# HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH)

Source: https://onco.cc/drugs/her2-testing-assays/  
OnCo record `her2-testing-assays` (Treatment). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

The tests that grade a breast or stomach cancer's HER2 level, from the original trastuzumab test in 1998 to the new 'HER2-low' and 'ultralow' cut-offs.

## Summary

HercepTest (Dako, PMA P980018, 25 September 1998) was the first companion diagnostic ever approved, launched with trastuzumab. Roche's PATHWAY anti-HER2/neu (4B5) and INFORM HER2 Dual ISH are the other FDA-approved workhorses, extended to gastric cancer with the ToGA trial. The HER2-low era changed what a score means: in October 2022 the FDA approved PATHWAY 4B5 as the companion diagnostic for trastuzumab deruxtecan in HER2-low (IHC 1+ or 2+/ISH-negative) metastatic breast cancer after DESTINY-Breast04, and in January 2025 extended it to HER2-ultralow (IHC 0 with membrane staining) after DESTINY-Breast06. A score of 0 versus 1+ that once meant nothing now decides eligibility for an antibody-drug conjugate, which has forced pathologists to re-standardise the low end of the scale.

## Fields

- Kind: Treatment
- Status: approved
- Last checked: 2026-09-10
- Tags: test
- Brand: HercepTest / PATHWAY anti-HER2 (4B5) / INFORM HER2 Dual ISH
- Modality: HER2 immunohistochemistry and in situ hybridisation companion diagnostic assays
- Mechanism: Immunohistochemistry for HER2 protein scored 0, 1+, 2+ or 3+ under ASCO/CAP guidelines, with in situ hybridisation (FISH or dual-colour chromogenic ISH) to resolve 2+ cases by measuring ERBB2 gene amplification.
- Approvals: US 1998: HercepTest: HER2 overexpression to select trastuzumab in breast cancer (first companion diagnostic); US 2010: HER2 testing extended to gastric and GEJ adenocarcinoma with trastuzumab (ToGA); US 2022: PATHWAY 4B5 as companion diagnostic for trastuzumab deruxtecan in HER2-low breast cancer; US 2025: PATHWAY 4B5 extended to HER2-ultralow (IHC 0 with membrane staining)

## Notes

- What a result means: 3+ (or 2+ with amplification) is HER2-positive and unlocks trastuzumab-based therapy; 1+ or 2+ without amplification is HER2-low and, in metastatic disease, unlocks trastuzumab deruxtecan.

## Sources

- FDA PMA P980018: https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfpma/pma.cfm?id=P980018
- ASCO/CAP HER2 testing guideline update (2023): https://doi.org/10.1200/JCO.22.02864

## Connected records

- cancers: [Gallbladder cancer](https://onco.cc/cancers/gallbladder/), [Gastric & gastro-oesophageal junction cancer](https://onco.cc/cancers/gastric/), [HER2-positive breast cancer](https://onco.cc/cancers/breast-her2-positive/), [HR-positive / HER2-negative breast cancer](https://onco.cc/cancers/breast-hr-positive/), [Triple-negative breast cancer (TNBC)](https://onco.cc/cancers/tnbc/)
- technologies: [Companion diagnostics](https://onco.cc/technologies/companion-diagnostic/), [Cytogenetics and FISH](https://onco.cc/technologies/cytogenetics-fish/), [Histopathology & immunohistochemistry](https://onco.cc/technologies/histopathology-ihc/)
- targets: [HER2](https://onco.cc/targets/her2/)
- drugs: [Trastuzumab](https://onco.cc/drugs/trastuzumab/), [Trastuzumab deruxtecan](https://onco.cc/drugs/trastuzumab-deruxtecan/)
- companies: [Agilent Technologies (Dako)](https://onco.cc/companies/agilent/), [Roche / Genentech](https://onco.cc/companies/roche-genentech/)
- terms: [FISH / ISH (in situ hybridisation)](https://onco.cc/terms/fish/), [HER2 and genomic testing for biliary cancer on the NHS](https://onco.cc/terms/genomic-testing-biliary-uk/), [HER2 testing in biliary tract cancer (IHC, ISH and NGS)](https://onco.cc/terms/her2-testing-in-biliary-cancer/), [HER2-low and HER2-ultralow](https://onco.cc/terms/her2-low/), [HER2-positive (IHC 3+ or ISH-amplified)](https://onco.cc/terms/her2-positive/), [Immunohistochemistry (IHC)](https://onco.cc/terms/ihc/)
- trials: [DESTINY-Breast04](https://onco.cc/trials/destiny-breast04/), [DESTINY-Breast06](https://onco.cc/trials/destiny-breast06/)
- key papers: [Clinical, pathological, and PAM50 gene expression features of HER2-low breast cancer](https://onco.cc/key-papers/paper-schettini-her2-low-features-npj-breast-cancer-2021/), [Exploring the spectrum of HER2 in non-metastatic triple negative breast cancer: from HER2-null to HER2-low, including HER2-ultralow status](https://onco.cc/key-papers/paper-boissiere-michot-her2-ultralow-tnbc-virchows-arch-2026/), [Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer: ASCO-College of American Pathologists Guideline Update](https://onco.cc/key-papers/paper-wolff-j-clin-oncol/)
- biomarkers: [HER2 IHC 0 (HER2-negative, including ultralow)](https://onco.cc/biomarkers/her2-ihc-0/), [HER2 IHC 1+](https://onco.cc/biomarkers/her2-ihc-1-plus/), [HER2 IHC 2+ (equivocal, reflex to ISH)](https://onco.cc/biomarkers/her2-ihc-2-plus/), [HER2 IHC 3+ (HER2-positive by immunohistochemistry)](https://onco.cc/biomarkers/her2-ihc-3-plus/), [HER2 ISH amplified (ERBB2 gene amplification)](https://onco.cc/biomarkers/her2-ish-amplified/), [HER2-low (IHC 1+ or IHC 2+/ISH-negative)](https://onco.cc/biomarkers/her2-low-ihc/), [HER2-ultralow (IHC 0 with membrane staining)](https://onco.cc/biomarkers/her2-ultralow/)
- roadmaps: [Diagnostics roadmap: stains → gene panels → blood tests that decide treatment](https://onco.cc/roadmaps/diagnostics-roadmap/)

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