The pH probe test, formally known as esophageal pH monitoring, is a specialized diagnostic procedure used in gastroenterology to measure the amount of acid flowing back from the stomach into the esophagus. The test is considered the gold standard for objectively documenting acid reflux, which is the underlying cause of Gastroesophageal Reflux Disease (GERD). The test uses a sensor to track the acidity level, or pH, in the lower esophagus over an extended period, typically 24 to 96 hours.
Stomach acid generally has a very low pH, less than 4.0. The test records every instance the esophageal pH drops to this acidic level, indicating a reflux event. This comprehensive measurement helps physicians determine how often and for how long the esophagus is exposed to harmful gastric acid. The resulting data provides an objective, physiological measurement.
The Diagnostic Role of Esophageal pH Monitoring
Physicians utilize esophageal pH monitoring when the diagnosis of Gastroesophageal Reflux Disease is unclear or when a patient’s symptoms are unusual or persistent. It is often ordered if an initial endoscopy, a procedure that examines the esophagus with a camera, does not show clear signs of acid-related damage, leaving the cause of symptoms uncertain. The test establishes an objective link between a patient’s reported symptoms and actual acid reflux events.
Monitoring is also helpful for evaluating patients who have not found relief with standard treatments, such as proton-pump inhibitor (PPI) medications. Performing the test while the patient is taking PPIs helps doctors determine if persistent symptoms are due to continued acid exposure or if they are caused by non-acidic reflux, which requires a different treatment approach. This information is important for guiding the next steps, including potential anti-reflux surgery.
The test plays a role in diagnosing Laryngopharyngeal Reflux (LPR), sometimes called “silent reflux.” LPR occurs when stomach contents travel high enough to affect the throat, voice box, and nasal passages. Symptoms of LPR, such as chronic cough, hoarseness, and frequent throat clearing, are often atypical for GERD. Quantifying the acid exposure provides objective evidence to support an LPR diagnosis and personalize a treatment plan.
How the Test is Performed
There are two primary methods for esophageal pH monitoring, each offering a different approach to data collection: catheter-based monitoring and wireless capsule monitoring.
Catheter-Based Monitoring
The catheter-based method involves inserting a thin, flexible tube, or catheter, through the patient’s nose. The sensor end is positioned in the lower esophagus, approximately five centimeters above the lower esophageal sphincter. The external end connects to a small, portable recording device worn for about 24 hours.
During this period, the patient maintains their normal daily routine, including eating and sleeping habits, to capture a realistic view of reflux activity. The patient keeps a detailed diary, manually logging the times they eat, drink, lie down, and experience any symptoms like heartburn or chest pain. After monitoring, the catheter is gently removed, and the data recorder and diary are returned to the physician for analysis.
Wireless Capsule Monitoring
The wireless method, such as the Bravo system, uses a small, capsule-like pH sensor, roughly the size of a gel cap. This capsule is temporarily attached to the lining of the esophagus during an upper endoscopy, typically performed with sedation. The capsule is secured to the esophageal wall with a tiny pin and suction, and wirelessly transmits pH data to a small receiver worn by the patient, often for 48 to 96 hours.
Patients using the wireless capsule must also record their activities and symptoms in a diary or by pressing corresponding buttons on the external receiver. The primary benefit of the wireless capsule is increased comfort and convenience, as there is no visible tube, allowing the patient to carry out more normal activities. The capsule naturally detaches from the esophageal wall after several days and passes harmlessly through the digestive tract.
Understanding the Results and Acid Exposure Metrics
Once the data is collected, a physician analyzes the readings to determine if the acid exposure in the esophagus is considered pathological.
The most direct metric is the Total Acid Exposure Time (AET), which is the percentage of the total monitoring time that the pH in the esophagus was below 4.0. An AET less than four percent is generally considered normal, while an AET greater than six percent is definitively considered abnormal and diagnostic of GERD.
The analysis also includes the correlation between recorded reflux events and the patient’s logged symptoms. Metrics like the Symptom Index (SI) and Symptom Association Probability (SAP) are calculated to quantify the likelihood that a patient’s discomfort is directly caused by a drop in esophageal pH. A high correlation strengthens the evidence that acid reflux is the actual cause of the patient’s complaints.
The data is often summarized into a single numerical value called the DeMeester Score, a composite numerical value calculated from six different parameters of acid exposure. These parameters include:
- The total number of reflux episodes.
- The percentage of time the esophagus was acidic in both upright and lying positions.
- The duration of the longest reflux episode.
A DeMeester Score greater than 14.7 is often used to indicate pathological reflux, providing a standardized measure of the overall severity of acid exposure.