BACKGROUND: In 2023, the diagnostic criteria for gastroesophageal reflux disease (GERD) were updated in the Lyon Consensus 2.0. It is important to investigate the diagnostic accuracy of complementary tests used for GERD diagnosis. OBJECTIVES: To determine the diagnostic accuracy of mean nocturnal basal impedance (MNBI) measured by pH-metry in patients with suspected GERD compared to the updated 2023 Lyon Consensus Diagnostic Criteria and to identify the optimal cutoff point for diagnosis. DESIGN: Diagnostic test study based on a retrospective cohort of patients with suspected GERD. METHODS: Diagnosis was confirmed using the updated Lyon criteria as the gold standard. We evaluated sensitivity, specificity, and discriminatory ability via area under the receiver operating characteristic curve (AUC-ROC). The best cutoff point was determined using Liu's method for MNBI, number of reflux episodes, number of acid reflux episodes, DeMeester score, and bolus exposure time. ROC curves were compared using DeLong's method. RESULTS: A total of 55 patients were included, with 26 diagnosed positive and 29 negative for GERD. MNBI yielded an AUC-ROC of 0.77 (95% confidence interval (CI): 0.64-0.89); at 1590 ohms, sensitivity and specificity were 69.2% and 72.4%, respectively. The DeMeester score demonstrated superior discriminatory ability over MNBI ( = 0.03), with an AUC-ROC of 0.90 (95% CI: 0.80-1.00), sensitivity of 92%, and specificity of 93% at a cutoff of 15.6. No significant differences in discriminatory ability were observed when comparing MNBI with other complementary tests ( < 0.05). CONCLUSION: MNBI and other complementary tests showed suboptimal characteristics for GERD diagnosis. These tests should be interpreted alongside clinical, endoscopic, and pH-metry findings. The DeMeester score seems promising, but further prospective studies are needed to prove its utility.