Interpretation of Insulin, C‑Peptide, and Proinsulin Measurements in Hypoglycemia

Normal Physiological Response to Documented Hypoglycemia

• During true hypoglycemia (plasma glucose ≤55 mg/dL or 3.0 mmol/L), the normal endocrine response is complete suppression of endogenous insulin secretion, resulting in low insulin and low C‑peptide concentrations. [1][2]

• Any measurable elevation of insulin or C‑peptide in the setting of confirmed hypoglycemia indicates inappropriate insulin secretion and warrants further evaluation. [1][2]

Diagnostic Patterns

Endogenous Hyperinsulinism (Insulinoma / Nesidioblastosis)

• Simultaneous elevation of insulin and C‑peptide is diagnostic for endogenous hyperinsulinism when glucose is ≤45 mg/dL (2.5 mmol/L); this pattern has approximately 100 % sensitivity and specificity. 1

• More recent data (Praxis Medical Insights) suggest that a C‑peptide cutoff of ≥0.30 nmol/L at glucose <2.3 mmol/L yields sensitivity ≈ 96 % and specificity ≈ 100 % (strength: high). 3

Exogenous Insulin (Factitious Hypoglycemia)

• A pattern of markedly elevated insulin with suppressed C‑peptide is diagnostic for surreptitious insulin injection, because exogenous insulin lacks C‑peptide and suppresses endogenous secretion. 3

• Certain rapid‑acting or long‑acting insulin analogs may escape detection by some insulin assays, producing falsely low insulin values; therefore, C‑peptide measurement is mandatory to avoid missed factitious hypoglycemia. 3

Sulfonylurea / Meglitinide Ingestion

• Both insulin and C‑peptide are elevated, mimicking endogenous hyperinsulinism; however, a positive oral hypoglycemic agent screen distinguishes drug‑induced hyperinsulinemic hypoglycemia. Routine screening for sulfonylureas and meglitinides is recommended in any unexplained hyperinsulinemic episode. 1

Insulin Autoimmune Syndrome (Hirata Disease)

• Insulin and C‑peptide appear elevated, but the presence of insulin antibodies confirms the autoimmune etiology. Routine measurement of insulin antibodies is advised in the work‑up of unexplained hypoglycemia. 1

Relative Utility of the Biochemical Markers

C‑Peptide

• Considered the most reliable marker for endogenous insulin secretion because it is released in equimolar amounts with insulin, is not cleared by the liver, has a longer half‑life, and is unaffected by exogenous insulin administration. (Evidence: high‑quality guideline recommendation). 3

Insulin

• Insulin measurement is less reliable due to hepatic clearance, assay cross‑reactivity with proinsulin (up to 40 % in some assays), and potential underestimation in hemolyzed samples. Small insulinomas may produce insulin levels that fall below conventional cutoffs. (Evidence: moderate). 3

Proinsulin

• Although not routinely required, elevated proinsulin (>5 pmol/L when glucose <2.5 mmol/L) is highly specific for endogenous hyperinsulinism and may remain elevated even when insulin measurements are borderline. (Evidence: moderate). 3

Recommended Diagnostic Approach

• The Endocrine Society recommends simultaneous measurement of plasma glucose, insulin, C‑peptide, and proinsulin during a documented hypoglycemic episode (glucose <55 mg/dL or 3.0 mmol/L). 1

• If spontaneous hypoglycemia cannot be captured, a supervised 72‑hour fast (or mixed‑meal test) should be performed to provoke the biochemical abnormalities. 1

• Additional mandatory tests include:

Critical Thresholds Summary

*Thresholds are based on the 2009 Endocrine Society guidance; individual laboratories must validate cut‑offs on their specific assay platforms. [1][2]

Caveats and Special Populations

• Renal insufficiency: C‑peptide clearance is reduced, invalidating standard thresholds; β‑hydroxybutyrate and glucagon stimulation testing are preferred. 3

• Hepatic insufficiency and post‑pancreatectomy patients: Thresholds have not been validated; interpret results with caution. 3

• Assay variability: Laboratories must verify assay‑specific cut‑offs because inter‑assay differences can be substantial. 3