BIOMARKER UPDATE
Gastric Cancer Biomarker Testing in 2026: HER2, PD-L1, MSI/MMR & CLDN18.2
A practical 2026 update on the tests that increasingly determine treatment pathways in gastric and gastroesophageal junction cancer.
October 7, 2026 · Vialen Health
A September 2026 multidisciplinary practice guideline from Chinese pathology and gastric-cancer societies systematized biomarker testing for gastric cancer, including HER2, CLDN18.2, MSI/MMR and PD-L1. For an international patient, the useful takeaway is not simply to “get biomarker testing.” The exact specimen, assay, score, date and treatment context can change what a result means and whether it can support a treatment review.
BIOMARKER TESTING IS A TREATMENT-ROUTING PROBLEM
Modern gastric-cancer treatment increasingly depends on pathology and biomarker data before a treatment path can be compared. HER2 can define HER2-directed treatment. PD-L1 and MSI/MMR can influence immunotherapy decisions. CLDN18.2 can define access to CLDN18.2-directed therapy. The 2026 guideline emphasizes who should be tested, when testing should occur, how tests should be performed and the importance of quality control.
HER2: KEEP THE ORIGINAL REPORT, NOT JUST “POSITIVE” OR “NEGATIVE”
HER2 assessment may involve immunohistochemistry and, in appropriate cases, confirmatory in-situ hybridization. A useful case packet should preserve the original pathology report, specimen source, test method and score. A shorthand summary can lose information a receiving center may need to interpret the result.
PD-L1 AND MSI/MMR ANSWER DIFFERENT QUESTIONS
PD-L1 and mismatch-repair or microsatellite-instability testing are not interchangeable. Their clinical use depends on disease setting, treatment history and the treating team’s protocol. An international review should therefore capture the actual PD-L1 scoring system and value, plus the underlying MSI or MMR result rather than a generic “immunotherapy biomarker” label.
CLDN18.2: PERCENTAGE, INTENSITY AND ASSAY DETAILS MATTER
CLDN18.2 is especially easy to oversimplify. In the United States, VYLOY uses an FDA-approved companion diagnostic and a defined threshold for first-line treatment. Other research programs and treatment settings can use different thresholds. A report that only says “CLDN18.2 positive” may be insufficient; the percentage of tumor cells with moderate-to-strong membranous staining, the antibody or assay, and the specimen should be retained whenever available.
WHY SPECIMEN AND TIMING MATTER
Biomarker expression can be heterogeneous across tumor sites and can evolve during treatment. Biopsy tissue can also be limited. A 2026 gastrointestinal-pathology position paper highlighted pre-analytic handling, biomarker heterogeneity and tissue stewardship as practical constraints on predictive testing. This is one reason a current treatment review may need the original slide or block information, not just a copied biomarker line.
WHAT TO COLLECT BEFORE AN INTERNATIONAL REVIEW
Before opening a cross-border treatment review, collect the pathology diagnosis, specimen date and site, HER2 report, PD-L1 score and assay, MSI/MMR result, CLDN18.2 report with percentage and intensity if available, prior systemic treatments, and the most recent disease-status imaging. Missing data do not automatically exclude a review, but they should be identified explicitly before anyone discusses travel.
VIALEN’S BOUNDARY
Vialen organizes evidence, records and treatment-access questions for formal review. We do not interpret a biomarker as proof of treatment eligibility. Testing decisions, pathology interpretation and treatment selection remain with qualified laboratories, treating physicians and receiving institutions.
References
FDA: VYLOY (zolbetuximab-clzb) approval and companion diagnostic
Educational information only. Testing, diagnosis, clinical eligibility and treatment decisions remain with qualified laboratories, licensed treating physicians and receiving institutions.
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