Relying solely on Next-Generation Sequencing (NGS) is insufficient for HER2 testing in biliary tract cancers. Data shows NGS misses up to 15% of patients with HER2 overexpression detected by immunohistochemistry (IHC). Performing both tests is essential to avoid denying patients effective targeted therapies.

Waiting to test for HER2 overexpression until a patient progresses creates significant delays for biopsies and results. This can force clinicians to start less effective treatments while waiting and risks exhausting scarce tissue samples needed for multiple biomarker tests. Testing upfront streamlines second-line therapy decisions.

Effective treatment of HER2-driven NSCLC requires more than just identifying mutations. HER2 is a multiplexed biomarker where both genetic mutations (TKD and non-TKD) and protein overexpression (via IHC) are independently actionable. Comprehensive testing is crucial to ensure patients are eligible for the full range of available targeted therapies, including TKIs and ADCs.

When a colorectal tumor loses HER2 protein expression (IHC 0) but retains HER2 gene amplification via NGS, the decision to continue HER2-targeted therapy is guided by the amplification copy number. A low copy number argues against continuing the targeted regimen.

While PD-L1 testing by IHC is standard, the diagnostic workup for metastatic NSCLC must now also include IHC for HER2 and CMET. This is because specific antibody-drug conjugates and other agents are now approved and tied directly to the protein expression levels identified by these tests.

Driven by T-DXd's high efficacy and the known variability of IHC testing, oncologists are pragmatically retesting HER2-negative tumors and treating HER2-low (1+) patients. This blurs official indications to maximize patient access to a transformative drug.

For certain therapies like Enhertu, eligibility is based on immunohistochemistry (IHC), not NGS. Labs must run HER2 IHC in parallel because NGS, as a population-based test, can miss intratumoral heterogeneity (small clusters of positive cells) that IHC can detect, thus identifying more eligible patients for targeted therapy.

While Next-Gen Sequencing (NGS) provides genetic data, IHC directly measures the protein, is faster, cheaper, and requires less tissue. This makes it more scalable for routine clinical use, especially with small biopsy samples. High-level IHC loss correlates well with genetic loss seen on NGS.

The standard HER2 tests were developed to identify HER2-positive tumors, not to precisely quantify low levels of expression. This creates a diagnostic challenge for identifying patients eligible for HER2-low targeted ADCs, requiring closer collaboration with pathology to interpret results that may be near the threshold, such as HER2-zero but with some minimal staining.