficiency testing samples. DM1, a dominant disorder, has a prevalence of 1:20,000 due to the expansion of CTG trinucleotide repeats in the DMPK gene.
  
  
  
  External proficiency testing administered by the College of American Pathologists/American College of Medical Genetics and Genomics distributes three samples twice yearly. Responses from 2003 through the first distribution of 2013 were analyzed after stratification by location (United States/international). Both the repeat sizes (analytic validity) and clinical interpretations were assessed.
  
  
  
  Over the 21 distributions, 45 US and 29 international laboratories participated. Analytic sensitivity for detecting and reporting expanded repeats (≥50) was 99.2% (382/385 challenges) and 97.1% (133/137 challenges), respectively. Analytic specificity (to within two repeats of the consensus) was 99.2% (1,790/1,805 alleles) and 98.6% (702/712 alleles), respectively. Clinical interpretations were correct for 99.3% (450/453) and 98.2% (224/228) of positive challenges and in 99.9% (936/937) and 99.6% (455/457) of negative challenges, respectively. Of four incorrect interpretations made in the United States, two were probably due to sample mix-up.
  
  
  
  This review of laboratory performance regarding laboratory-developed genetic tests indicates very high performance for both the analytic and interpretative challenges for DM1.Genet Med 18 12, 1290-1294.
authors.affiliation: Department of Molecular and Medical Genetics, Oregon Health & Science University, Portland, Oregon, USA.
authors.forename: C Sue
authors.identifier: 0000-0001-6154-8449
authors.initials: CS
authors.lastname: Richards
authors.affiliation: Department of Pathology and Laboratory Medicine, Women & Infants Hospital and the Warren Alpert Medical School of Brown University, Providence, Rhode Island, USA.
authors.forename: Glenn E
authors.initials: GE
authors.lastname: Palomaki
authors.affiliation: Department of Human Genetics, Emory University School of Medicine, Atlanta, Georgia, USA.
authors.forename: Madhuri
authors.initials: M
authors.lastname: Hegde
mesh_terms: D000483:Alleles; D005820:Genetic Testing; D006801:Humans; D059021:Laboratory Proficiency Testing; D009223:Myotonic Dystrophy / Q000175:diagnosis / Q000453:epidemiology / Q000235:genetics*; D065636:Myotonin-Protein Kinase / Q000235:genetics*; D019680:Trinucleotide Repeat Expansion / Q000235:genetics
chemical_list: C585517:DMPK protein, human; D065636:Myotonin-Protein Kinase