PATHOLOGY UPDATE
Can Ascites or Pleural Fluid Help With Gastric Cancer Biomarker Testing?
A new 2026 study adds evidence for using malignant-effusion cell blocks in advanced gastric cancer—while showing why tissue and fluid results cannot always be treated as equivalent.
October 7, 2026 · Vialen Health
Ascites and pleural effusions are common in advanced gastric cancer. When malignant cells are present, laboratories can sometimes prepare a cell block from the fluid and use it for biomarker testing. A May 2026 study from Peking University Cancer Hospital compared effusion-derived cell blocks with matched gastric-cancer tissue for HER2, PD-L1, CLDN18.2 and FGFR2b. The results are useful, but they do not mean fluid testing automatically replaces tissue.
WHAT THE 2026 STUDY TESTED
The investigators retrospectively evaluated malignant-effusion cell blocks and paired primary or metastatic gastric adenocarcinoma tissue. They used immunohistochemistry for HER2, PD-L1, CLDN18.2 and FGFR2b, with confirmatory fluorescence in-situ hybridization for HER2-positive cases.
THE IMPORTANT CLDN18.2 FINDING
CLDN18.2 had the highest positivity rate in the effusion cell blocks at 34.0%, but CLDN18.2 positivity was significantly lower in effusion cell blocks than in matched primary tumors. Metastatic tissue also showed higher CLDN18.2 positivity and expression than effusion cell blocks. That difference matters when a treatment pathway depends on a specific CLDN18.2 threshold.
SERIAL FLUID SAMPLES WERE FAIRLY CONSISTENT
Among serially collected effusion cell blocks, reported concordance was high over time: 92.2% for CLDN18.2, 98.1% for HER2, 94.8% for tumor PD-L1 and 99.1% for FGFR2b. This supports the idea that an adequate malignant-effusion cell block may provide useful longitudinal information in selected patients.
WHY THIS DOES NOT MAKE TISSUE OPTIONAL
A fluid sample and a tissue biopsy sample different tumor compartments, and tumor-marker expression can be heterogeneous. The same study found a meaningful CLDN18.2 difference between fluid and tissue. Earlier comparative work also found high but imperfect concordance. The specimen that should guide a treatment decision depends on the clinical question, sample quality and the receiving laboratory or treatment program.
WHAT TO ASK THE PATHOLOGY LAB
If tissue is limited but malignant ascites or pleural fluid is available, useful questions include: Was a cell block prepared? Is there enough viable tumor for the requested assay? Which antibody or platform was used? Is the result reported with the same scoring system required by the treatment program? Is matched tissue available for comparison? The treating oncologist and pathologist should decide whether repeat tissue sampling is necessary.
INTERNATIONAL REVIEW IMPLICATION
For a cross-border review, send the exact specimen source and report rather than summarizing the result as simply “positive.” If a destination program has a threshold or assay requirement, that requirement should be checked before travel. A receiving center may accept existing material, request a new block or slide, or require repeat testing.
VIALEN’S BOUNDARY
Vialen can organize specimen history, pathology reports and destination-center requirements. We do not decide whether effusion cytology is sufficient for an individual treatment decision. That determination belongs to the pathology laboratory and treating or receiving medical team.
References
Educational information only. Testing, diagnosis, clinical eligibility and treatment decisions remain with qualified laboratories, licensed treating physicians and receiving institutions.
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