A 46-year-old man comes to the clinic for evaluation of a 6-month history of persistent, b… — USMLE Step 3

A $46$ -year-old man comes to the clinic for evaluation of a $6$ -month history of persistent, burning epigastric pain and voluminous, watery diarrhea. He reports no weight loss, hematemesis, or melena. Two months ago, an esophagogastroduodenoscopy performed at an outside facility showed prominent gastric folds and multiple ulcerations in the duodenal bulb and postbulbar duodenum. He was prescribed high-dose pantoprazole, which initially provided partial relief, but his symptoms recurred. He was instructed to discontinue pantoprazole $14$ days prior to today's visit. Vital signs are within normal limits. Physical examination reveals mild epigastric tenderness but is otherwise unremarkable.

Laboratory studies show:
Serum calcium: $9.2$ mg/dL
Serum creatinine: $0.9$ mg/dL
Fasting serum gastrin: $680$ pg/mL (Normal: $< 100$ pg/mL)

A nasogastric tube is placed, and gastric fluid analysis reveals a basal acid output of $22$ mEq/h (Normal: $< 10$ mEq/h) and a gastric fluid pH of $1.5$ .

Which of the following is the most appropriate next step in the diagnostic workup?

Secretin stimulation testAnswer
B
Gallium-68 DOTATATE PET/CT scan
C
Endoscopic ultrasound of the pancreas
D
Measurement of anti-parietal cell antibodies
E
Genetic testing for MEN1 gene mutations

Answer

The correct answer is the secretin stimulation test.

The correct sequence of diagnostic testing is to perform a secretin stimulation test. The patient presents with clinical features highly suggestive of Zollinger-Ellison syndrome (ZES). While his fasting serum gastrin is elevated (

680

pg/mL) and his gastric pH is appropriately low (

1.5

, ruling out achlorhydria), a fasting gastrin level is only definitively diagnostic of ZES if it is greater than

10

times the upper limit of normal (typically

> 1000

pg/mL). For values in the equivocal range (

100-1000

pg/mL), biochemical confirmation is required using the secretin stimulation test. Secretin normally inhibits gastrin release from normal gastric G cells but paradoxically stimulates gastrin release from gastrinoma cells.

Step-by-Step Solution

Analyze the clinical presentation and initial endoscopy.

The patient has refractory peptic ulcer disease in atypical locations (postbulbar duodenum), prominent gastric folds, and diarrhea, which are classic signs of Zollinger-Ellison syndrome (ZES).

Identifying the clinical syndrome guides the selection of the appropriate biochemical workup.

Interpret the laboratory and gastric fluid analyses.

The fasting serum gastrin is elevated at

680

pg/mL. The gastric pH is

< 2.0

(

1.5

), which confirms primary hyperchlorhydria and rules out secondary hypergastrinemia caused by achlorhydria.

Differentiating between primary hypersecretion (gastrinoma, retained antrum) and secondary hypersecretion (atrophic gastritis, PPI use) is the critical first diagnostic branch point.

Evaluate the fasting gastrin level against diagnostic thresholds.

A fasting gastrin level

> 1000

pg/mL combined with a gastric pH

< 2.0

is definitively diagnostic of ZES. However, this patient's level of

680

pg/mL falls into the equivocal range (

100

1000

pg/mL).

Recognizing that the laboratory values are highly suspicious but not definitively diagnostic dictates the need for further biochemical confirmation rather than proceeding immediately to imaging.

Select the correct confirmatory test sequence.

The secretin stimulation test must be performed. A rise in serum gastrin of

> 120

pg/mL above basal levels following secretin administration will definitively confirm the diagnosis of ZES.

Proper test sequencing dictates that biochemical confirmation must be absolutely secured prior to ordering expensive, high-radiation, or invasive localizing imaging.

Key Concept

In patients with suspected Zollinger-Ellison syndrome who have a gastric pH

< 2.0

but an equivocal fasting serum gastrin level (

100

1000

pg/mL), the diagnosis must be biochemically confirmed using a secretin stimulation test before proceeding to tumor localization imaging.

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