Insulin and C-peptide are secreted into portal vein in equimolar amounts, but serum ratio = 1:5 to 1:15 due to removal of approximately 50% of insulin from blood during initial passage through the liver. C-peptide half-life = approximately 30 minutes.
Normal range: fasting molar ratio insulin to C-peptide = 1.0.
Use
To differentiate insulinoma from factitious hypoglycemia due to insulin
Interpretation
Less than 1.0 in molarity units (or >47.17 µg/ng in conventional units)
Greater than 1.0 in molarity units (or <47.17 µg/ng in conventional units)
Exogenous insulin administration
Cirrhosis
Limitations
There are ethnic differences in insulin/C-peptide ratio in both fasting and glucose-stimulated conditions in normal young nondiabetic pregnant women. Compared with their Caucasian and Hispanic counterparts, African American women had indices suggestive of lower insulin production and greater insulin resistance (i.e., a lower C-peptide concentration, a lower C/I ratio, and elevations in insulin and the I/G ratio).