The CURRENT equivocal range for FISH assays is 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.