. Author manuscript; available in PMC: 2022 Apr 21.

Published in final edited form as: Arch Pathol Lab Med. 2016 Apr 15;140(11):1250–1258. doi: 10.5858/arpa.2016-0009-OA

Table 4.

Summary of Associations Between American Society of Clinical Oncology/College of American Pathologists (ASCO-CAP) In Situ Hybridization (ISH) Guidelines Groups, HER2 Protein Expression, and Diagnostic Status in Our Consult Practice

ASCO-CAP FISH Group	ASCO-CAP Designation	Predominant Pattern of HER2 Protein Expression	Consult Practice Diagnoses

1. Ratio ≥2.0, HER2 copies ≥4.0	ISH positive	HER2 overexpression (IHC 2+/3+); P < .001	HER2 amplified
2. Ratio ≥2.0, HER2 copies <4.0	ISH positive	HER2 low expression (IHC 0/1+); P = .02	HER2 not amplified
3. Ratio <2.0, HER2 copies ≥6.0	ISH positive	HER2 mixed protein expression	Mixed category of HER2 not amplified and HER2 amplified cases
4. Ratio <2.0, HER2 copies ≥4.0 and <6.0	ISH equivocal	HER2 low expression (IHC 0/1+); P < .001	HER2 not amplified
5. Ratio <2.0, HER2 copies <4.0	ISH negative	HER2 low expression (IHC 0/1+); P < .001^a	HER2 not amplified

Abbreviations: FISH, fluorescence in situ hybridization; IHC, immunohistochemistry.

P-value is based on the chi-square statistic that tests the hypothesis that equal proportions of tumors (0.25 in each) fall into the 4 IHC expression groups.