This test is used to measure how often and for how long acid enters the esophagus from the stomach. It is done to evaluate the presence of gastroesophageal reflux disease (GERD), as well as measure how effective GERD is being treated by medication. It is performed by placing a very thin tube through the nose into the esophagus. This is connected to a small recorder that the patient wears for 24 hours. At the end of the test, the tube is removed and data is collected and analyzed from the recorder.