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Difficulty: MediumOrdering Laboratory Tests

An outpatient consultation is requested for a patient experiencing persistent, voluminous diarrhea and burning epigastric pain that frequently awakens them at night. An esophagogastroduodenoscopy performed three weeks ago demonstrated prominent gastric folds and multiple ulcerations extending into the second portion of the duodenum. Symptoms have persisted despite strict adherence to high-dose proton pump inhibitor therapy. The patient is instructed to appropriately discontinue the acid-suppressing medication, after which laboratory evaluation reveals a fasting serum gastrin level of 650 pg/mL (reference range: < 100 pg/mL) alongside a basal gastric pH of 1.5. Which of the following is the most appropriate next step in the diagnostic evaluation to establish the etiology of this patient's condition?

  1. A
    Diagnostic laparoscopy with biopsy
  2. Secretin stimulation testAnswer
  3. C
    Fasting serum chromogranin A level
  4. D
    Somatostatin receptor scintigraphy
  5. E
    Serum vasoactive intestinal peptide assay

Answer

The correct answer is the secretin stimulation test.
The correct answer is the secretin stimulation test. This patient's presentation of persistent diarrhea, refractory peptic ulcer disease in an atypical location, and prominent gastric folds is highly suspicious for Zollinger-Ellison syndrome (ZES), caused by a gastrin-secreting neuroendocrine tumor (gastrinoma). The initial evaluation for ZES involves measuring a fasting serum gastrin level alongside gastric pH. While a fasting gastrin level greater than 1,000 pg/mL in the presence of a gastric pH less than 2.0 is diagnostic, this patient has an indeterminate elevation (650 pg/mL). In patients with non-diagnostic gastrin elevations (typically between 100 and 1,000 pg/mL) and confirmed gastric acidity, a secretin stimulation test is the most appropriate next step. Exogenous secretin stimulates the release of gastrin from neoplastic gastrinoma cells while inhibiting its release from normal gastric G cells. A paradoxical increase in serum gastrin of more than 120 pg/mL above the basal level definitively confirms the diagnosis.

Step-by-Step Solution

1
Identify the clinical syndrome from the patient's presentation.
The patient has refractory peptic ulcer disease in an atypical location (distal duodenum) and diarrhea, highly suggesting Zollinger-Ellison syndrome (ZES).
Recognizing the classic triad of gastric acid hypersecretion, severe peptic ulcer disease, and non-beta islet cell tumors of the pancreas/duodenum is the first step in directing the workup.
2
Evaluate the initial laboratory findings provided in the stem.
The fasting serum gastrin is elevated (650 pg/mL) but not definitively diagnostic (which requires >1,000 pg/mL), while the gastric pH is appropriately acidic (< 2.0).
Gastrin levels between 100 and 1,000 pg/mL are indeterminate, as they can be seen in other conditions; confirming the pH is < 2.0 rules out achlorhydria as a cause of hypergastrinemia.
3
Determine the standard diagnostic algorithm for indeterminate hypergastrinemia.
A provocative biochemical test is required to distinguish a gastrinoma from other etiologies.
Exogenous secretin stimulates gastrin release from gastrinoma cells while paradoxically inhibiting its release from normal gastric G cells. An increase in gastrin of > 120 pg/mL confirms the diagnosis.

Key Concept

In patients with suspected Zollinger-Ellison syndrome who have an indeterminate fasting serum gastrin level (100 to 1,000 pg/mL) and acidic gastric pH, a secretin stimulation test is required to establish the diagnosis.
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Ordering Laboratory Tests

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