Conventional Cytogenetic Analysis of Constitutional Abnormalities A 20-Year Review of Proficiency Test Results From the College of American Pathologists/American College of Medical Genetics and Genomics Cytogenetics Committee

Context.—The joint College of American Pathologists/ American College of Medical Genetics and Genomics Cytogenetics Committee works to ensure competency and proficiency of clinical cytogenetics testing laboratories through proficiency testing programs for various clinical tests offered by such laboratories, including the evaluation of constitutional abnormalities. Objective.—To review and analyze 20 years of constitutional chromosome analysis proficiency testing results (2003–2022), primarily utilizing G-banded karyograms. Design.—A retrospective review of results from 2003 through 2022 was performed, identifying challenges addressing constitutional disorders. The chromosomal abnormalities and overall performance were evaluated. Results.—A total of 184 cases from 161 proficiency testing challenges were administered from 2003 through 2022. Challenges consisted of metaphase images and accompanying clinical history for evaluation of numerical and/ or structural abnormalities. Of the 184 cases, only 2 (1%) failed to reach an 80% grading consensus for recognition of the abnormality. Both cases illustrated the limitations of correctly characterizing some chromosomal abnormalities, including recombinant chromosomal abnormalities and isochromosome identification. In addition, 2 cases failed to reach a consensus for nomenclature reporting: 1 with an isochromosome and another with a duplication. Conclusions.—This 20-year review illustrates the high rate of competency and proficiency of cytogenetic laboratories in the correct identification of constitutional chromosome abnormalities.