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The page below is a sample from the LabCE course HER2 Testing in Breast Cancer. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Fluorescence in Situ Hybridization (FISH): Equivocal Results

The CURRENT equivocal range for FISH assays are defined by the American Society of Clinical Oncologists (ASCO) and College of American Pathologists (CAP) as:
      • ISH group 2 = HER2/CEP17 ratio ≥ 2.0; average HER2 copy number < 4.0 signals per cell
      • ISH group 3 = HER2/CEP17 ratio <2.0; average HER2 copy number ≥ 6.0 signals per cell
      • ISH group 4 = HER2/CEP17 ratio < 2.0; average HER2 copy number ≥ 4.0 and < 6.0 signals per cell
*NOTE - ISH Group1 represents clearly positive HER2 ISH staining results (average HER2 copy number ≥ 4.0 signals/cell), while ISH Group 5 represents HER2 ISH staining that is definitively negative (average HER2 copy number < 4.0 signals/cell).
  • Patients with a HER2/CEP17 FISH ratio between 1.8 - 2.2 were formerly considered HER2-positive and were eligible for treatment in the adjuvant trastuzumab trials.
  • Equivocal results of a single test (IHC or FISH) require additional action. The same specimen can be used for an alternate test or a new test can be performed (new specimen using same or alternate test).
  • Equivocal IHC samples must be confirmed by FISH analysis of the sample.
  • Equivocal FISH samples are confirmed by counting additional cells or repeating the FISH test.
  • If FISH remains equivocal after additional cells counted or assay repeated, confirmatory IHC is recommended so that HER2 protein expression is known for the sample with true equivocal gene amplification.