Parallel, not sequential: Why upfront multi-biomarker testing should define first-line gastric cancer care

First-line treatment of unresectable or metastatic gastric adenocarcinoma now depends on HER2, programmed death-ligand 1 (PD-L1), mismatch repair/microsatellite instability (MMR/MSI), and CLDN18.2 results, and guidance recommends that results be available as soon as possible. Implementation, however, often remains fragmented: serial ordering consumes limited biopsy tissue and can force empiric treatment before the full biomarker profile is known. Chemotherapy should not be withheld while results are pending, but delays may postpone evidence-aligned treatment additions and complicate choices that registration trials evaluated from treatment initiation. We outline a resource-adaptive biomarker-first workflow based on coordinated upfront testing, pathology-led tissue allocation, central referral when local assays are unavailable, and assay-specific reporting. Testing should be parallel; treatment interpretation should remain evidence-structured and individualized when markers overlap. The immediate task is not to establish another biomarker list, but to make an accepted panel reliable across settings.