Endocrine dynamic function tests - 72 hour fast

Role

The 72-hour supervised fast is the diagnostic gold-standard test for suspected insulinoma or other causes of endogenous hyperinsulinaemic hypoglycaemia - monitoring for the development of hypoglycaemia with inappropriately elevated insulin/C-peptide during a prolonged fast under close medical supervision
Reserved for the specific work-up of confirmed or strongly suspected fasting hypoglycaemia where an insulinoma or similar cause is suspected, given the resource intensity and hypoglycaemia risk requiring close inpatient monitoring

In health, insulin secretion is appropriately suppressed as blood glucose falls during fasting; an insulin-secreting tumour (or other endogenous hyperinsulinism cause) fails to suppress insulin secretion appropriately, causing hypoglycaemia to develop as the fast progresses - the diagnostic test exploits this failure of normal physiological insulin suppression
The test exploits the normal physiological suppression of insulin secretion during fasting - an insulinoma's defining diagnostic feature is its failure to suppress insulin output appropriately as glucose falls, distinguishing pathological autonomous insulin secretion from the normal fasting physiological response

Blood glucose, insulin, C-peptide, and proinsulin are measured at intervals throughout the fast (and at the point of any hypoglycaemic symptoms), with the fast terminated once glucose falls to a diagnostic threshold with symptoms, or at 72 hours if hypoglycaemia has not developed
A supervised, safety-monitored inpatient setting is essential, given the risk of significant hypoglycaemia developing, sometimes requiring urgent glucose administration to terminate the test safely

Documented hypoglycaemia with inappropriately elevated insulin/C-peptide (failure of suppression) confirms endogenous hyperinsulinism, directing further tumour localisation imaging
A normal fast (no hypoglycaemia by 72 hours, or hypoglycaemia with appropriately suppressed insulin) makes an insulin-secreting tumour unlikely and redirects the differential toward other causes of hypoglycaemia

Insulinoma (the primary indication), sulfonylurea-induced hypoglycaemia (a mimicker requiring specific screening, since it produces a similar biochemical pattern), factitious insulin administration (proinsulin/C-peptide pattern helps distinguish from endogenous secretion)

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