How do you diagnose an insulinoma?
Insulinomas can be diagnosed through a simple fasting blood test. Your NET specialist will look for a certain combination of:
Low blood sugar (less than 2.5 mmol/L)*
High insulin (<20.8 pmol/L or higher)*
High levels of C peptide (0.2nmol/L or higher)*, an inactive amino acid that in a healthy body will be produced in equal amounts to insulin.
*European Neuroendocrine Tumor Society 2023
Guidance paper for functioning pancreatic neuroendocrine tumour syndromes
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 hypoglycaemia
Blood sugar levels below 2.5 mmol/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, and also whenever you show symptoms of low blood sugar, to look at the key levels of blood glucose, insulin, C peptides and sulphonylurea.
This hospital fasting test will diagnose insulinomas in more than 90% of cases.
Read the recommended protocol for carrying out a 72-hour fast
The Blood Tests for an insulinoma
Insulinomas can be diagnosed through a simple fasting blood test. The blood samples must be handled and stored correctly. Failure to do so can result in the blood haemolysing and the 72-hour fast being repeated because of invalid or no results.
READ: How to correctly handle bloods to test for an insulinoma