Methodologies
HER2 status can be tested by IHC, which uses antibodies to determine the level of HER2 protein expression, or ISH techniques (including fluorescence, chromogenic and silver ISH), which employ DNA probes to determine the level of HER2 gene amplification. Samples are classified as HER2 positive if they are IHC 3 or have a positive result with ISH.
IHC assessment of HER2 status is subjective, with scoring on a 0–3 scale.
Figure 5.1 Commercial HercepTest™ kit (Dako) scoring system for immunohistochemical characterisation of HER2 status in breast tumoursa (reproduced with permission from Dako)
ISH techniques provide a quantitative assessment of HER2 status, determined by the number of HER2 signals per cell nucleus. Manufacturers’ scoring recommendations vary depending on whether the kit uses a CEP17 probe in addition to the HER2 probe.
Figure 5.2 HER2-positive status in breast tumours using ISH
With all methodologies, it is essential that every HER2 test used is standardised and validated within each laboratory, and that
quality control (QC) and quality assurance (QA) programmes are in place.
For details on when to use the different HER2-testing methodologies, see the
Testing algorithm.