A 38-year-old woman from Mumbai presents with recurrent peptic ulcer disease unresponsive to standard proton pump inhibitor therapy. Endoscopy shows severe gastritis with prominent rugae. You suspect atypical gastric pathology. Which investigation is most specific for diagnosing chronic atrophic gastritis with achlorhydria?

Explanation

## Diagnosis of Chronic Atrophic Gastritis with Achlorhydria **Key Point:** Fasting serum gastrin level combined with direct gastric pH measurement is the most specific investigation for confirming achlorhydria (gastric pH >6) and secondary hypergastrinaemia in atrophic gastritis. ### Pathophysiology of Achlorhydria-Induced Hypergastrinaemia 1. **Parietal cell loss** → decreased HCl secretion 2. **Loss of negative feedback** → G-cells are not inhibited by acid 3. **Paradoxical gastrin rise** → fasting gastrin often >1000 pg/mL 4. **Gastric pH elevation** → pH typically >6 (confirms achlorhydria) ### Diagnostic Approach | Investigation | Achlorhydria | ZES | Normal | |---|---|---|---| | **Fasting gastrin** | Elevated (often >1000) | Elevated (>1000) | <100 | | **Gastric pH** | >6 (high) | <2 (low) | 1.5–3.5 | | **Secretin stimulation** | No rise or falls | Rise ≥200 pg/mL | No rise | | **Parietal cell antibody** | Positive (autoimmune) | Negative | Negative | | **Intrinsic factor antibody** | Positive (pernicious anaemia) | Negative | Negative | **High-Yield:** The combination of **elevated fasting gastrin + gastric pH >6** is diagnostic of achlorhydria. This differentiates it from ZES (where pH <2 despite high gastrin). **Clinical Pearl:** In atrophic gastritis, gastric pH measurement is critical—it directly proves the absence of acid secretion. Fasting gastrin alone can be misleading because both ZES and achlorhydria cause hypergastrinaemia; pH is the discriminator. ### Diagnostic Algorithm for Hypergastrinaemia ```mermaid flowchart TD A[Elevated fasting gastrin]:::outcome --> B[Measure gastric pH]:::action B --> C{pH <2?}:::decision C -->|Yes| D[Secretin stimulation]:::action C -->|No| E[pH >6 = Achlorhydria]:::outcome D --> F{Gastrin rise ≥200?}:::decision F -->|Yes| G[ZES confirmed]:::outcome F -->|No| H[Achlorhydria confirmed]:::outcome E --> I[Check parietal cell & IF antibodies]:::action I --> J{Antibodies positive?}:::decision J -->|Yes| K[Autoimmune atrophic gastritis]:::outcome J -->|No| L[Environmental atrophic gastritis]:::outcome ``` ### Why Gastric pH is Essential **Warning:** Fasting gastrin alone CANNOT distinguish ZES from achlorhydria. Both present with elevated gastrin. Gastric pH measurement is the key discriminator: - **ZES:** pH <2 (acid-secreting gastrinoma) - **Achlorhydria:** pH >6 (parietal cell failure) [cite:Harrison 21e Ch 297]