Diagnostic criteria for ZES include the following:
- Elevated levels of Basal Acid Output (BAO), greater than 15 mEq in unoperated patients and greater than 5 mEq if previous acid-reducing surgery has been performed;
- Elevated level of fasting serum gastrin (>100 pg/mL until 1994, >200 pg/mL since 1994);
- Abnormal results from stimulation testing with secretin (an increase of >200 pg/mL postinjection) or with calcium (an increase >395 pg/mL);
- Positive histologic confirmation of gastrinoma; or
- A combination of these criteria.
The tests used most commonly to establish a diagnosis of ZES are the fasting serum gastrin concentration and BAO evaluation. Measurement of gastric pH is important to exclude achlorhydria as a cause of secondary hypergastrinemia. Patients taking Proton Pump Inhibitors (PPIs), those who have undergone massive small bowel resection, or those who have renal insufficiency, G-cell hyperplasia, or gastric outlet obstruction may have gastrin levels between 150 and 1000 pg/mL Hence, for patients with suspected ZES with an equivocal fasting serum gastrin concentration, a secretin stimulation test should be performed.