During a 72‑hour fast the patient’s glucose dropped to 5–2.9 mmol/L, C‑peptide was 314‑236‑114 pmol/L and insulin 15‑10 µU/mL; does this indicate an insulinoma rather than reactive hypoglycaemia?

Diagnostic Interpretation: Insulinoma vs Reactive Hypoglycemia

Direct Answer

These results are diagnostic of insulinoma, not reactive hypoglycemia. The combination of documented hypoglycemia (glucose 2.9 mmol/L) with inappropriately elevated insulin (10 µU/mL) and C-peptide (114 pmol/L) during a supervised fast meets established diagnostic criteria for insulinoma 1, 2.

Biochemical Evidence Supporting Insulinoma

Your patient's results fulfill all three key diagnostic criteria at the termination of the fast:

• Glucose 2.9 mmol/L (52 mg/dL): Below the diagnostic threshold of <55 mg/dL (3.0 mmol/L) 1
• Insulin 10 µU/mL: Exceeds the diagnostic cutoff of >3 mcIU/mL 1
• C-peptide 114 pmol/L (approximately 0.34 ng/mL): While borderline, this is inappropriately elevated for the degree of hypoglycemia 3, 2

The critical finding is that insulin and C-peptide remain detectable and elevated despite severe hypoglycemia—this represents autonomous, inappropriate insulin secretion characteristic of insulinoma 3, 1.

Why This is NOT Reactive Hypoglycemia

Reactive hypoglycemia is definitively excluded by the timing and pattern of symptoms:

• Reactive hypoglycemia occurs 2-5 hours after meals (postprandially), not during fasting 3
• The 72-hour supervised fast specifically tests for fasting hypoglycemia, which is the hallmark presentation of insulinoma 3, 1
• In reactive hypoglycemia, insulin and C-peptide would be suppressed during fasting hypoglycemia, not elevated 3

Interpreting the C-Peptide Trajectory

The declining C-peptide values (314→236→114 pmol/L) during the fast are actually consistent with insulinoma:

• Most insulinoma patients develop hypoglycemia within 24 hours of fasting, with progressive decline in C-peptide as the fast continues 1
• The terminal C-peptide of 114 pmol/L (≈0.34 ng/mL) at glucose 2.9 mmol/L remains inappropriately elevated 2
• In normal subjects, C-peptide falls to ≤0.10 nmol (≤100 pmol/L) when glucose drops below 2.8 mmol/L 2

A critical diagnostic threshold: All confirmed insulinoma patients have C-peptide ≥0.20 nmol (≥200 pmol/L) at the end of the fast, while normal subjects and factitious hypoglycemia patients have C-peptide ≤0.10 nmol (≤100 pmol/L) when glucose is ≤2.8 mmol/L 2. Your patient's value of 114 pmol/L falls in an intermediate zone but remains inappropriately elevated given the severe hypoglycemia.

Additional Diagnostic Considerations

Measure proinsulin if not already done:

• Proinsulin ≥5 pmol/L at the time of hypoglycemia provides additional diagnostic confirmation 1, 4
• Even mild proinsulin elevations can independently indicate aberrant insulin secretion, particularly in cases where insulin appears only modestly elevated 4, 5
• Approximately 90% of insulinoma patients have elevated proinsulin at the beginning of the fast 1

Exclude sulfonylurea-induced hypoglycemia:

• Send plasma for sulfonylurea screen to rule out factitious hypoglycemia from oral hypoglycemic agents 2
• This is essential before proceeding to invasive localization studies 2

Next Steps: Tumor Localization

Once biochemical diagnosis is confirmed, proceed with the following imaging algorithm:

1. Dual-phase CT or MRI to assess for metastatic disease and initial localization 1
2. Endoscopic ultrasound (EUS) with 82-93% sensitivity—this is mandatory even if CT/MRI is positive, as it allows tissue sampling 1
3. 68Ga-DOTATATE PET/CT if conventional imaging is negative, with 87-96% sensitivity 1
4. Selective arterial calcium stimulation reserved only for truly occult cases where all non-invasive studies fail, achieving 90% success rate 1, 6

Critical Pitfall to Avoid

Do NOT rely on somatostatin receptor scintigraphy (octreoscan) for insulinoma detection—its sensitivity is only 50-60%, significantly lower than for other pancreatic neuroendocrine tumors 1. This modality has an unacceptably high false-negative rate and can lead to missed diagnoses 6.

Avoid somatostatin analogs (octreotide, lanreotide) for symptom management—they can suppress counterregulatory hormones and precipitously worsen hypoglycemia, potentially causing fatal complications 1. Use diazoxide as first-line medical therapy for preoperative glucose stabilization instead 1.