Background: FGFR2 status of a patient with metastatic gastric adenocarcinoma could be an important factor in determining optimal treatment strategy with FGFR2 inhibitors and antibodies. The question remains how well different assays agree on the FGFR2 status of the same patient and whether one test can be substituted by another. Methods: Pairwise comparison of 4 tests based on the same patient population was performed: 3 IHC assays [Abcam clone EPR24075-418, R&D clone 98706, Santa Cruz clone C-8] and one FISH test. One hundred and nine formalin-fixed, paraffin embedded samples (including 64 primary tumors and 45 metastases of same patients) were obtained and were stained with FGFR2 IHC assays. Two trained pathologists independently evaluated the percentages of tumor staining and their intensity. FGFR2 FISH was performed as described previously [Su, BJC 2014]. The concordance analysis was performed to assess (1) correlation of FGFR2 expression/amplification between different assays in primary and metastases, (2) the predictive properties of one test of another. Results: After evaluating the expression in the first 19 patients, further study was carried out only using the Abсam assay due to pronounced nuclear staining with other IHC tests. FGFR2 any level expression was detected in 29 (47%) primary tumors and 18 (40%) metastases with concordance of 91%. The prevalence of FGFR2 amplification was 9.4% and intratumoral heterogeneity was observed in 33% of FGFR2 amplified cases. Pearson Correlation Coefficients (PCC) were: 0.89, 0.38 and 0.35 between IHC3+/FISH, IHC≥1% stained cells/FISH and IHC≥10%/FISH, respectively. The table represents how well one assay can predict the same outcome (positivity or negativity) of another assay. Conclusions: Among patients who were negative by FISH, 86%-93% of the patients were negative by IHC assay (Abcam). Among patients who were positive by FISH, 75-80% of them were positive by IHC. FISH should not be recommended as a substitute for a FGFR2 IHC assay due to high probability of false negative prediction as a result of intratumoral heterogeneity and low PCC.