HER2 overexpression has been demonstrated to play a major role in the onset, development, and progression of breast cancer (BC). About 15-20% of BC patients show HER2 amplification and/or HER2 over-expression, which are associated with increased tumor aggression and poor prognosis. Immunohistochemistry (IHC) is the method of choice to analyze HER2 status in BC patients. Due to the potential broader applicability of current anti-HER2-targeting drugs, the sensitivity of these assays is now of greater importance for selecting eligible patients.

In this context, it has become necessary to evaluate the diagnostic utility of HER2 assays with respect to the detection of not only HER2-positive (IHC 3+, 2+ and FISH positive) but also HER2-low (HER2 IHC 2+ / FISH negative or IHC 1 +) BC cases. We performed an IHC concordance study comparing the HercepTest (mAb) run on the Dako Omnis platform and the PATHWAY 4B5 assay run on the Ventana BenchMark ULTRA using a BC cohort of 119 samples and assessing assay sensitivity and specificity with respect to amplification status and inter-assay & -observer variations.