Diagnosing an Insulinoma
Confirming low blood glucose
Doctors look for symptoms during documented low blood glucose (hypoglycaemia, or a hypo) that improve after carbohydrate. This is called Whipple’s triad.
Blood tests
Laboratory blood samples taken during a hypo measure glucose, insulin and C-peptide, sometimes also proinsulin and ketones. Results must be read together: insulin within the laboratory’s normal range may still be too high when glucose is low.
Doctors exclude other causes, including medicines and hormone deficiencies. Sensor readings alone cannot diagnose an insulinoma.
Biochemical criteria in ENETS 2023
Table 4 gives the following insulinoma cut-offs. The values and inequality signs below are reproduced exactly as printed. These are diagnostic thresholds, not normal laboratory reference ranges.
Glucose (mmol/L): <2.5
C-peptide (nmol/L): >0.2
Insulin (pmol/L): >20.8
Pro-Insulin (pmol/L): >5
BHOB (mmol/L): <2.7
ΔGlucose after glucagon: >+1.4
What these results mean
The symbol < means less than; > means greater than. Blood glucose, insulin and C-peptide must be assessed from samples taken at the same time during a hypo.
Insulin should fall when blood glucose is low. Insulin and C-peptide above these cut-offs suggest that the body is continuing to release insulin. Proinsulin is the precursor of insulin. BHOB, or beta-hydroxybutyrate, is a ketone measured as supporting evidence.
The glucagon result means a blood glucose rise of more than 1.4 mmol/L after glucagon, if this supervised test is used.
This pattern is not unique to an insulinoma. Specialists must exclude other causes, including sulphonylurea or meglitinide medicines, before making the diagnosis.
Supervised tests
If blood tests during a naturally occurring hypo are inconclusive, a supervised hospital fast lasting up to 72 hours may be needed. Clear evidence of insulin excess during a hypo can avoid a fasting test. Symptoms after meals may require a mixed meal test, with blood samples collected after eating. Do not attempt diagnostic fasting at home.
Finding the cause
After blood tests confirm excess insulin during hypoglycaemia, CT, MRI, endoscopic ultrasound or specialist imaging helps locate the cause.
Full reference
Hofland J, Falconi M, Christ E, Castaño JP, Faggiano A, Lamarca A, Perren A, Petrucci S, Prasad V, Ruszniewski P, Thirlwell C, Vullierme MP, Welin S, Bartsch DK. European Neuroendocrine Tumor Society 2023 guidance paper for functioning pancreatic neuroendocrine tumour syndromes. Journal of Neuroendocrinology. 2023;35(8):e13318. Question 1, Figure 1 and Table 4; Question 5. https://doi.org/10.1111/jne.13318
Read the full ENETS 2023 guidance (PDF).
Updated 27 September 2026.