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C-Peptide Fasting

Diabetes
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Report in 4Hrs

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Fasting Required

Details

C-Peptide is an indirect marker of insulin production by the pancreas

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C-Peptide Fasting Test Information Guide

  • Why is it done?
    • Measures endogenous insulin production by quantifying C-peptide, a byproduct released when the pancreas converts proinsulin to insulin
    • Distinguishes between endogenous and exogenous insulin production (helps differentiate Type 1 from Type 2 diabetes)
    • Evaluates pancreatic beta cell function and reserve capacity
    • Diagnoses hypoglycemia etiology (distinguishes insulin-mediated from non-insulin-mediated causes)
    • Assesses residual beta cell function in established diabetes mellitus
    • Detects insulinomas and other neuroendocrine tumors that inappropriately secrete insulin
    • Evaluates complications related to exogenous insulin administration (factitious hypoglycemia)
    • Performed during fasting state to establish baseline endogenous insulin secretion and assess beta cell response
  • Normal Range
    • Reference Range (Fasting): 0.8–3.1 ng/mL (0.27–1.03 nmol/L)
    • Units of Measurement: Nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L)
    • Normal Result Interpretation: Normal C-peptide indicates intact pancreatic beta cell function and appropriate endogenous insulin secretion response to fasting state
    • Low/Abnormal Result (<0.8 ng/mL): Indicates impaired pancreatic beta cell function; may suggest Type 1 diabetes, advanced Type 2 diabetes, chronic pancreatitis, or complete beta cell destruction
    • High/Elevated Result (>3.1 ng/mL): Suggests excessive endogenous insulin production; may indicate insulin resistance, early Type 2 diabetes, metabolic syndrome, insulinoma, or factitious hyperinsulinemia
    • Note: Reference ranges may vary by laboratory; always consult specific laboratory reference values for accurate interpretation
  • Interpretation
    • Low C-Peptide with High Blood Glucose: Characteristic of Type 1 diabetes mellitus or advanced Type 2 diabetes with significant beta cell exhaustion; indicates minimal to absent endogenous insulin production
    • High C-Peptide with Normal/Elevated Blood Glucose: Suggests insulin resistance (beta cells compensating with increased insulin production); typical of metabolic syndrome, obesity, and early Type 2 diabetes
    • Hypoglycemia with Detectable C-Peptide: Indicates endogenous insulin secretion is responsible for hypoglycemia; suggestive of insulinoma, autoimmune insulin syndrome, or sulfonylurea use
    • Hypoglycemia with Suppressed C-Peptide (<0.5 ng/mL): Rules out insulinoma and endogenous insulin secretion; indicates non-insulin-mediated hypoglycemia (critical illness, malnutrition, exogenous insulin/medication use)
    • Factors Affecting Results:
      • Renal function: Impaired kidney function increases C-peptide levels (reduced clearance)
      • Obesity: Often associated with elevated C-peptide due to insulin resistance
      • Medications: Certain drugs (thiazide diuretics, corticosteroids, estrogen) may elevate C-peptide
      • Physical stress: Acute illness, surgery, or infection may temporarily alter results
      • Timing of collection: Must be truly fasting to establish accurate baseline
      • Age and gender: Minimal variation; values relatively stable across demographics
    • Clinical Significance: C-peptide fasting is superior to insulin measurement for assessing endogenous pancreatic function because it is not affected by exogenous insulin therapy and has longer half-life (5-30 minutes vs 3-5 minutes for insulin), providing more stable and accurate assessment of beta cell reserve
  • Associated Organs
    • Primary Organ System: Pancreas (beta cells within the islets of Langerhans)
    • Secondary Organs Involved: Kidneys (C-peptide clearance); liver (glucose metabolism and regulation)
    • Diseases Diagnosed or Monitored:
      • Type 1 Diabetes Mellitus: Autoimmune destruction of beta cells; low C-peptide indicates advanced disease
      • Type 2 Diabetes Mellitus: Initial high C-peptide from compensatory hyperinsulinemia; eventually decreases with disease progression and beta cell failure
      • Insulinomas: Neuroendocrine tumors producing excessive insulin; elevated C-peptide with hypoglycemia
      • Chronic Pancreatitis: Progressive pancreatic damage reduces functional beta cell mass; low C-peptide
      • Pancreatic Cancer: May destroy beta cells; low C-peptide with malignancy
      • Metabolic Syndrome and Insulin Resistance: High C-peptide reflects compensatory beta cell hyperfunction
