The diagnosis of an insulinoma is usually made biochemically with low blood glucose, elevated insulin, proinsulin, and C-peptide levels, and confirmed by localizing the tumour with medical imaging or angiography.

These include recurrent headache, lethargy, diplopia, altered mental status, and blurred vision, particularly with exercise or fasting.

At that point, or when the patient has symptoms of hypoglycemia, a blood test is drawn for serum glucose, insulin, proinsulin, and C-peptide levels.

[citation needed] The insulinoma might be localized by noninvasive means, using ultrasound, CT scan, or MRI techniques.

An indium-111 pentetreotide scan is more sensitive than ultrasound, CT, or MRI for detection of somatostatin receptor positive tumours, but not a good diagnostic tool for insulinomas.

[10] Sometimes, angiography with percutaneous transhepatic pancreatic vein catheterization to sample the blood for insulin levels is required.

Calcium can be injected into selected arteries to stimulate insulin release from various parts of the pancreas, which can be measured by sampling blood from their respective veins.

The use of calcium stimulation improves the specificity of this test.During surgery to remove an insulinoma, an intraoperative ultrasound can sometimes localize the tumour, which helps guide the surgeon in the operation and has a higher sensitivity than noninvasive imaging tests.

Medications such as diazoxide and somatostatin can be used to block the release of insulin for patients who are not surgical candidates or who otherwise have inoperable tumours.

[citation needed] Insulinomas are rare neuroendocrine tumours with an incidence estimated at one to four new cases per million persons per year.

[citation needed] An insulinoma removed from a woman in Munich provided insulin mRNA that was used in the first human gene cloning experiment.