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Acid- and Reflux-Related Tests
Acid- and reflux-related tests serve mainly to diagnose acid reflux into the esophagus. Other less commonly used tests include gastric analysis and testing to assess bile reflux. Complications are very rare for these procedures. Patients must have nothing by mouth (npo) after midnight but are free to eat as usual after the monitoring device is placed.
Ambulatory 24-h esophageal pH monitoring with or without intraluminal impedance testing is currently the most common test for quantifying gastroesophageal reflux. The principal indications are
Tests may use a transnasal continuous reflux-monitoring catheter or a wireless pH-monitoring device that is endoscopically attached to the distal esophagus.
A thin tube containing a pH probe is positioned 5 cm above the lower esophageal sphincter. The patient records symptoms, meals, and sleep for 24 h. Esophageal acid exposure is defined by the percentage of the total recording time that the pH is < 4.0. A value > 4.3% is considered abnormal if the patient has not been taking a proton pump inhibitor, and a value > 1.3% is abnormal if the patient has been taking a proton pump inhibitor for the duration of the test. Additional sensors along the more proximal regions of the pH probe allow identification of proximal reflux episodes.
The newer pH-monitoring devices usually also do multichannel intraluminal impedance testing, which identifies weakly acidic reflux (pH between 4.0 and 7) and nonacidic reflux (pH > 7), which would be missed by conventional pH monitoring.
The correlation between patient-reported symptoms and reflux events can be assessed using the symptom index (SI) or symptom association probability (SAP). A significant SI value or SAP value suggests that the correlation between symptoms and reflux events is not due to chance. Excessive reflux and significant symptom-reflux correlation are positive predictors of a favorable outcome from antireflux surgery.
Ambulatory esophageal pH monitoring can also be done using a wireless pH-sensing capsule that is attached to the distal esophagus. The device is endoscopically placed 5 cm above the lower esophageal sphincter and continuously monitors esophageal acid exposure (defined as pH < 4.0) for 48 h. Similar to the probe-based test, patients record symptoms, meals, and sleep for the duration of the test and excessive acid exposure and symptom-reflux correlation (SI or SAP) are identified. However, because the capsule is a pure pH sensor, only acid reflux is detected. The capsule usually falls off within a week of placement and is spontaneously passed in the stool. Because the capsule transmits data wirelessly while attached, it does not need to be retrieved.
Gastric acid analysis is rarely done in current practice. When conducted, samples of stomach contents obtained via NGT are used to measure gastric acid output in a basal and stimulated state. This information may be useful in a patient who develops a recurrent ulcer after surgical vagotomy for peptic ulcer disease. In this case, a positive acid response to stimulation (sham feeding) indicates an incomplete vagotomy. The test also is used to evaluate a patient with elevated serum gastrin levels. Hyperchlorhydria in the presence of elevated gastrin usually indicates Zollinger-Ellison syndrome. Hypochlorhydria in the presence of elevated gastrin indicates impairment of acid output, such as occurs in pernicious anemia, atrophic gastritis, and Ménétrier disease and after inhibition of gastric acid secretion by potent antisecretory drugs.
To do gastric analysis, an NGT is inserted and the gastric contents are aspirated and discarded. Gastric juice is then collected for 1 h, divided into four 15-min samples. These samples represent basal acid output.
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