secretin stimulation test

**Physiology of secretin** - Secreted by S cells in the duodenal mucosa in response to acidic chyme (duodenal pH < 4.5) and fatty acids. - Actions: stimulates pancreatic ductal cells to secrete bicarbonate-rich fluid (neutralizes acid), stimulates biliary bicarbonate secretion, and decreases gastric acid secretion by inhibiting gastrin release and parietal cell activity. It also decreases gastric emptying. - Mnemonic: secretin = "Sec-retes bicarb" and "neutralizes acid."

**1. Secretin stimulation test for gastrinoma (Zollinger-Ellison syndrome, ZES)** - Indication: fasting serum gastrin is elevated (>1000 pg/mL is virtually diagnostic with gastric pH <4) or equivocal (e.g., 100–1000 pg/mL) in a patient with refractory or recurrent peptic ulcers, ulcers in unusual locations (distal duodenum or jejunum), diarrhea or steatorrhea, large gastric folds, or MEN1. - Method: give IV secretin (0.4 mcg/kg) and measure serum gastrin at baseline and at intervals (2, 5, 10, 15, 20 minutes). - Interpretation: - **Positive:** paradoxical rise in gastrin of ≥120 pg/mL over baseline (some use ≥200). Gastrinoma cells express secretin receptors, and secretin triggers gastrin release from them. - **Negative:** normal G cells and G-cell hyperplasia show no rise or a decrease, because secretin normally inhibits gastrin release from antral G cells. - Differential for hypergastrinemia: - With high acid (low pH): ZES, G-cell hyperplasia, retained antrum, gastric outlet obstruction, renal failure. - With low acid (high pH): atrophic gastritis, pernicious anemia, PPI use, because loss of negative feedback raises gastrin. - PPIs can cause false positives, so stop them beforehand if safe (about 1 week, with caution, because stopping can cause serious complications in ZES). - Next steps: localize the tumor (somatostatin receptor scintigraphy or DOTATATE PET, EUS, CT/MRI). Gastrinomas are most often in the "gastrinoma triangle" (cystic/common bile duct junction, 2nd–3rd part of the duodenum, pancreatic neck/body). About 25% are associated with MEN1 (parathyroid, pituitary, pancreas). Treat with high-dose PPIs and surgical resection when feasible. - Other findings in ZES: multiple, refractory ulcers, diarrhea or steatorrhea (acid inactivates pancreatic lipase and damages the mucosa), and thick gastric folds.

**2. Secretin stimulation test for pancreatic exocrine function** - Method: IV secretin (often with CCK) is given, and duodenal fluid is collected by endoscopy or tube to measure bicarbonate concentration and volume. - Interpretation: peak bicarbonate <80 mEq/L indicates impaired exocrine function (chronic pancreatitis, pancreatic insufficiency). It is more sensitive for early chronic pancreatitis than imaging or fecal elastase. - Secretin-enhanced MRCP can also be used to evaluate ductal anatomy (pancreas divisum, ductal strictures, and early chronic pancreatitis) and to estimate pancreatic secretory function. - Clinical context: chronic pancreatitis causes steatorrhea, weight loss, and diabetes, and imaging may show calcifications. Common causes are alcohol, hypertriglyceridemia, and cystic fibrosis.

**High-yield exam pearls** - The paradoxical gastrin rise after secretin is diagnostic of gastrinoma. Normally secretin lowers gastrin. - Contrast with the calcium infusion test (rarely used now) and the protein meal test, where G-cell hyperplasia shows an exaggerated gastrin rise after a meal. - Contrast with a PPI-related rise in gastrin, where the secretin test is negative and gastric pH is high. - Gastrinoma is the main cause of ZES, the most common functional pancreatic neuroendocrine tumor syndrome in MEN1.