The most appropriate diagnostic test is endoscopic ultrasound. Hypoglycemia is best diagnosed when three conditions coexist (Whipple triad): (1) hypoglycemic symptoms are present, (2) the plasma glucose level is low, and (3) resolution of symptoms occurs promptly after glucose ingestion. When Whipple triad is observed, further evaluation for the cause of pathologic hypoglycemia should include an inpatient 72-hour fast with regular measurement of simultaneous plasma glucose, insulin, and C-peptide (a measure of endogenous insulin secretion). If the results are consistent with insulinoma, only then should CT imaging of the pancreas be performed to localize the tumor and direct its surgical resection. Because insulinomas are often small, further testing (with, for example, endoscopic ultrasonography) may be necessary for localization, as is required in this patient.

Endoscopic retrograde cholangiopancreatography (ERCP) is an endoscopic test that allows for visualization and therapeutic interventions of the bile and pancreatic ducts. It is an invasive procedure that will not identify small solid tumors of the pancreas such as an insulinoma.

Insulinomas almost always have inadequate expression of somatostatin receptors to be detected by octreotide scanning; therefore, this is not a useful diagnostic test for insulinomas.

Although abdominal ultrasonography is noninvasive, it does not have the resolution to identify small pancreatic tumors, particularly tumors not detected by a higher-resolution imaging procedure such as CT scanning.