How do you diagnose an insulinoma?

Insulinomas can be diagnosed through a simple fasting blood test.

Three blood test should be undertaken:

• low blood sugar (less than 2.8 mmol/l)
• high insulin (6 microunits/ml or higher)
• and high levels of C peptide (0.2 nmol/l or higher), an inactive amino acid that in a healthy body will be produced in equal amounts to insulin

The doctor may also use a 'rule of thumb' guide called the Whipple's Triad. Under this guide an insulinoma will be considered if you experience

• Symptoms and signs of hypoglycemia
• Blood sugar levels below 2.8 mmmol/l
• Recovery from an attack after eating something sugary

If you are on medication for diabetes you can still be tested for insulinomas.

• If someone takes insulin, doctors will be looking at the levels of C peptides in the blood. Commercially used insulin does not contain C peptides, so a test will look for certain levels that might suggest a tumour is present.
• If someone takes sulphonylurea tablets (that lower blood glucose levels) the doctor will be looking at the level of sulphonylurea in the blood in relation to insulin, blood sugar and C peptide levels. If it is normal an insulinoma will be suspected.

The long fast

If further confirmation is needed you may be invited into hospital for a special fasting test that can take between 48 and 72 hours. You will not be allowed to eat or drink, apart from water, throughout this period. You will have blood tests at intervals of between three to six hours, an also whenever you show symptoms of low blood sugar, to look at the key levels of blood glucose, insulin, C peptides and sulphonylurea. The hospital fasting test will diagnose insulinomas in more than 90% of cases.