Diagnosis, Labs, and Pathology

Diagnosing a duodenal gastrinoma is a meticulous process because “high gastrin” does not automatically mean you have a tumor. Your care team must prove two things at once: that your body is making too much gastrin and that this gastrin is causing your stomach to produce dangerous levels of acid ^[1].

The Core Diagnostic Criteria

The gold standard for a Zollinger-Ellison Syndrome (ZES) diagnosis is a combination of two specific lab results:

1. Fasting Serum Gastrin (FSG): A markedly elevated level (often defined in relation to the laboratory upper limit, such as more than 10 times the upper limit) is highly suggestive of a gastrinoma ^[2][1].
2. Gastric pH: Your stomach acid must be measured. A pH of 2.0 or less strongly supports that the high gastrin is actually driving acid production ^[2][3].

If you are not on a PPI and your gastrin is high but your stomach pH is high (not acidic), you likely do not have ZES. Instead, your body may be raising gastrin levels to compensate for a lack of acid, which can be caused by chronic kidney issues, previous gastric surgery, or a condition called atrophic gastritis ^[4][1]. However, if you are taking a Proton Pump Inhibitor (PPI), your medication intentionally raises your gastric pH, which in turn raises your gastrin levels. Because of this medication effect, a high pH while on a PPI cannot reliably exclude ZES, making interpretation by a specialist critical.

The Secretin Stimulation Test

If your fasting gastrin is elevated but not quite at the definitive mark, doctors often use a secretin stimulation test. In this test, a hormone called secretin is injected, and your blood is tested at specific intervals. In a person with ZES, secretin causes gastrin levels to spike significantly (often more than 120 pg/mL above the starting point); in a healthy person, gastrin levels would stay the same or even drop ^[1][5].

Historically, patients were told they must stop their PPI medications for at least one week before this test. However, we now know that abruptly stopping PPIs can trigger life-threatening acid spikes ^[6]. Current evidence shows testing can sometimes be done safely without abruptly stopping your medication, but there is no universal patient-directed protocol ^[7]. Because remaining on the PPI can confound the results, the decision to use a carefully supervised withdrawal, a bridge to an H2 blocker, or to test while on treatment must be managed by a specialized center with an explicit monitoring plan. Never stop your medication for a test unless a specialist is supervising you closely.

Hunting for the Tumor: Localization

Duodenal gastrinomas are notoriously small—often less than 1 centimeter—making them difficult to find on standard CT or MRI scans ^[1]. A highly useful tool for finding these tiny growths is the Ga-68 DOTATATE PET/CT ^[8]. This scan uses a radioactive tracer that sticks to specific “docks” (somatostatin receptors) found on the surface of most neuroendocrine tumors, allowing them to glow on the image ^[9]. While it is an excellent scan for staging, very small lesions or those with low receptor expression can still be missed. Therefore, it is used alongside Endoscopic Ultrasound (EUS) and multiphasic CT or MRI when appropriate. A negative scan does not rule out disease.

Reading Your Pathology Report

If a piece of the tumor was removed (biopsy), your pathology report will contain critical data points that predict how the tumor might behave:

• WHO Grade: Tumors are graded from G1 (slowest growing) to G3 (most aggressive) ^[10].
• Ki-67 Index: This is a percentage that tells you how many cells are actively dividing. For gastrointestinal NETs, a Ki-67 below 3% is typically a G1 tumor; between 3% and 20% is an intermediate G2; and above 20% is G3 ^[10][11]. The mitotic rate (another measure of cell division) is also relevant.
• Differentiation: “Well-differentiated” means the cells still look somewhat like normal cells, which is common for duodenal gastrinomas ^[12][13].
• Gastrin Staining: A “positive” stain for gastrin confirms the tumor has a gastrin-producing phenotype ^[14]. However, this does not by itself prove the tumor is the cause of your hypergastrinemia; the clinical findings must be integrated.

Be aware that a small biopsy can sometimes “underestimate” the grade. A tiny sample might show a low Ki-67, while the rest of the tumor (if removed later) could have a higher, more active area called a hotspot ^[15]. Always discuss the full context of your report with a neuroendocrine specialist.