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

OBJECTIVE: The 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/METHODS: We 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.

RESULTS: The 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.

CONCLUSIONS: F-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.