To investigate the clinical and pathobiological significance of distinguishing score 0 and score 1+ within the group of immunohistochemistry (IHC)-negative invasive breast cancers.
We studied HER2 status using both IHC and fluorescence in situ hybridization (FISH) in 150 consecutive breast tumors submitted to our laboratory after a negative IHC result in local testing centers.
We were able to discern a group of score 0 tumors that had a lower HER2 copy number than the group consisting of score 1+ tumors. In contrast with the group of score 1+ tumors, HER2 FISH was consistently negative for both copy number-based and ratio-based tumors without equivocal results.
In a setting with stringent quality assurance, score 0 and score 1+ tumors emerge as distinct and clinically important subgroups within the HER2 IHC-negative population.