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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.
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.
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. Connects to HER2.
1.HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH) slips into a pocket on HER2.
| Region | Year | Indication |
|---|---|---|
| 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) |
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Three-quarters of chemotherapy-naive TNBC would be low or ultralow by the DESTINY-Breast06 definitions, but ultralow is unlabelled in TNBC and carries no prognostic weight.
One treatment page on OnCo cites this paper by its DOI; this record gives the citation a page of its own so a reader can follow it without leaving OnCo. Read the abstract above alongside the citing page listed under Related; the record was created automatically from the Europe PMC entry and its figures have not been checked by hand.
HER2-low is a drug eligibility label, not a biological subtype, in triple-negative disease; because a third of patients qualify for trastuzumab deruxtecan on a score pathologists disagree about, re-scoring and digital assistance for HER2 0 versus 1+ is a practical gap.
Query for this drug: (TITLE:"HER2 IHC and ISH companion assays" OR ABSTRACT:"HER2 IHC and ISH companion assays" OR TITLE:"HercepTest, PATHWAY 4B5, HER2 Dual ISH" OR ABSTRACT:"HercepTest, PATHWAY 4B5, HER2 Dual ISH" OR TITLE:"HercepTest" OR ABSTRACT:"HercepTest" OR TITLE:"PATHWAY anti-HER2" OR ABSTRACT:"PATHWAY anti-HER2" OR TITLE:"INFORM HER2 Dual ISH" OR ABSTRACT:"INFORM HER2 Dual ISH") AND (cancer OR tumor OR tumour OR oncology OR carcinoma OR lymphoma OR leukemia OR leukaemia OR myeloma OR sarcoma OR melanoma OR glioma). Results are unfiltered search hits about HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH), not a curated reading list.
Shares Agilent Technologies (Dako), Diagnostics roadmap: stains → gene panels → blood tests that decide treatment, Companion diagnostics, Immunohistochemistry (IHC) and the tag test.
Shares Companion diagnostics, Immunohistochemistry (IHC), Histopathology & immunohistochemistry, Roche / Genentech and the tag test.
Shares Companion diagnostics, Immunohistochemistry (IHC), Histopathology & immunohistochemistry, Roche / Genentech and the tag test.
Shares Agilent Technologies (Dako), Companion diagnostics and the tag test.
Shares Companion diagnostics, Histopathology & immunohistochemistry, Roche / Genentech, Gallbladder cancer and the tag test.
Shares HER2 testing in biliary tract cancer (IHC, ISH and NGS), Diagnostics roadmap: stains → gene panels → blood tests that decide treatment, Companion diagnostics, Gallbladder cancer and the tag test.
Shares Diagnostics roadmap: stains → gene panels → blood tests that decide treatment, Companion diagnostics, Roche / Genentech and the tag test.
Shares Diagnostics roadmap: stains → gene panels → blood tests that decide treatment, Companion diagnostics, Triple-negative breast cancer (TNBC) and the tag test.