26th May 2025

Investigations

Bloods in insulinoma should show: 

• Symptomatic hypoglycaemia <3.0mmol/L
• Insulin > 3.0uU/mL
• C-peptide > 0.6ng/mL
• Proinsulin ≥ 5 pmol/L

C-peptide is released when endogenous proinsulin is broken down into insulin. Therefore this will be raised only with endogenous insulin production and not with injectable/exogenous insulin use. Sulphonylureas (eg gliclazide) will cause the body to make insulin so this will also release C-peptide.

The below table summarises the interpretation of blood tests in determining the cause of hypoglycaemia. 

Table 1: interpreting blood test results in hypoglycaemia

To determine between insulinoma and sulphonylurea use (factitious hypoglycaemia) we can test for serum proinsulin levels (which would be significantly raised in insulinoma due to excess insulin production) or for serum or urine sulphonylurea.   

48-72 hour supervised fast: 

• in insulinoma this will invoke hypoglycaemia at which point the test stops
• Glucose, C-peptide and insulin levels should be sent off once hypoglycaemic

MRI/CT imaging

Endoscopic ultrasound if no visible lesion on imaging

GLP-1 receptor imaging (nuclear medicine scan specific for insulinoma)