Diagnosis

Most patients with a gastrinoma have fasting serum gastrin levels >200 pg/mL. Serum gastrin levels >1000 pg/mL are strongly suggestive of a gastrinoma. However, demonstration of elevated serum gastrin levels is not sufficient to make the diagnosis of a gastrinoma as there are other causes of hypergastrinemia.

Ulcerogenic causes of hypergastrinemia include antral G-cell hyperplasia, gastric outlet obstruction, retained gastric antrum syndrome, and gastrinoma. Nonulcerogenic causes include atrophic gastritis, pernicious anemia, previous vago-tomy, renal failure, and short-gut syndrome.

After confirming hypergastrinemia, the next step diagnostically is gastric acid analysis. To obtain accurate data patients must abstain from antisecretory medications prior to this analysis. Basal acid output >15 mEq/hr in nonoperated patients, >5 mEq/hr in patients with previous vagotomy, or a ratio of basal to maximal acid output >0.6 supports the conclusion that hypergastrinemia is associated with hypersecretion of gastric acid. Once this is determined, provocative testing with secretin may be performed to differentiate among the various causes of ulcerogenic hypergastrinemia. Secretin stimulation is done in the fasting state by obtaining serum samples for gastrin in the basal period and after secretin administration in 5-minute intervals for 30 minutes. Increase in serum gastrin levels of >200 pg/mL above the basal level supports the diagnosis of gastrinoma. After biochemical diagnosis is made, localization studies are then conducted. The initial imaging technique recommended for the identification of a gastrinoma is a dynamic abdominal CT scan with 5-mm pancreatic cuts and both oral and intravenous contrast.

There are a number of clinical situations in which there is a heightened suspicion of a gastrinoma. Serum gastrin levels should be obtained in patients with peptic ulcer disease when ulcers are recurrent on antisecretory therapy or when ulcers fail to heal on appropriate medical therapy. Postoperative ulcers, post-bulbar duodenal ulcers, peptic ulcers associated with diarrhea, and family history of peptic ulcer disease should also stimulate one to consider measuring serum gastrin levels. Other indications for measuring serum gastrin include prolonged undiagnosed diarrhea, observation of prominent gastric rugal folds, and the presence of other pancreatic endocrine tumors. MEN kindred should also be screened.

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