Gastric analysis is a technique for measuring acid production of the stomach. Gastric analysis may, in this context, be classified as qualitative or quantitative. A qualitative gastric analysis is undertaken to ascertain whether the gastric glands can secrete acid. A quantitative gastric analysis seeks to determine the amount of HCl secreted by the stomach and is carried out by determining the basal secretion level or the secretory response to insulin hypoglycemia.
Historically, the measurement of acid production provided a useful clinical tool in peptic ulcer disease, pernicious anemia, and management of postvagotomy patients. Because of the discovery of H. pylori, the improvements in endoscopy, and the development of proton pump inhibitors, the use of gastric analysis has diminished; it remains a useful diagnostic tool in patients with hypergastrinemia and gastrinoma (Zollinger-Ellison syndrome).
Gastric analysis measures the gastric production of acid over a period of time to determine the basal acid output of the stomach. The patient fasts overnight and on the day of the examination, because food in the stomach stimulates acid release and interferes with the determination of basal acid production. A radiopaque nasogastric tube is placed in the most dependent portion of the stomach. If it is positioned correctly, only 5% to 10% of the stomach acid will enter the duodenum and not be collected. The position can be verified with fluoroscopy or through water retrieval. For water retrieval, 20 to 50 mL of water is instilled into the tube and aspirated. Retrieval of at least 90% of the instilled water indicates successful positioning of the tube. Once positioning is verified, continuous gastric aspiration is maintained with negative pressure at 40 to 50 mm Hg. The gastric contents are collected into separate containers at intervals of 15 minutes. Töpfer reagent (dimethylaminoazobenzene that changes color from red to yellow over the pH range of 2.9 to 4.0) is added to ensure the acidity of the specimen. Once collected, two methods can be used to calculate hydrogen concentration. One method involves titrating gastric contents with sodium hydroxide to a pH of 7.0. The amount of sodium hydroxide required will determine the milli- moles or milliequivalents of acid collected. Another method uses a pH meter to determine the hydrogen activity of the gastric fluid, which can be used to calculate the concentration of H+. The total output of acid by the stomach over an hour is the basal acid output; a normal output is less than 10 mEq/hour for men and 5 mEq/hour for women.
In the past, after the basal acid output had been determined, the maximum acid output could be determined by stimulation with pentagastrin, a synthetic form of gastrin, and measurement of the gastric output. The maximum acid output was often increased in patients with duodenal ulcers. The test is no longer routinely obtained, however, because pentagastrin is not commercially available.
The histamine test is indicated whenever no acid is found in the residual and basal secretions. After the subcutaneous injection of histamine (0.01 mg per kilogram of body weight) or of ametazole hydrochloride (0.5 mg/kg), which is preferred by some authorities because its side effects are less severe, testing for acid continues at 15-minute intervals and is terminated with the first 15-minute specimen in which acid appears. If it has not appeared by the end of 90 minutes, a further attempt to verify the inability to secrete acid is made by the augmented histamine test, in which the injection of a much larger dose of histamine is made possible by the prior administration of an antihistamine drug. (This procedure is based on the fact that the antihistamines block all but the acid secretory effects of histamine.) Thirty minutes after the administration of the antihistaminic, 0.04 mg/kg of histamine diphosphate is given subcutaneously; the continuously aspirated gastric content is tested at 15-minute intervals with Töpfer reagent. When this reagent, added to the specimens, turns yellow, indicating a hydrogen-ion concentration with a pH above 4.0, it becomes necessary to determine the pH electrometrically with a pH meter before “absolute achlorhydria” can be pronounced.
Gastric analysis, in combination with clinical features and serum gastrin levels, is used in the diagnosis of Zollinger-Ellison syndrome, a hypersecretory disorder characterized by peptic ulceration in the upper intestine distal to the duodenal bulb, multiple endocrinopathies, and secretion of such enormous quantities of HCl as to require total gastrectomy to abolish the ulcerative process. A basal acid output of more than 15 mEq/hour, or more than 5 mEq/hour after stomach surgery, is consistent with a diagnosis of the syndrome.