Gastric Analysis

Dr. Arvind Kumar

Introduction

  • Gastric analysis is a laboratory test used to examine the quantity and composition of gastric juice.
  • It mainly evaluates the amount of hydrochloric acid (HCl) present in gastric juice.
  • The test provides information about the acid-secreting function of the stomach.
  • Gastric analysis mainly measures free acidity and total acidity.
  • It helps identify abnormal gastric acid secretion.
  • The test may be useful in the evaluation of certain gastric disorders.
  • Gastric juice is usually collected using a nasogastric tube for laboratory analysis.

Types of Gastric Acidity

1. Free Hydrochloric Acid

  • Free HCl is hydrochloric acid present in gastric juice in the free state.
  • It is mainly responsible for maintaining the low pH of gastric contents.

2. Combined Hydrochloric Acid

  • This refers to HCl that is combined with proteins and other substances in the gastric contents.

3. Total Acidity

  • Total acidity represents the combined contribution of free HCl and other acid components present in gastric juice.

Specimen Required

  • Specimen: Gastric juice
  • Gastric contents are collected using a nasogastric tube.
  • The patient is generally prepared according to the laboratory protocol, often involving an overnight fast.
  • Gastric juice may be collected as basal secretion and, when required, after stimulation of acid secretion.

Gastric Juice Collection

  1. The patient is prepared according to the test protocol.
  2. A nasogastric tube is introduced into the stomach.
  3. The gastric contents are aspirated.
  4. Basal gastric secretion may be collected over a defined period.
  5. A gastric stimulant may be administered when stimulated acid secretion is being evaluated.
  6. Additional gastric samples are collected at specified intervals.
  7. Samples are labeled and sent to the laboratory for analysis.

Methods of Gastric Analysis

Gastric acidity is commonly estimated by titration.

Principle

A measured volume of gastric juice is titrated against a standard alkali solution, usually sodium hydroxide (NaOH), using suitable indicators.

The amount of alkali required to neutralize the acid is proportional to the acid concentration in the gastric juice.

Estimation of Free Hydrochloric Acid

  • A measured volume of gastric juice is taken.
  • A suitable indicator is added.
  • The sample is titrated with standard NaOH.
  • The endpoint indicates neutralization of free HCl.
  • The result is calculated and reported as gastric acidity.

Estimation of Total Acidity

  • Gastric juice is titrated with standard NaOH.
  • The titration is continued to the endpoint representing neutralization of total acid.
  • The amount of NaOH used is used to calculate total acidity.

 

Requirements

  1. Sample:
    • Gastric aspirate is obtained from the stomach using a nasogastric tube (NGT) or endoscopy.
  2. Reagents:
    • Titration agents (e.g., 0.1 N NaOH for titration of gastric acid).
    • Histamine or pentagastrin (for stimulation of acid secretion).
    • Indicator (such as methyl orange or bromothymol blue) for titration.
    • pH indicator strips or a pH meter for determining the pH of gastric contents.
  3. Apparatus:
    • Nasogastric tube (NGT) or endoscopy equipment for collecting gastric aspirates.
    • Titration setup for acid quantification.
    • pH meter for measuring the acidity of gastric juice.
    • Syringes or vacutainers for collecting samples.

