Esophageal pH monitoring is a test that measures how often and how long stomach acid is entering the esophagus. A small thin tube is introduced through the nose or mouth and into the stomach, which is then drawn back up into the esophagus. The tube is attached to a monitor which records the level of acidity in the esophagus. The patient records symptoms and activity while the tube is left in place for the next 24 hours. The information from the monitor is compared to the diary the patient provides. This test is helpful in determining the amount of stomach acid entering the esophagus.