Guidelines for Post‑Meal C‑Peptide Testing and Interpretation

1. Standardized Testing Protocol

Mixed‑meal composition: Administer a standardized mixed meal providing ≈500 kcal with 50 % of calories from carbohydrate after an overnight fast of ≥8 hours. 1
Patient preparation: Hold all non‑essential medications before the test. 1
Sampling schedule: Obtain blood samples immediately before meal ingestion and then every 30 minutes for a total of 300 minutes (5 hours). 1
Analytes measured at each time point: Plasma glucose (laboratory‑measured), insulin, C‑peptide, and proinsulin. 1
Glucose measurement method: Use laboratory‑based plasma glucose assays; point‑of‑care glucose meters are insufficient for diagnostic decisions. 1

90‑minute stimulated C‑peptide:
- ≥ 0.2 nmol/L (≈0.6 ng/mL) indicates significant endogenous insulin secretion. 1
- < 0.2 nmol/L indicates severe insulin deficiency. 1
Diagnostic performance of the 90‑minute threshold: 98 % sensitivity and 100 % specificity for detecting clinically meaningful insulin secretion. 1
Diabetes classification (post‑meal C‑peptide):
- < 0.2 nmol/L suggests type 1 diabetes. [3][4]
- > 0.6 nmol/L suggests type 2 diabetes or preserved β‑cell function. [3][4]

Inappropriate endogenous insulin secretion: C‑peptide ≥ 0.6 ng/mL (≥ 0.2 nmol/L) together with insulin ≥ 3 µU/mL points to insulinoma, nesidioblastosis, or autoimmune hypoglycemia. 5
Exogenous insulin administration: C‑peptide < 0.6 ng/mL with insulin > 3 µU/mL indicates that the insulin is exogenous. 5
Diagnostic accuracy of this combination: Approximately 100 % sensitivity and specificity for detecting inappropriate insulin secretion. 6

Hypoglycemic episodes typically occur 2–4 hours after meals, distinguishing them from fasting hypoglycemia of insulinoma. 7

The normal biphasic insulin secretion pattern (first phase 3–5 min, second phase 60–120 min) is lost in type 2 diabetes and pre‑diabetes. 8

Assay variability: Significant inter‑assay variability exists despite use of the same WHO calibrator; laboratories must validate cut‑offs on their specific platform. 6
Interpretation with exogenous insulin: Exogenous insulin does not cross‑react with C‑peptide assays, and its presence reduces peak C‑peptide by ~20 % without compromising the 0.2 nmol/L threshold’s classification accuracy. 6

Renal insufficiency: Standard C‑peptide thresholds are not applicable; alternative stimulation (β‑hydroxybutyrate, glucagon) should be used. 6
Hepatic insufficiency, post‑partial pancreatectomy, and insulin autoimmune syndrome: Thresholds have not been validated; interpret results with caution and consider additional markers (e.g., extremely high insulin > 100 µU/mL, positive insulin antibodies). [6][7]

Oral glucose tolerance test (OGTT): Not recommended for evaluating post‑prandial hypoglycemia; mixed‑meal testing is preferred. 5
Point‑of‑care glucose meters: Should not be relied upon for diagnostic interpretation of post‑meal C‑peptide testing. 1