title: Discrepancies in reporting the CAG repeat lengths for Huntington's disease.
abstract: Huntington's disease results from a CAG repeat expansion within the Huntingtin gene; this is measured routinely in diagnostic laboratories. The European Huntington's Disease Network REGISTRY project centrally measures CAG repeat lengths on fresh samples; these were compared with the original results from 121 laboratories across 15 countries. We report on 1326 duplicate results; a discrepancy in reporting the upper allele occurred in 51% of cases, this reduced to 13.3% and 9.7% when we applied acceptable measurement errors proposed by the American College of Medical Genetics and the Draft European Best Practice Guidelines, respectively. Duplicate results were available for 1250 lower alleles; discrepancies occurred in 40% of cases. Clinically significant discrepancies occurred in 4.0% of cases with a potential unexplained misdiagnosis rate of 0.3%. There was considerable variation in the discrepancy rate among 10 of the countries participating in this study. Out of 1326 samples, 348 were re-analysed by an accredited diagnostic laboratory, based in Germany, with concordance rates of 93% and 94% for the upper and lower alleles, respectively. This became 100% if the acceptable measurement errors were applied. The central laboratory correctly reported allele sizes for six standard reference samples, blind to the known result. Our study differs from external quality assessment (EQA) schemes in that these are duplicate results obtained from a large sample of patients across the whole diagnostic range. We strongly recommend that laboratories state an error rate for their measurement on the report, participate in EQA schemes and use reference materials regularly to adjust their own internal standards.
authors.affiliation: Department of Clinical Genetics, Sheffield Children's Hospital, Sheffield, UK. Oliver.quarrell@sch.nhs.uk
authors.forename: Oliver W
authors.initials: OW
authors.lastname: Quarrell
authors.forename: Olivia
authors.initials: O
authors.lastname: Handley
authors.forename: Kirsty
authors.initials: K
authors.lastname: O'Donovan
authors.forename: Christine
authors.initials: C
authors.lastname: Dumoulin
authors.forename: Maria
authors.initials: M
authors.lastname: Ramos-Arroyo
authors.forename: Ida
authors.initials: I
authors.lastname: Biunno
authors.forename: Peter
authors.initials: P
authors.lastname: Bauer
authors.forename: Margaret
authors.initials: M
authors.lastname: Kline
authors.forename: G Bernhard
authors.initials: GB
authors.lastname: Landwehrmeyer
mesh_terms: D000483:Alleles; D003951:Diagnostic Errors / Q000706:statistics & numerical data*; D005820:Genetic Testing / Q000379:methods / Q000592:standards*; D017408:Guidelines as Topic; D006801:Humans; D000071058:Huntingtin Protein; D006816:Huntington Disease / Q000175:diagnosis / Q000235:genetics*; D007391:International Cooperation; D009154:Mutation; D009419:Nerve Tissue Proteins / Q000235:genetics*; D009687:Nuclear Proteins / Q000235:genetics*; D012015:Reference Standards; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D018911:Trinucleotide Repeats
chemical_list: C086055:HTT protein, human; D000071058:Huntingtin Protein; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins