Purpose <p>The European Working Group on Sarcopenia in Older People (EWGSOP) revised the consensus in 2018, including that using computed tomography (CT) imaging of the lumbar third vertebra (L3) for the evaluation of muscle mass. However, there is currently discrepancy and confusion in the application of specific cross-sectional and cutoff values for L3. This study aimed to standardize the diagnosis of low muscle mass using L3-CT.</p> Materials and methods <p>This study included patients who underwent radical gastrectomy for gastric cancer between July 2014 and February 2019. Sarcopenia factors were measured preoperatively. Patients were followed up to obtain actual clinical outcomes. We used the cutoff values obtained based on the inferior aspect of L3-CT images to diagnose sarcopenia in three aspects, respectively. Univariate and multivariate analyses were used to compare long-term and short-term postoperative prognostic differences.</p> Results <p>Sarcopenia was found to be an independent risk factor for postoperative complications and overall survival in patients with all three diagnoses of sarcopenia. According to the multivariate model for predicting postoperative complications, patients with inferior-L3 sarcopenia (<i>n</i> = 154,13.8%) had a greater odds ratio (OR) than patients with superior-L3 sarcopenia (<i>n</i> = 220,19.7%) or transverse-L3 sarcopenia (<i>n</i> = 194,17.4%) did (OR, inferior sarcopenia vs. superior sarcopenia, transverse sarcopenia, 2.030 vs. 1.608, 1.679). Furthermore, patients with inferior-L3 sarcopenia had the highest hazard ratio (HR) (HR, inferior sarcopenia vs. superior sarcopenia, transverse sarcopenia, 1.491 vs. 1.408, 1.376) in the multivariate model for predicting overall survival.</p> Conclusion <p>We recommend that when diagnosing low muscle mass using L3-CT, the intercepted cross section should be uniform and consistent with the aspect on which the cutoff value is based.</p>