Abstract Aim To investigate the Lyon Consensus Criteria impact on Ph-Impedance diagnosis of GERD and their relation to high resolution manometric tracing results. Background & Methods GERD is a complex disease with a heterogeneous symptom profile and a multifaceted pathogenic basis that defies a simple diagnostic algorithm or categorical classification. The Lyon Consensus defines parameters on esophageal testing that conclusively establish the presence of GERD and characteristics that rule out GERD. We retrospectively evaluated according the new criteria our patients underwent pH-Impedance one year before the publication of Lyon Consensus Criteria and analyzed the differences in relation to the older diagnostic criteria. Results 130 patients (56 M, median age 52,03 R18-77) Ph-impedance exams for suspected typical and atypical GERD were re-evaluated. According to Lyon criteria for conclusive evidence of GERD 15,38% of them were over-diagnosticated and 1,53% had inconclusive evidence of the disease using the past criteria. In the evaluation of 94 patients with typical symptoms 23,4% were over-diagnosticated in according to the past diagnostic criteria. Statistical analysis of manometric values of the patients highlight that LES basal pressure (median 108.95, R38.7-283.4) (p=0,049) and UES residual pressure (median 7.8 R0.6-63.5) (p=0,003) significantly predict the Lyon criteria pH-impedance outcome. Analysis of manometric values of the patients with typical symptoms revealed that LES residual pressure (median 5,21 R0,1-20,7) (p=0,007) and Total Number of refluxes (median 28,03 R1-122) (p=0,013) significantly predict the Lyon criteria pH-impedance outcome. Conclusion Lyon Consensus Criteria significantly changes the pH-impedance diagnosis in patients undergoing testing for suspected GERD. In patients with typical symptoms new criteria allow not to overdiagnose 25% of GERD patient. Manometric values analysis significantly correlates and predict the Lyon criteria pH-impedance outcome.