These tests are used to measure how often material refluxes from the stomach back into the esophagus (gastroesophageal reflux). The pH probe measures acid reflux. The impedance catheter measures both acid and nonacid reflux. Your child’s doctor will decide which test will be most helpful. For both tests, a small, flexible catheter is passed through the nose and into the esophagus. The catheter is attached to a recording device. The catheter is left in place for 24 hours. During this time your child can eat and drink normally.