C-Peptide in the Evaluation of Hypoglycaemias

What is C-Peptide?

1. Not present in commercial insulin preparations - exogenous insulin is C-peptide-free.
2. Negligible hepatic extraction - unlike insulin (50% first-pass hepatic removal), C-peptide reflects beta-cell secretion more accurately. Its half-life (~30 minutes) is longer than insulin's (4-9 minutes).

The Core Diagnostic Principle

Diagnostic Cut-offs During Hypoglycaemia

• Plasma insulin ≥3 µU/mL (≥18 pmol/L)
• Plasma C-peptide ≥0.6 ng/mL (≥0.2 nmol/L)
• Plasma proinsulin ≥5.0 pmol/L
• Beta-hydroxybutyrate ≤2.7 mmol/L (ketone suppression = insulin action present)
• IV glucagon 1 mg → rise in plasma glucose >25 mg/dL (confirms glycogen stores intact under insulin)

Causes of Hypoglycaemia and C-Peptide Pattern

1. Insulinoma (Endogenous Hyperinsulinism)

• Most common functional pancreatic neuroendocrine tumour (incidence 1-3/100,000/year).
• During the 72-hour supervised fast, both insulin AND C-peptide are elevated at the time of symptomatic hypoglycaemia.
• Whipple's triad: symptoms of hypoglycaemia + plasma glucose <45 mg/dL + resolution with glucose.
• C-peptide elevation confirms the insulin is endogenous - distinguishing insulinoma from factitious hypoglycaemia.
• (Fischer's Mastery of Surgery, 8th ed., p. 5141; Goldman-Cecil Medicine)

2. Exogenous Insulin Administration (Factitious / Surreptitious)

• Elevated insulin with low or undetectable C-peptide is pathognomonic.
• Insulin levels may reach >1000 µU/mL.
• "Exogenous insulin does not contain C-peptide, so an elevated insulin level without detectable C-peptide is indicative of exogenous administration." (Fischer's Mastery of Surgery)
• (Rosen's Emergency Medicine; Washington Manual of Medical Therapeutics)

3. Sulfonylurea / Insulin Secretagogue Ingestion

• These drugs stimulate native beta cells, so both insulin AND C-peptide are elevated - mimicking insulinoma biochemically.
• The differentiator is a positive plasma/urine screen for sulfonylureas or meglitinides.
• Assays must be sent to laboratories capable of detecting very low drug concentrations, as standard assays may miss it. (Henry's, p. 273)
• Sulfonamide-induced hypoglycaemia similarly causes raised insulin + C-peptide.

4. Autoimmune Insulin Syndrome (Hirata Disease)

• Antibodies to endogenous insulin bind secreted insulin postprandially, then release it later causing delayed hypoglycaemia.
• C-peptide is incompletely suppressed (still measurable because the antibodies are blocking feedback, not eliminating secretion).
• Extremely high plasma insulin with high insulin-to-C-peptide molar ratio is characteristic.
• Associated with sulfhydryl drugs (methimazole, penicillamine, captopril) and autoimmune conditions. (Henry's, p. 275)

5. Insulin Receptor Antibody Hypoglycaemia (Type B Insulin Resistance)

• Antibodies activate the insulin receptor directly.
• C-peptide is suppressed appropriately (insulin secretion is inhibited by the resulting hypoglycaemia).

6. Non-Beta-Cell Tumour Hypoglycaemia (Big IGF-II)

• Large mesenchymal or hepatic tumours produce "big IGF-II," which mimics insulin action.
• Insulin secretion is appropriately suppressed → both insulin and C-peptide are low.
• Plasma IGF-II:IGF-I ratio is elevated. (Harrison's, 22nd ed.)

7. Drug-Induced (Non-Secretagogue) Hypoglycaemia

• Pentamidine: destroys beta cells acutely → initial insulin leak → high insulin, then low C-peptide later.
• Quinine, quinolones, beta-blockers: various mechanisms, not primarily insulin-secretory, so C-peptide pattern is variable/not diagnostically pivotal. (Henry's, p. 273)

8. Post-Bariatric Surgery Hypoglycaemia

• Usually >1 year post Roux-en-Y gastric bypass.
• Rapid gastric emptying causes exaggerated insulin + GLP-1 surge.
• Postprandial hypoglycaemia; C-peptide will be elevated (endogenous hyperinsulinism).

