Endocrine Abstracts

Background: Insulin, derived from a single chain precursor pro-insulin, is cleaved by convertases to form insulin (t1/2 – 4 min) and C-peptide (t1/2 – 30 min). The ratio of insulin to C-peptide levels, which is usually less than one, is reversed in presence of exogenous insulin and insulin autoimmune syndrome. We present the significance of insulin-C-peptide ratio (ICR) in the diagnostic algorithm for the investigation of endogenous hyperinsulinaemia.

Methods: All requests for measurement of insulin and C-peptide levels were retrospectively reviewed over a 4 years period from 2006 to 2009 from two hospitals. Information regarding the indications for the requisitions, the clinical history and the final diagnosis was retrieved after review of the clinical notes.

Results: Of 52 samples from 21 patients were analysed. 11/21 patients were being investigated for hypoglycaemia (Table 1) and 10 patients were investigated for determining the type of diabetes mellitus.

ICR was <1 (Mean-0.05) in all cases except in two cases (exogenous insulin administration and insulin autoimmune syndrome (IAS)). In the former, insulin was elevated (445 pmol/l) with undetectable C-peptide levels (<94 pmol/l). In the patient with IAS, insulin levels was disproportionately high (17 750 pmol/l) along with corresponding high levels of C-peptide levels (8520 pmol/l) i.e. ICR >1.

Conclusion: When exogenous insulin administration is excluded, the reversal of ICR (normally <1) can be considered as a surrogate marker for causes associated with elevated insulin antibody levels. This may negate the necessity of measuring insulin antibody levels in all cases of endogenous hyperinsulinaemia.