C‑Peptide Measurement in Patients Receiving Exogenous Insulin

1. Fundamental Principles

C‑peptide remains a reliable marker of endogenous insulin secretion even when patients are on exogenous insulin, because administered insulin does not cross‑react with C‑peptide assays and C‑peptide is released in equimolar amounts with endogenous insulin. 1, 2
C‑peptide has no biological activity; it is produced in equal quantities to endogenous insulin during proinsulin processing. 1
Exogenous insulin does not directly alter measured C‑peptide concentrations, as standard assays detect only endogenous C‑peptide. 1, 2
Compared with insulin levels, C‑peptide offers superior diagnostic value because it is not cleared by the liver and exhibits a more constant excretion rate over a longer period. 1

An insulin concentration markedly above 3 mIU/L together with a C‑peptide level < 0.6 ng/mL strongly suggests exogenous insulin administration and is diagnostic for factitious hypoglycemia caused by injected insulin. 3, 4
Certain rapid‑acting or long‑acting insulin analogues (e.g., lispro, aspart, glargine) may be missed by some insulin assays, producing a low measured insulin despite exogenous administration and thereby mimicking non‑insulin hypoglycemia. 3, 4

Simultaneous insulin ≥ 3 mIU/L and C‑peptide ≥ 0.6 ng/mL indicate endogenous hyperinsulinism (e.g., insulinoma or nesidioblastosis). 3, 4
Differential diagnoses for this pattern include inadvertent sulfonylurea/meglitinide ingestion and the presence of anti‑insulin antibodies. 3, 4

C‑peptide < 0.2 nmol/L (≈ 0.6 ng/mL) is characteristic of type 1 diabetes with severe insulin deficiency. 2, 5
In hypoglycemia (plasma glucose ≤ 45 mg/dL ≈ 2.5 mmol/L), a C‑peptide ≥ 0.6 ng/mL provides ≈ 100 % sensitivity and specificity for detecting inappropriate endogenous insulin secretion. 1, 6, 7
C‑peptide > 0.6 nmol/L (≈ 0.2 nmol/L) suggests type 2 diabetes or preserved β‑cell function. 5
Conversion: pmol/L × 0.003 = ng/mL. 1, 6

Fasting or glucagon‑stimulated C‑peptide helps differentiate type 1 from type 2 diabetes in ambiguous presentations (e.g., a patient with a type 2 phenotype presenting in ketoacidosis). 5

C‑peptide measurement is essential for evaluating non‑diabetic hypoglycemia to exclude surreptitious insulin use. 5
Blood samples should be drawn during a documented hypoglycemic episode (plasma glucose < 55 mg/dL ≈ 3.0 mmol/L). 8, 2

For insurance approval of insulin‑pump therapy, fasting C‑peptide should be measured when the concurrent fasting plasma glucose is ≤ 220 mg/dL (≈ 12.5 mmol/L). 5

Population / Condition	Recommended Approach
Renal insufficiency	Standard C‑peptide thresholds are not applicable; use plasma β‑hydroxybutyrate assay and glucagon stimulation test.	[6, 7, 9]
Hepatic insufficiency	Thresholds have not been validated; consider alternative functional testing.	[6, 7, 9]
Post‑partial pancreatectomy	Thresholds have not been validated; interpret with caution.	[6, 7, 9]
Certain genetic forms of nesidioblastosis	May show inappropriate insulin secretion with suppressed C‑peptide at lower glucose levels; adjust interpretation accordingly.	[6, 7, 9]

Laboratories must validate C‑peptide cut‑offs using their specific assay platforms and kits, as inter‑assay variability remains significant despite standardization efforts. 6, 7, 4
Immunochemiluminometric assay (ICMA) is commonly employed; the ADVIA Centaur kit is an example of a widely used platform. 4

Anti‑insulin antibodies can elevate both insulin and C‑peptide, mimicking endogenous hyperinsulinism; therefore, insulin‑antibody testing should be performed when evaluating unexplained hypoglycemia. 3, 4
Routine measurement of insulin antibodies is recommended in the work‑up of hypoglycemia. 8, 2