Forty-eight-hour fast: the diagnostic test for insulinoma

Area of Science:

• Endocrinology
• Clinical Diagnostics

Background:

• Insulinoma, a tumor causing fasting hypoglycemia due to excess insulin secretion, is traditionally diagnosed using Whipple's triad during a 72-hour supervised fast.
• The efficacy of the prolonged 72-hour fast has not been re-evaluated with contemporary insulin, C-peptide, and proinsulin assays.

Purpose of the Study:

• To assess the necessity of a full 72-hour fast for diagnosing insulinoma in light of advancements in biochemical assays.

Main Methods:

• Retrospective review of data from 127 patients with pathologically confirmed insulinoma undergoing supervised fasting at the NIH between 1970 and 2000.
• Analysis of glucose, insulin, C-peptide, and proinsulin levels during fasting, noting termination criteria and symptom onset.

Main Results:

• Hypoglycemia necessitating fast termination occurred by 12 hours in 42.5%, by 24 hours in 66.9%, and by 48 hours in 94.5% of patients.
• Elevated immunoreactive proinsulin was present at the start of the fast in 90% of tested patients, with suppressibility differentiating insulinoma from non-insulinoma cases.
• Seven patients required fasting beyond 48 hours despite meeting diagnostic criteria earlier.

Conclusions:

• With current insulin and proinsulin assays, a diagnosis of insulinoma can be reliably established within 48 hours.
• The 48-hour supervised fast should be adopted as the standard diagnostic protocol, replacing the traditional 72-hour fast.