Gastric analysis consists of a series of tests used to analyze the contents of the stomach. The complete series involves:
collecting residual gastric fluid from a fasting patient
collecting basal secretions every 15 minutes for four hours
intramuscular administration of a drug that stimulates gastric acid output
collecting stomach secretions every 15 minutes for 90 minutes
The appearance, blood, bile, pH, volume, millimoles of H+ per liter, millimoles of H+ per volume, and millimoles of H+ per hour of each specimen is then evaluated.
Purpose
A gastric analysis is performed to evaluate gastric function by measuring the contents of a fasting patient's stomach the for acidity, appearance, and volume. The basal gastric secretion test is indicated for patients with obscure gastric pain, loss of appetite, and weight loss. It is also utilized for suspected peptic ulcer, severe gastritis, and Zollinger-Ellison (Z-E) syndrome.
The gastric acid stimulation test is indicated when abnormalities are found during the basal secretion test. These abnormalities can be caused by a number of disorders, including duodenal ulcer, pernicious anemia, and gastric cancer. While this test will detect abnormalities, x rays and other studies are necessary to obtain a definitive diagnosis.
Precautions
Because both the basal acid output test and the gastric acid stimulation test require gastric intubation through the mouth or nasal passage, neither test is recommended for patients with esophageal problems, aortic aneurysm, severe gastric hemorrhage, or congestive heart failure. The gastric acid output test is also not recommended in patients who are sensitive to pentagastrin (the drug used to stimulate gastric acid output).
Description
This test, whether performed for basal gastric acid secretion, gastric acid stimulation, or both, requires gastric intubation by mouth or through the nasal passage.
Basal gastric acid secretion
The patient should be fasting overnight (12 hours) prior to intubation. After allowing approximately 10 to 15 minutes for the patient to adjust to the presence of the tube, and with the patient in a sitting position, specimens are obtained every 15 minutes for a period of 90 minutes. The first two specimens are examined visibly for blood and volume but are discarded to eliminate gastric contents that might be affected by the stress of the intubation process. The patient is allowed no liquids during the test, and saliva must be ejected to avoid diluting the stomach contents.
The final four specimens collected during the test constitute the basal acid output. Each sample is titrated to pH 3.5 using 0.1 N sodium hydroxide. The millimoles of hydrogen ion in each sample are calculated from the
KEY TERMS
Achlorhydria—An abnormal condition in which hydrochloric acid is absent from the secretions of the gastric glands in the stomach.
Intubation—Insertion of a tube into a body canal or hollow organ, as into the stomach.
Pernicious anemia—One of the main types of anemia, caused by inadequate absorption of vitamin B12 Symptoms include tingling in the hands, legs, and feet, spastic movements, weight loss, confusion, depression, and decreased intellectual function.
Subcutaneously—Under the skin.
Zollinger-Ellison syndrome—A rare condition characterized by severe and recurrent peptic ulcers in the stomach, duodenum, and upper small intestine, caused by a tumor or tumors, usually found in the pancreas. The tumor secretes the hormone gastrin, which stimulates the stomach and duodenum to produce large quantities of acid, leading to ulceration. Most often cancerous, the tumor must be removed surgically; otherwise total surgical removal of the stomach is necessary.
amount of base used to neutralize the stomach acid of each. The results of the closest three samples are averaged and multipled by four to give the millimoles of free hydrogen ions per hour. If analysis suggests abnormally low gastric secretion, the maximum acid output test is performed immediately afterward.
Gastric acid stimulation test
After the basal samples have been collected, the tube remains in place for the gastric acid stimulation test. Pentagastrin, or a similar drug that stimulates gastric acid output, is injected subcutaneously. After 15 minutes, a specimen is collected every 15 minutes for one hour. These specimens are called the post-stimulation specimens. As is the case with the basal gastric secretion test, the patient can have no liquids during this test, and their saliva must be ejected to avoid diluting the stomach contents. The maximal acid output (MAO) is determined by titrating each of the four specimens and averaging the results. The average is used to determine the millimoles of hydrogen ion produced per hour. Alternatively, the peak acid output (PAO) is determined by titrating each specimen and using the average hydrogen ion concentration of the highest two to calculate the acid produced in millimoles per hour.
