Gastrin
Gastrin is secreted by G cells in the gastric antrum in response to a meal. It stimulates parietal cells in the fundus of the stomach to secrete gastric acid and also stimulates antral motility and secretion of pepsin and intrinsic factor. In addition to digested food, gastrin secretion is stimulated by calcium, alcohol and high intragastric pH. Gastrin release is inhibited by low gastric pH, secretin, glucagon, calcitonin, somatostatin and vasoactive intestinal peptide. Gastrin levels exhibit a circadian rhythm with lowest values between 03:00 and 07:00. Measurement of serum gastrin concentration is useful in the work-up of the Zollinger Ellison syndrome (ZE).
Gastrin levels should be measured together with gastric pH after an overnight fast. Normal fasting gastrin levels usually rule out a gastrinoma, while gastrin levels above 1000 pg/mL with a gastric pH <2.5 are diagnostic of a gastrinoma. Most patients with fasting serum gastrin levels greater than 1500 pg/mL are found to have metastases. However, 60% of patients with ZE have gastrin levels that are elevated but less than 1000 pg/mL. In these cases a secretin provocative test is helpful. A baseline gastrin level is drawn and then an intravenous bolus of 2 to 3 U secretin per kilogram of body weight is administered over 30 seconds. Serum gastrin levels are then measured at 2, 5, 10, 15, and 20 minutes. Normally, no change or a slight suppression of gastrin concentration will be seen. Patients with gastrinoma demonstrate a paradoxical increase of 100 to 200 pg/mL in serum gastrin at 5 minutes with maximum levels reaching several thousand pg/mL. Approximately 90% of ZE patients have a positive secretin test.
Other disorders can elevate serum gastrin levels into the range seen in ZE.
- A few patients with gastric or duodenal ulcers may have gastrin levels between 100 and 500 pg/mL.
- Patients with pernicious anemia, pyloric stenosis, chronic atrophic gastritis, vagotomy without gastric resection, short bowel syndrome, chronic renal failure, and hyperparathyroidism may have moderately increased levels.
- H2 blocker medications and steroid therapy produce mild to moderate gastrin elevation.
- Proton pump inhibitors inhibit gastric acid secretion, resulting in gastrin levels as high as 500 to 1000 pg/mL.
- Postprandial levels are usually between 95 and 140 pg/mL and seldom exceed 250 pg/mL.
- A high protein meal can produce a marked increase in serum gastrin.
Commercial kits for measuring gastrin usually use antibodies directed against gastrin-17 peptide. Gastrinoma tumors often secrete a heterogenous mixture of small and large gastrin molecules. Some kits may not detect gastrin molecules secreted by some tumors, giving a false negative result (Clinical Chemistry2012;58:831-36).
Reference value is usually 0 - 200 pg/mL.
Specimen requirement is one SST tube of blood. Gastrin in serum is unstable at room temperature or refrigerated temperatures. Serum specimens can lose up to 50% of their gastrin in a period of 48 hours at 4oC because of proteolytic enzymes in the serum.
