IHC is a surrogate marker for gene amplification.
- Patients with 0 to 1+ IHC staining are more than likely negative for HER2 gene amplification
- Patients with 3+ IHC staining are more than likely positive for HER2 gene amplification
The benefits of IHC include low cost and rapid turnaround time.
Patients with 2+ IHC staining are not reliably negative or positive for HER2 gene amplification. Therefore, FISH testing must be performed for gene enumeration in 2+ cases. In 3% of cases, protein expression does not correlate with gene amplification.
The primary benefit of FISH is accuracy. FISH drawbacks include high complexity of test performance, cost of the test, storage/archival over any extended amount of time causes loss of signal and slides are interpretable, and need for a specialized microscope.