What does human epidermal growth factor receptor 2 (HER2) testing for breast cancer?

Updated: May 14, 2019
  • Author: Maurie Markman, MD, MS; Chief Editor: Karl S Roth, MD  more...
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Breast cancer specimens with equivocal IHC should undergo validation using a HER2 gene amplification method, such as fluorescence in situ hybridization (FISH). More centers are relying on FISH alone for determining HER2 status.

In general, FISH testing is thought to be more reliable than IHC, but it is more expensive. Equivocal IHC results can be seen in 15% of invasive breast cancers, whereas equivocal HER2 FISH results are seen in less than 3% of invasive breast cancer specimens and those that had previously been considered HER2 positive. Discordant results (IHC 3+/FISH negative or IHC less than 3+/FISH positive) have been observed in approximately 4% of specimens. Currently, no data support excluding this group from treatment with trastuzumab.

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