How the immunoassay transformed C-peptide from a duckling into a swan.

This edition of 'Then and now' re-examines Lise Heding's very highly cited paper 'Radioimmunological determination of human C-peptide in serum', which was published in Diabetologia in 1975. We show how this article and other related articles by Heding contributed to heightened respect for C-peptide (and transformation of Heding's research programme). Initially thought of as an inert discard, C-peptide in blood is now recognised as an excellent surrogate measure of insulin secretion under a wide range of conditions. The assay is especially valuable for acute ascertainment of the insulin secretory capabilities of patients with type 1 diabetes or of transplanted beta cells. The assay is also being used to monitor endogenous beta cell loss or in vivo expansion of beta cell mass over the long term. We conclude with two promising future applications: (1) measurements of C-peptide in blood (along with insulin, glucose, and HbA(1c)) at annual intervals as a potential approach to earlier diagnosis of diabetes; and (2) among many recent advances in recognising properties of C-peptide (including status as a candidate hormone), most promising is C-peptide as a possible therapy for diabetic neuropathy and nephropathy.