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discordance between IHC and ISH (single probe silver ISH, SISH; or dual probe fluorescence ISH, FISH). Of the 1,701 HER2 IHC 0 test results retrieved, 99.2% were HER2 SISH negative, 0.8% equivocal, and no SISH positive cases were identified. Of the IHC 0 and SISH equivocal cases, all were subsequently shown to be HER2 negative by further FISH testing. Of the 881 IHC 1+ test results, 96.7% were SISH negative, 3.0% equivocal, and 0.3% were positive. The majority of IHC 1+ cases with equivocal or positive SISH were ultimately shown to be HER2 negative by FISH. The IHC 1+ cases that were subsequently shown to be HER2 amplified by FISH were reviewed and some exhibited either tumour heterogeneity or were in the neoadjuvant setting. Our review supports the current recommendation against reflexive ISH testing of IHC 0 and 1+ cases, and referral for further testing in cases of tumour heterogeneity.

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