Any of several methods for diagnosing gastroesophageal reflux disease, including endoscopy and direct measurement of esophageal pH. An electrode for detecting the pH is placed in the stomach and a reading taken; then the electrode is withdrawn until it is in the esophagus. Normally, the pH will become more alkaline (i.e., rise) as the electrode is moved from the stomach into the esophagus. If there is acid reflux, the pH will be acid in both the stomach and esophagus.