Postprandial serum C-peptide to plasma glucose concentration ratio correlates with oral glucose tolerance test- and glucose clamp-based disposition indexes

ObjectiveThe C-peptide index (CPI), a ratio of serum C-peptide to plasma glucose levels, is a readily measured index of β-cell function. The difference in the physiological features reflected by the index measured under fasting (F-CPI) or postprandial (PP-CPI) conditions has remained unclear, however.Materials/MethodsWe investigated the relationship of the two CPIs to indexes of insulin secretion measured with an oral glucose tolerance test (OGTT) or with hyperglycemic and hyperinsulinemic–euglycemic clamp analyses as well as to disposition indexes (indexes of insulin secretion adjusted for insulin sensitivity) calculated from OGTT- or clamp-based analyses. We also examined the relationship between glucose tolerance and the clamp-based disposition index.ResultsThe clamp-based disposition index declined progressively from normal glucose tolerance to impaired glucose tolerance to Type 2 diabetes, and it strongly correlated with the 2-h plasma glucose level during an OGTT. For patients with Type 2 diabetes, both F-CPI and PP-CPI correlated with indexes of insulin secretion including HOMA-β, the insulinogenic index, the ratio of the area under the insulin curve to that under the glucose curve during an OGTT, the serum C-peptide level after glucagon challenge, as well as early and total insulin secretion measured with a hyperglycemic clamp. PP-CPI, but not F-CPI, was significantly correlated with clamp-based and OGTT-based disposition indexes.ConclusionsF-CPI was correlated only with unadjusted indexes of insulin secretion, whereas PP-CPI was correlated with such indexes as well as with those adjusted for insulin sensitivity. The better clinical utility of PP-CPI might be attributable to these physiological characteristics.