CLINICAL UTILITY OF MEASURING Insulin, Proinsulin, C-peptide and Glucagon

INSULIN

·         Evaluation of fasting hypoglycemia and insulin resistance

·    Evaluation of polycystic ovary syndrome (here women have insulin resistance and abnormal carbohydrate metabolism)

·         Predict diabetes mellitus

·         Select optimal therapy for diabetes

PROINSULIN

·         Diagnosis of beta cell tumor

·         Familial hyperproinsulinemia due to impaired conversion

C-PEPTIDE

Measurement of C-peptide has number of advantage over insulin measurement. Because hepatic metabolism is negligible, C-peptide concentrations are better indicators of beta cell function. C-peptide assay only measured endogenous insulin production so do not cross-react with insulin antibodies which interfere with insulin immunoassay. The primary indication for measuring C-peptide is for evaluation of fasting hypoglycemia. Some patients with hyperinsulinism may have high C-peptide with normal insulin concentrations. When hypoglycemia is due to exogenous insulin administration then there is low level of C-peptide but insulin will be high. Measurement of C-peptide can be used to monitor patients’ response to pancreatic surgery; C-peptide should be undetectable after surgery. It is also used to classify diabetes mellitus, in type 2 there is high level of C-peptide unlike type 1 where reduced level are seen.

The term immunoreactive insulin is referred to assay that may recognize in addition to insulin, substrates that share antigenic epitopes with insulin. E.g. proinsulin, proinsulin conversion intermediates, insulin derivatives produced by glycation or dimerization.

GLUCAGON

Very high level of glucagon is seen with alpha cell tumors of pancreas called glucagonomas. 

The reference interval of insulin after overnight fasting is 2-25 uIM/mL, Fasting serum C-peptide 0.25-0.6 nmol/L, fasting serum glucagon 70-180 ng/L.