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PURPOSE: ERBB2 amplification (human epidermal growth factor receptor 2 positivity [HER2+]) in metastatic colorectal cancer (mCRC) has important therapeutic implications that necessitate the need for accurate diagnostics. The study purpose was to establish and validate harmonized diagnostic criteria for HER2+ mCRC among 3 groups (GI-SCREEN-Japan, NCTN-SWOG-USA, and Korea). PATIENTS AND METHODS: We assessed HER2 status by immunohistochemistry (IHC), ERBB2/CEP17 ratio, gene copy number (GCN) by fluorescence in situ hybridization (FISH), and copy number variation (CNV) using two targeted next-generation sequencing (NGS) panels. Tumor samples from 475 and 16 patients with mCRC in exploratory and validation cohorts, respectively, were used for cross-validation of the NGS panels. RESULTS: Consensus diagnostic criteria among the 3 groups for HER2+ mCRC were established as follows: IHC 3+ or IHC 2+ and ERBB2/CEP17 ratio by FISH ≥ 2.0, tumor content > 10% for surgically resected specimens, and necessary tumor content not defined for biopsy specimens. The median GCN and CNV for HER2+ patients were 10.9 and 27.7 compared with 2.5 (P <.0001) and 3.5 (P <.0001), respectively, in HER2-negative patients. These findings were validated in a validation cohort (GCN, 16.2 v 2.4 [ P =.0002]; CNV, 42.5 v 2.0 [ P =.0003]). GCN correlated with CNV in both cohorts (exploratory, r = 0.90; validation, r = 0.97; P <.0001). CNV in cross-validation of the 2 NGS panels also showed a strong correlation (r = 0.98; P <.0001). CNV in patients who fulfilled the consensus criteria was > 4.0 in all, which demonstrates the accuracy of the IHC/FISH criteria and cross-validation of NGS panels. CONCLUSION: We have established and verified harmonized diagnostic criteria for HER2+ and demonstrated consistency between IHC/FISH and CNV determined by NGS in mCRC. [ABSTRACT FROM AUTHOR]