Amplification of the HER2 gene in breast cancers testing 2+ weak positive by HercepTest immunohistochemistry: false-positive or false-negative immunohistochemistry?

Background: The majority of cases of breast cancer scoring HER2 weak positive (2+) on immunohistochemistry (IHC) using the HercepTest are not associated with amplification of the HER2/neu gene.

Aim: To examine the reproducibility of IHC in cases scoring 2+ subsequently shown to have gene amplification by fluorescence in-situ hybridisation (FISH).

Methods: A retrospective analysis of 153 cases referred for FISH confirmation of a weak positive HercepTest (2+) result was performed.