      • Autoimmune Polyglandular Syndrome: Multiple endocrine gland involvement; may affect pancreatic function
      • Neonatal Hyperinsulinism: Genetic condition causing excessive beta cell insulin secretion in infants
    • Associated Complications:
      • Severe Hypoglycemia: Recurrent hypoglycemic episodes from uncontrolled endogenous insulin secretion; risk of seizures and loss of consciousness
      • Hyperglycemia and Diabetic Complications: Including neuropathy, nephropathy, retinopathy, and cardiovascular disease
      • Acute Pancreatitis: From insulinoma or pancreatic disease; life-threatening inflammation
      • Diabetic Ketoacidosis: In uncontrolled Type 1 diabetes with severe hyperglycemia
      • Chronic Kidney Disease: From diabetic nephropathy; impairs C-peptide clearance and increases levels
  • Follow-up Tests
    • Recommended Follow-up Testing:
      • Fasting Blood Glucose: Essential baseline to correlate with C-peptide level for diabetes diagnosis
      • HbA1c (Glycated Hemoglobin): Assesses long-term glucose control over 2-3 months; useful for diabetes monitoring
      • Fasting Serum Insulin: Directly measures circulating insulin; used alongside C-peptide for insulin resistance assessment
      • HOMA-IR (Homeostatic Model Assessment of Insulin Resistance): Calculated from fasting glucose and insulin; quantifies insulin resistance
      • Oral Glucose Tolerance Test (OGTT): Evaluates pancreatic response to glucose stimulation; useful for impaired glucose tolerance
      • Stimulated C-Peptide Testing: Using glucagon or glucose stimulation to assess beta cell reserve and function
      • 72-Hour Fast with C-Peptide and Insulin: Gold standard for insulinoma diagnosis; monitors glucose, insulin, and C-peptide during controlled fasting
      • Serum/Urine Sulfonylureas and Meglitinides: Screen for factitious hypoglycemia from medication use
      • Pancreatic Imaging (CT/MRI): Localize insulinomas or assess pancreatic pathology if C-peptide abnormalities detected
      • Pancreatic Autoantibodies (GAD, IA-2, ZnT8): Assess for autoimmune Type 1 diabetes or LADA (Latent Autoimmune Diabetes in Adults)
      • Renal Function Tests (Creatinine, eGFR): Essential because renal impairment affects C-peptide clearance and interpretation
    • Monitoring Frequency:
      • Type 1 Diabetes: Yearly assessment of C-peptide to monitor residual beta cell function; more frequent during early 'honeymoon phase'
      • Type 2 Diabetes: Baseline C-peptide assessment; repeat every 1-2 years to monitor beta cell reserve decline
      • Insulinoma Monitoring: Post-surgical follow-up every 3-6 months initially; less frequent if complete remission
      • Hypoglycemia Workup: Single diagnostic testing; repeat if results inconclusive or clinical suspicion remains high
  • Fasting Required?
    • Fasting Required: YES
    • Fasting Duration: Minimum 8 hours overnight fast; ideally 10-12 hours for optimal standardization
    • Timing of Test: Early morning collection (7:00-9:00 AM) preferred after fasting overnight from midnight
    • Allowed During Fasting:
      • Water (plain water only, no added flavoring or electrolytes)
      • Necessary medications with small sips of water if medically required
    • NOT Allowed During Fasting:
      • Any food or beverages containing calories (including coffee, tea, juice, milk)
      • Chewing gum, mints, or candy
      • Smoking or tobacco use
    • Medications to Discuss:
      • Insulin and anti-diabetic agents: Generally continue as prescribed; inform clinician
      • Corticosteroids: May affect C-peptide levels; inform laboratory if recently used
      • Thiazide diuretics and estrogen: Can elevate C-peptide; document in test requisition
      • Beta-blockers: Generally safe but may affect metabolic response
    • Patient Preparation Instructions:
      • Stop eating and drinking (except water) at midnight the night before the test
      • Maintain normal physical activity the day before; avoid strenuous exercise on test day
      • Manage stress: Avoid unusual stress or anxiety before testing as this affects glucose and C-peptide
      • Avoid alcohol for 24 hours before test; can affect glucose metabolism
      • Arrive at laboratory well-rested; allow adequate time to sit quietly for 5-10 minutes before blood draw
      • Bring identification and insurance information
      • Wear loose clothing to facilitate venipuncture
      • Inform phlebotomist of any recent acute illness, infections, or significant physical stress
    • Special Considerations: Fasting requirement is critical for accurate C-peptide interpretation as fed state significantly elevates C-peptide from postprandial beta cell stimulation; falsely elevated results will occur if patient has consumed food/beverages before blood draw

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