Does a 72‑hour supervised fast showing hypoglycemia (glucose 2.9 mmol/L) on night 3, with insulin decreasing from 15 to 10 µU/mL (still >3 µU/mL), C‑peptide falling to ~102 pmol/L, and marked ketonuria (+4), still support a diagnosis of insulinoma in this female patient?

Diagnostic Interpretation: This Case Does NOT Support Insulinoma

The presence of marked ketonuria (+4) on day 3 of the fast, combined with declining insulin levels (15→10 µU/mL) and falling C-peptide (to 102 pmol/L ≈ 0.34 ng/mL), argues strongly AGAINST insulinoma and instead suggests appropriate physiologic suppression of insulin secretion during prolonged fasting. 1

Critical Biochemical Analysis

Why This Pattern Excludes Insulinoma

• Ketonuria is the key discriminator: The presence of +4 ketones in urine on day 3 indicates that the patient has appropriately shifted to fat metabolism and ketone production—a normal physiologic response to fasting that should NOT occur in insulinoma, where autonomous insulin secretion suppresses lipolysis and ketogenesis. 1

• Insulin levels are declining appropriately: While the insulin level of 10 µU/mL at hypoglycemia (2.9 mmol/L) is technically above the diagnostic threshold of >3 µU/mL, the critical finding is that insulin decreased from 15 to 10 during the fast, demonstrating appropriate suppression rather than autonomous secretion. 1, 2

• C-peptide at 102 pmol/L (≈0.34 ng/mL) is borderline: Although one guideline-based source notes that a C-peptide of approximately 0.34 ng/mL at glucose 2.9 mmol/L could be "inappropriately elevated" for insulinoma 1, this must be interpreted in the full clinical context—particularly the presence of marked ketonuria, which definitively excludes autonomous insulin hypersecretion. 3

Diagnostic Criteria for Insulinoma (Not Met Here)

Required Biochemical Triad at Fast Termination

The National Comprehensive Cancer Network diagnostic criteria for insulinoma require ALL of the following at the time of documented hypoglycemia: 1

• Insulin level >3 µU/mL (often >6 µU/mL for high specificity)
• C-peptide ≥0.6 ng/mL (≥200 pmol/L)
• Proinsulin ≥5 pmol/L
• Absence of ketonuria—this is the critical missing element in your patient 1, 2

What the Literature Shows

• In a landmark study of 37 confirmed insulinoma patients, all had C-peptide ≥0.20 nmol/L (≥200 pmol/L, equivalent to ≥0.60 ng/mL) at fast termination, whereas normal subjects had C-peptide ≤0.10 nmol/L when glucose was ≤2.8 mmol/L. 3

• Your patient's C-peptide of 102 pmol/L falls well below the 200 pmol/L threshold established for insulinoma diagnosis. 3

Alternative Diagnoses to Consider

Reactive (Postprandial) Hypoglycemia

• Fasting hypoglycemia during a 72-hour supervised fast with appropriate insulin suppression (evidenced by ketonuria) suggests that the patient's hypoglycemic episodes may be postprandial rather than fasting in nature. 1, 2

• If the patient has predominantly post-prandial symptoms (1–4 hours after eating), a mixed-meal tolerance test (MMTT) should be performed next, not additional fasting studies. 2

Non-Insulinoma Causes of Fasting Hypoglycemia

• Consider non-islet cell tumor hypoglycemia (NICTH), medication-induced hypoglycemia (verify negative sulfonylurea screen), adrenal insufficiency, or factitious hypoglycemia from exogenous insulin (though elevated C-peptide argues against this). 2, 4

Recommended Next Steps

Immediate Actions

• Discontinue the fasting test—the presence of marked ketonuria indicates the patient has achieved appropriate metabolic adaptation to fasting and further prolongation risks harm without diagnostic benefit. 1

• Obtain a detailed symptom diary: Document the precise timing of hypoglycemic episodes relative to meals (fasting vs. 1–4 hours postprandial) to guide the next diagnostic test. 2

If Symptoms Are Postprandial

• Perform a mixed-meal tolerance test (MMTT) with blood sampling at baseline and every 30 minutes for 180 minutes, measuring glucose, insulin, and C-peptide. 2

• A positive MMTT (hypoglycemia with inappropriately elevated insulin/C-peptide between 60–180 minutes post-meal) would suggest non-insulinoma pancreatogenous hypoglycemia syndrome (NIPHS) or post-bariatric hypoglycemia if the patient has prior gastric surgery. 2

If True Fasting Hypoglycemia Persists

• Rule out medication-induced hypoglycemia: Confirm negative urine/plasma sulfonylurea screen. 2, 4

• Evaluate for adrenal insufficiency: Measure morning cortisol and consider ACTH stimulation testing.

• Assess for non-islet cell tumor hypoglycemia: Measure IGF-II and IGF-I levels, and perform imaging to evaluate for large mesenchymal or epithelial tumors. 4

Common Pitfalls to Avoid

• Do not rely solely on insulin >3 µU/mL as diagnostic of insulinoma—this threshold has poor specificity when used in isolation and must be accompanied by elevated C-peptide (≥0.6 ng/mL), elevated proinsulin, and absence of ketones. 1, 3

• Do not proceed to imaging studies (CT, MRI, EUS) based on this biochemical profile, as the presence of ketonuria excludes autonomous insulin hypersecretion and any pancreatic lesion found would likely be an incidental finding. 1, 4

• Avoid octreotide or somatostatin analogs in any patient with suspected insulinoma until octreoscan positivity is confirmed, as these agents can paradoxically worsen hypoglycemia by suppressing counterregulatory hormones. 1, 4