Discrepancies in reporting the CAG repeat lengths for Huntington's disease
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Published on: December 10, 2021
Oliver W Quarrell1, Olivia Handley, Kirsty O'Donovan
1Department of Clinical Genetics, Sheffield Children's Hospital, Sheffield, UK. Oliver.quarrell@sch.nhs.uk
Insights
Huntington's disease diagnostic testing shows significant CAG repeat length discrepancies between labs. Applying European and American guidelines greatly reduces errors, highlighting the need for standardized reporting and quality control in genetic testing.
Area of Science:
- Genetics
- Molecular Biology
- Neurology
Background:
- Huntington's disease is caused by CAG repeat expansion in the Huntingtin gene.
- Accurate CAG repeat length measurement is crucial for diagnosis and patient management.
- Variability in laboratory testing can impact diagnostic accuracy.
Purpose of the Study:
- To assess the accuracy and consistency of CAG repeat length measurements in Huntington's disease diagnostics.
- To compare results from routine diagnostic laboratories with central reference testing.
- To evaluate the impact of proposed error margins on reported discrepancies.
Main Methods:
- Comparison of 1326 duplicate CAG repeat length results from 121 laboratories across 15 countries.
- Analysis of upper and lower allele discrepancies.
- Re-analysis of 348 samples by an accredited German laboratory.
- Evaluation against American College of Medical Genetics and European Best Practice Guidelines.
Main Results:
- 51% discrepancy in upper allele reporting, reduced to 13.3% and 9.7% with guideline application.
- 40% discrepancy in lower allele reporting.
- Clinically significant discrepancies in 4.0% of cases, with a 0.3% potential misdiagnosis rate.
- High concordance (93-100%) when accredited labs applied error margins.
Conclusions:
- Significant inter-laboratory variability exists in Huntington's disease genetic testing.
- Adherence to standardized error margins is essential for improving diagnostic accuracy.
- Laboratories should report measurement error rates, participate in EQA, and use reference materials.
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.
