8
Optimal Algorithm
Figure 2. Evaluation of HER2 Gene Amplification by ISH
Assay of the Invasive Component of a Breast
Cancer Specimen Using a Single-signal (HER2
Gene) Assay (Single-probe ISH)
HER2 testing (invasive component)
by validated single-probe ISH assay
Batch controls and on-slide controls
show appropriate hybridization
Average HER2
copy number ≥6.0
signals/cell
a
Concurrent IHC 3+
and/or
Concurrent dual-
probe ISH Group 1
c
Average HER2
copy number <4.0
signals/cell
a
Concurrent
IHC 2+
Perform dual-probe
ISH for final result
c
NOTE. e final reported results assume that there is no apparent histopathologic discordance
observed by the pathologist.
a
It is recommended that concomitant IHC review should become part of the interpretation of single-
probe ISH results. e expert panel also preferentially recommends the use of dual-probe instead of
single-probe ISH assays.
b
Using sections from the same tissue samples used for single-probe ISH, perform IHC (if not already
performed) and/or dual-probe ISH. If IHC results are 2+ equivocal, it is recommended to also
performdual probe ISH.
c
If initial assessment of dual-probe ISH suggestive of Groups 2, 3, or 4, follow the algorithm
described in Figure3.
Average HER2 copy
number ≥4.0 and
<6.0 signals/cell
b
Concurrent IHC 0,
1+ and/or
Concurrent dual-
probe ISH Group 5
c
ISH
positive
ISH
negative