The 72-Hour Supervised Fast Protocol

1. Fast begins after the last meal; patient may drink water.
2. Measure glucose, insulin, C-peptide, beta-hydroxybutyrate every 6 hours; increase to every 1-2 hours when glucose reaches 60 mg/dL.
3. End the fast when: glucose ≤45 mg/dL with symptoms, or ≤55 mg/dL if Whipple's triad was previously documented.
4. At termination: draw glucose, insulin, C-peptide, beta-hydroxybutyrate, and sulfonylurea screen simultaneously.
5. Administer glucagon 1 mg IV; measure glucose at 10, 20, 30 minutes.
6. If hypoglycaemia develops with measurable insulin and C-peptide, no SFU detected → insulinoma workup (imaging with CT/MRI/EUS, ⁶⁸Ga-DOTATATE PET/CT).

Summary Interpretation Table

Additional Uses of C-Peptide Beyond Hypoglycaemia Diagnosis

• Beta-cell function in diabetes: stimulated C-peptide >1.8 ng/mL (0.60 nmol/L) → type 2 behaviour; <0.5 µg/L → type 1 pattern (Tietz, 7th ed.).
• Post-pancreatectomy monitoring: should be undetectable after radical pancreatectomy; rises after successful islet/pancreas transplant.
• Renal disease: C-peptide (unlike insulin) is renally cleared, so levels are elevated in ESRD and must be interpreted cautiously. (Henry's)
• Clinical trial endpoint for beta-cell preservation in new-onset T1D studies.

C-Peptide Role in Hypoglycaemia

Why C-Peptide?

• It is absent from all commercial insulin preparations
• It undergoes negligible hepatic extraction (unlike insulin, which has ~50% first-pass removal), so it more accurately reflects beta-cell secretory activity

The Core Question It Answers

Is the excess insulin endogenous (beta-cell origin) or exogenous (injected)?

Diagnostic Cut-offs (at plasma glucose <55 mg/dL with symptoms)

• Plasma C-peptide ≥0.6 ng/mL (≥0.2 nmol/L)
• Plasma insulin ≥3 µU/mL (≥18 pmol/L)
• Proinsulin ≥5.0 pmol/L
• Beta-hydroxybutyrate ≤2.7 mmol/L (ketones suppressed = insulin effect present)
• IV glucagon 1 mg → glucose rise >25 mg/dL

C-Peptide Pattern Across Each Hypoglycaemia Cause

The 72-Hour Fast - Role of C-Peptide

Glucose + Insulin + C-peptide + Proinsulin + Beta-hydroxybutyrate + Sulfonylurea screen

Quick Interpretation Summary

Hypoglycaemia Evaluation: 40-Year-Old Female, Family History of T2DM in Both Parents

Why the Family History Matters Here

• Insulin resistance compels the beta cells to over-secrete insulin in response to carbohydrate loads
• The insulin response may be delayed in onset but excessive in magnitude (late-phase hyperinsulinism)
• This produces postprandial (reactive) hypoglycaemia 1-3 hours after meals - the most likely pattern in this patient
• As insulin secretory capacity later declines, the patient may progress to prediabetes and eventually T2DM

Step 1 - Confirm Hypoglycaemia: Whipple's Triad

1. Symptoms consistent with hypoglycaemia (see below)
2. Documented low plasma glucose (<55 mg/dL / <3.0 mmol/L) at time of symptoms
3. Relief of symptoms with glucose administration

"A diagnosis of hypoglycaemia should not be made unless a patient meets the criteria of Whipple's triad." (Tietz 7th ed.)

• Plasma glucose >70 mg/dL during a symptomatic episode rules out hypoglycaemia as the cause.
• Many patients, especially middle-aged women, report symptoms they attribute to hypoglycaemia that are actually anxiety, palpitations, or fatigue without true glycaemic drop.