Procedure

  1. Preparation of the Patient
    • The patient must fast for 12-14 hours before the test to ensure that the stomach is empty and the results are not affected by food.
    • The patient should refrain from taking antacids, H2 blockers, or proton pump inhibitors for a period before the test, as these can interfere with acid secretion.
  1. Basal Gastric Secretion
    • The patient is positioned in a semi-recumbent position.
    • Nasogastric intubation is performed to place a tube in the stomach. Once the tube is in place, the stomach contents are aspirated and measured for volume, pH, and acid content (using titration).
    • The gastric aspirate is examined to determine gastric acid secretion, typically by titrating the sample with 0.1 N NaOH and determining the titrant required to neutralize the gastric acid.
  1. Gastric Secretion After Stimulation
    • Stimulation can be induced using histamine, pentagastrin, or food.
      • Histamine or pentagastrin is administered to stimulate the H2 receptors or gastrin receptors, respectively, and to increase gastric acid secretion.
      • Gastric contents are then aspirated again after stimulation, typically at 30-minute intervals to measure the acid output.
    • The samples are then titrated again for volume and pH analysis.
  1. Measurement of Gastric Acid Output
    • Gastric volume is measured using a graduated cylinder or similar device.
    • The pH of the gastric aspirate is determined using pH strips or a pH meter.
    • The acid output is calculated based on the volume and titration results. This can be expressed as mEq/hour (milli-equivalents of acid per hour).
  1. Interpretation of Results
    • Normal Gastric Acid Secretion: In a healthy individual, basal acid secretion is typically around 30-50 mL of gastric juice per hour with a pH of 1.5-3.5.
    • After stimulation, the acid output should increase, often up to 5 times the basal secretion, depending on the stimulant used.

Interpretation of Results

  1. Normal Findings:
    • A normal gastric analysis shows a balanced gastric acid secretion in response to stimulation.
    • Basal secretion is usually around 30-50 mL/h with a pH of 1.5-3.5.
    • After stimulation with pentagastrin or histamine, there should be a substantial increase in gastric acid output, and the gastric pH should remain low (indicating high acidity).
  2. Hypochlorhydria (Low Acid Secretion):
    • Basal secretion may show a higher pH, and the total acid output is reduced compared to normal.
    • The patient may have conditions like atrophic gastritis or gastric cancer or maybe taking medications like proton pump inhibitors (PPIs).
  3. Achlorhydria (No Acid Secretion):
    • In conditions like autoimmune gastritis or complete gastric atrophy, the gastric aspirate will show no acid production, and the pH will be neutral or alkaline.
    • No increase in acid output is observed after stimulation.
  4. Hyperchlorhydria (Excess Acid Secretion):
    • In conditions like Zollinger-Ellison syndrome (a gastrin-secreting tumor), there is an excessive increase in gastric acid production.
    • Basal acid secretion and stimulated acid output are higher than normal.
  5. Peptic Ulcer Disease:
    • A typical finding is increased basal secretion, especially if the ulcer is in the duodenum (because duodenal ulcers are often associated with increased acid secretion).
  6. Gastritis:
    • Basal secretion may be low or normal, but after stimulation, there may be an increased secretion of gastric acid.

Clinical Significance

  1. Diagnosis of Gastric Disorders:
    • Gastric analysis is crucial for diagnosing peptic ulcers, gastritis, and Zollinger-Ellison syndrome.
    • It can help assess the severity of acid reflux or gastric bleeding.
  2. Evaluation of Hypochlorhydria:
    • Hypochlorhydria (low stomach acid) is commonly seen in patients with atrophic gastritis, H. pylori infection, or as a side effect of proton pump inhibitors (PPIs). Gastric analysis helps assess the functional status of the stomach in these patients.
  3. Assessment of Acid Secretion in Peptic Ulcer Disease:
    • Gastric analysis can help determine whether there is increased acid secretion (often seen in duodenal ulcers) or decreased acid secretion (as seen in gastric ulcers).
  4. Monitoring of Gastric Function:
    • The test can be used to monitor the effects of therapy for gastric ulcers, gastritis, and other related conditions.
  5. Zollinger-Ellison Syndrome:
    • This syndrome, caused by gastrin-secreting tumors, leads to excessive acid production, which can be diagnosed through elevated gastric acid output in the test.

Limitations

  1. Invasive: Gastric analysis requires nasogastric tube insertion, which can be uncomfortable for patients.
  2. Risk of Complications: The test carries a slight risk of aspiration, especially if the patient is not positioned correctly during the procedure.
  3. Altered Results Due to Medications: Medications such as antacids, H2 blockers, and proton pump inhibitors can alter gastric secretion, making it difficult to obtain accurate results.
  4. Availability: Gastric analysis is not as widely available as some other diagnostic tests, and it is sometimes replaced by endoscopy or pH monitoring in clinical practice.