During a hypoglycemic episode, what does a C‑peptide level <0.05 ng/mL with an insulin level of 5.5 mIU/L indicate?

Exogenous Insulin Administration (Factitious Hypoglycemia)

A C-peptide level <0.05 ng/mL (essentially undetectable) combined with an insulin level of 5.5 mIU/L during hypoglycemia indicates exogenous insulin administration—either factitious (self-administered) or iatrogenic—rather than endogenous hyperinsulinism.

Diagnostic Interpretation

During spontaneous hypoglycemia, the body's normal physiological response is to completely suppress endogenous insulin secretion. According to Endocrine Society guidelines, when plasma glucose falls below 55 mg/dL, insulin levels should be below 3 μU/mL and C-peptide levels should be below 0.6 ng/mL (0.2 nmol/liter) 1. Your patient's C-peptide is far below this threshold at <0.05 ng/mL, indicating virtually complete suppression of endogenous insulin production.

However, the measurable insulin level of 5.5 mIU/L is inappropriately elevated for the degree of C-peptide suppression. This dissociation is pathognomonic for exogenous insulin exposure 2, 3.

Key Diagnostic Features

The critical finding here is the insulin-to-C-peptide mismatch:

• Normal physiology: Pancreatic beta cells secrete insulin and C-peptide in equimolar (1:1) ratios 4
• Your patient: High insulin with suppressed C-peptide = exogenous insulin source
• Endogenous hyperinsulinism (like insulinoma) would show: insulin ≥3.0 μU/mL AND C-peptide ≥0.6 ng/mL (or ≥0.3 nmol/L by more recent data) 1, 5

Recent high-quality research from 2023 established that C-peptide ≥0.30 nmol/L (approximately 0.9 ng/mL) with hypoglycemia <2.3 mmol/L has 96% sensitivity and 100% specificity for endogenous hyperinsulinism 5. Your patient's C-peptide is nowhere near this threshold.

Clinical Implications

This pattern requires immediate investigation for:

1. Factitious disorder (Munchausen syndrome) - particularly if the patient has healthcare access or knowledge 3
2. Medication error - accidental or intentional insulin administration
3. Malicious administration - consider in vulnerable patients 2

Important Caveats

• Ensure blood samples were collected in fluoride-oxalate tubes to prevent in vitro glycolysis, which can falsely lower glucose readings 6
• Confirm hypoglycemia was measured using glucose oxidase technique on venous plasma, not capillary glucometer readings 6
• Screen for oral hypoglycemic agents (sulfonylureas, meglitinides) which would cause high C-peptide, not suppressed C-peptide 1, 2
• Check for insulin antibodies if there's history of prior insulin exposure, though this wouldn't explain the suppressed C-peptide 1

What This Rules Out

This pattern definitively excludes:

• Insulinoma or other endogenous hyperinsulinism (would have elevated C-peptide)
• Oral hypoglycemic agent overdose (would have elevated C-peptide) 2
• IGF-mediated hypoglycemia (would have suppressed both insulin and C-peptide)

The combination of measurable insulin with profoundly suppressed C-peptide during documented hypoglycemia is diagnostic of exogenous insulin exposure and warrants psychiatric evaluation and safety assessment 3.