Step 2 - History and Symptoms

Type of symptoms

Key questions for this patient:

• When do symptoms occur? Fasting (morning, after exercise) vs. postprandial (1-3 hours after meals)?
• What was the last meal before the episode? High-carbohydrate load?
• Is there alcohol use (inhibits gluconeogenesis)?
• Any medications - including prescribed, OTC, herbal? (ACE inhibitors, beta-blockers, quinolones, sulfonylureas if prescribed for prediabetes/T2DM)
• Any recent weight changes, fatigue, polyuria, polydipsia (suggesting undiagnosed T2DM)?
• Any gastrointestinal surgery (gastric bypass, gastrectomy) - postprandial hypoglycaemia from rapid gastric emptying?
• Any autoimmune conditions (thyroid disease, vitiligo, rheumatoid arthritis) - risk for autoimmune insulin syndrome?
• Family history of MEN-1 (insulinoma association)?
• Psychiatric history - risk for factitious hypoglycaemia?

Step 3 - Physical Examination

• BMI and waist circumference - central obesity = strong marker of insulin resistance in this population
• Acanthosis nigricans (neck, axillae) - pathognomonic for insulin resistance
• Skin tags, hirsutism, PCOS features (also insulin resistance associations)
• Signs of Addison's disease - hyperpigmentation, postural hypotension
• Thyroid enlargement or signs of hypothyroidism
• Abdominal mass (large tumour causing IGF-II mediated hypoglycaemia)
• Signs of liver disease or alcohol use

Step 4 - Classify the Hypoglycaemia Type

A. Fasting Hypoglycaemia (overnight, after exercise, or >5 hours since last meal)

• Insulinoma
• Adrenal insufficiency / hypopituitarism
• Severe liver or renal disease
• Exogenous insulin or sulfonylurea use

B. Postprandial (Reactive) Hypoglycaemia (1-4 hours after meals)

• Insulin resistance / prediabetes / early T2DM - compensatory late hyperinsulinaemia
• Idiopathic reactive hypoglycaemia - delayed insulin response with exaggerated amplitude
• Post-gastric surgery (alimentary hypoglycaemia)
• Autoimmune insulin syndrome (Hirata disease)

Step 5 - Laboratory Workup

First-line tests (send together, ideally during a symptomatic episode):

For prediabetes/T2DM screening (given family history):

• Fasting plasma glucose ≥100 mg/dL = impaired fasting glucose
• HbA1c 5.7-6.4% = prediabetes
• Note: OGTT is NOT appropriate for diagnosing hypoglycaemia (Tietz 7th ed.; ADA statement) - it causes overdiagnosis of reactive hypoglycaemia and results are not reproducible

Step 6 - The 72-Hour Supervised Fast (if fasting hypoglycaemia suspected)

1. Begin after last meal; water allowed; patient active during waking hours
2. Measure glucose, insulin, C-peptide, beta-hydroxybutyrate every 6 hours
3. Increase to every 1-2 hours when glucose approaches 60 mg/dL
4. End fast when: glucose ≤45 mg/dL with symptoms OR glucose ≤55 mg/dL if Whipple's triad previously demonstrated
5. At termination: draw glucose + insulin + C-peptide + proinsulin + beta-hydroxybutyrate + sulfonylurea screen simultaneously
6. Administer glucagon 1 mg IV; measure glucose at 10, 20, 30 minutes

Step 7 - Mixed Meal Tolerance Test (if postprandial hypoglycaemia suspected)

• Draw glucose, insulin, C-peptide at 0, 30, 60, 90, 120, 180 minutes after a standardised mixed meal
• A glucose <50 mg/dL with symptoms = confirms reactive hypoglycaemia
• Elevated C-peptide at the nadir = confirms endogenous insulin excess
• The OGTT alone should NOT be used - it overdiagnoses hypoglycaemia and results are not reproducible (Tietz 7th ed.)

• Delayed insulin peak (instead of normal early first-phase response)
• Exaggerated late insulin surge (excess second-phase)
• Glucose nadir at 90-180 minutes

Step 8 - Imaging (only if biochemistry confirms endogenous hyperinsulinism)

Most Likely Differential Diagnoses for This Patient (Ranked)

Management Outline Based on Findings

• Dietary modification: small, frequent meals; reduce refined carbohydrate load; increase protein and fibre
• Weight loss if overweight (reduces insulin resistance)
• Regular moderate exercise
• Monitor HbA1c 6-monthly (high T2DM progression risk)
• Consider referral to dietitian and diabetes prevention programme

• Surgical resection (enucleation or pancreatectomy)
• Diazoxide or octreotide as bridge to surgery

• Hydrocortisone replacement