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🧪 C-Peptide – Fasting

Test Name	C-Peptide (Fasting)
Sample Type	Serum (Blood)
Fasting Required	✅ Yes – 8 to 10 hours of fasting
Ideal Collection Time	Morning (before insulin or oral medications)
Normal Range (Fasting)	0.78 – 1.89 ng/mL (varies by lab)
Method Used	CLIA / ECLIA (Electrochemiluminescence Immunoassay)
Reporting Time	1–2 working days
Department	Endocrinology / Diabetology

🧬 What is C-Peptide?

C-Peptide is a connecting peptide released in equal amounts with insulin when the pancreas processes proinsulin into:

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

🔍 Why is the C-Peptide Fasting Test Done?

Purpose	Explanation
Differentiate Type 1 vs Type 2 Diabetes	Low in Type 1 (due to beta-cell failure); normal/high in Type 2
Evaluate insulin production	Assesses endogenous insulin reserve
Diagnose insulinoma	High C-peptide + high insulin = likely insulin-secreting tumor
Monitor pancreatic function post-surgery	After partial pancreatectomy or islet transplant
Detect factitious hypoglycemia	High insulin + low C-peptide = exogenous insulin use

⚖️ Interpreting the Results

C-Peptide Level (Fasting)	Possible Interpretation
🔻 Low	Type 1 Diabetes, late-stage Type 2, or insulin overuse
✅ Normal	Typical for early Type 2 or healthy individuals
🔺 High	Insulin resistance, insulinoma, or sulfonylurea use

✅ Note: A stimulated C-Peptide test (with glucose or glucagon challenge) may be used for more accurate beta-cell function evaluation.

🔗 Tests Commonly Ordered Alongside

Test	Purpose
Fasting Insulin	Direct measure of insulin levels
Blood Glucose (Fasting/PP)	Correlate with insulin/C-peptide levels
HbA1c	Long-term blood glucose control
Anti-GAD Antibodies	Diagnose autoimmune Type 1 diabetes
Insulin Autoantibodies	Distinguish between exogenous insulin use and insulinoma

📋 Summary Table

Feature	Details
Test Type	Fasting Blood Test
Primary Purpose	Evaluate pancreatic insulin production
Useful For	Diagnosing diabetes type, insulinoma, hypoglycemia cause
Paired With	Glucose, Insulin, HbA1c, Autoantibodies

C-Peptide Fasting