<prompt> Best test to diagnose gastroesophageal reflux disease and quantify acid output is:EsohagogramEndoscopyManometry24 hr pH monitoring  <response>: 24 hr pH monitoring The gold standard, for diagnosing and quantifying acid reflux is the 24-hr pH test. -Sabiston Also Know Using 24-hour pH test an overall score is obtained. It is known as DeMeester score. 