Analyses and calculations
The appearance, blood, bile, pH, volume, millimoles of H+ per liter, millimoles of H+ per volume, and millimoles of free H+ per hour of each specimen are then evaluated. In addition, the basal acid output (BAO) is computed, as is the maximal acid output (MOA) or peak acid output (PAO). BAO is calculated by averaging the output of the three closest samples. MAO is calculated as the average of the four specimens. PAO is calculated by taking the mean of the two highest post-stimulation values.
Preparation
The patient should be fasting (nothing to eat or drink after the evening meal) on the day prior to the test, but may have water up to one hour before the test. Antacids, anticholinergics, cholinergics, alcohol, H2-receptor antagonists (Tagamet, Pepcid, Axid, Zantac), reserpine, adrenergic blockers, and adrenocorticosteroids should be withheld for one to three days before the test, as the physician requests. If pentagastrin is to be administered for the gastric acid secretion test, medical supervision should be maintained, as possible side effects may occur.
Additionally, because such external factors as the sight or odor of food, as well as psychological stress, can stimulate gastric secretion, accurate testing requires that the patient be relaxed and isolated from all sources of sensory stimulation.
Aftercare
Such complications as nausea, vomiting, and abdominal distention or pain are possible following removal of the gastric tube. If the patient has a sore throat, soothing lozenges may be given. The patient may also resume the usual diet and any medications that were withheld for the test(s).
Complications
There is a slight risk that the gastric tube may be inserted improperly, entering the trachea instead of the esophagus. If this happens, the patient may experience difficulty breathing or a coughing spell until the tube is properly inserted. Also, because the tube can be difficult to swallow, if a patient has an overactive gag reflex, there may be a transient rise in blood pressure due to anxiety. Other complications may include bleeding, dysrhythmias, esophageal perforation, layrngospasm and decreased mean pO2 (a measure of blood oxygen levels).
Results
Reference values for the basal acid output test and gastric acid stimulation test vary by laboratory, but are usually within the following ranges:
Fasting volume: 20-100 mL.
Fasting pH: less than 2.0.
BAO for men: 0 to 5 mmol/hour.
BAO for women: 0 to 4 mmol/hour.
MAO for men: 5 to 26 mmol/hour.
MAO for women: 7 to 15 mmol/hour.
An abnormal basal acid output is considered non-specific and must be evaluated in conjunction with the results of a gastric acid stimulation test. However, elevated secretion may suggest different types of ulcers; and markedly elevated results may be suggestive of Zollinger-Ellison syndrome. Depressed secretion may indicate a gastric cancer, while complete absence of secretion (achlorhydria) may suggest pernicious anemia.
Elevated gastric secretion levels in the gastric acid stimulation test may be indicative of duodenal ulcer; highest levels of secretion suggest Zollinger-Ellison syndrome, a gastrin-secreting tumor.
Measurement of plasma gastrin by radioimmunoassay is often performed when the gastric acid level is abnormal. Frankly elevated serum gastrin levels occur in pernicious anemia and atrophic gastritis, which are both associated with low gastic acid output; and in Zollinger-Ellison syndrome, which is associated with high gastric acid output. Gastrin levels are not elevated in persons with duodenal ulcers and are normal or slightly increased in persons with gastric ulcers.
Health care team roles
A physician orders the gastric analysis and interprets the results. The testing physician must obtain an accurate patient history, especially to determine if the patient is taking any drugs that can affect the test result and to learn about any recent illness, trauma, or symptoms that could be related to gastric function. The procedure should be explained to the patient by the unit nurse, who should be aware of the degree of seriousness of the patient's condition. Gastric analysis is performed by clinical laboratory scientists/medical technologists or by clinical laboratory technicians/medical laboratory technicians.
BOOKS
Chernecky, Cynthia C, and Barbara J. Berger, Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia, PA: W. B. Saunders Company, 2001.
Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests. 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.
