PURPOSE: To provide a detailed description of the recommended surgical technique for gas injection during Descemet's Membrane Endothelial Keratoplasty (DMEK) surgery and highlight intraoperative sign that emerged in case of fluid retention in the interface. METHODS: Gas injection recommended surgical technique is 2 steps procedure using a 1 ml syringe: the first step consists in injecting centrally to the graft and very slowly a bubble of gas smaller than the DMEK graft size, the second step, after checking the absence of fluid interface (no bagel sign), consists in enlarging the gas bubble to the desired filling. RESULTS: Bagel sign allows surgeons to look for interface fluid and correct for potential issues leading to graft detachment. The recommended surgical technique for gas injection also may help to prevent fluid retention in the interface and reduce the risk of graft detachment. CONCLUSION: Detecting a fluid interface retention at the end of DMEK surgery through iOCT or simply looking for the bagel sign may help to prevent post-operative complications.