External quality assessment programs in cytogenomics in the Lombardy Region, Italy (2018-2024): performance and trends.

BACKGROUND: This study summarizes seven years (2018-2024) of external quality assessment (EQA) activities in cytogenomics conducted in the Lombardy Region, Italy, across four diagnostic sectors: chromosomal microarray (CGH/SNP-array), prenatal cytogenetics, postnatal cytogenetics, and oncohematology. METHODS: A descriptive analysis was performed of the regional EQA cycles, focusing on analytical concordance and on the type and evolution of non-conformities over time. RESULTS: Analytical concordance remained consistently high across all EQA cycles, with correct identification of the primary findings in CGH-array and constitutional cytogenetics. Early rounds were mainly characterized by incomplete reporting, International system for human cytogenomic nomenclature (ISCN) inaccuracies, limited interpretive content, and insufficient communication of methodological limitations, whereas more recent cycles showed progressive convergence toward national (SIGU) and international (ACMG/ClinGen, European) standards. In microarray (CGH/SNP-array) exercises, inter-laboratory variability shifted from basic interpretive and reporting deficiencies to more advanced issues of standardization in 2023-2024. Prenatal and postnatal cytogenetics showed progressive improvement in report structure, ISCN compliance, and methodological completeness. Oncohematology remained the most challenging area, with persistent variability in complex karyotype reconstruction despite improved metaphase adequacy and more complete ISCN reporting. CONCLUSIONS: These findings support the role of regionally coordinated EQA schemes in promoting harmonization of cytogenomic practice, continuous quality improvement, and public health governance.