C-peptide to glucose ratio

When to use

• Assessing residual beta-cell function to help guide whether insulin therapy is likely required, e.g. when reclassifying diabetes type or choosing glucose-lowering therapy.

Formula

CGR = [fasting C-peptide (ng/mL) / fasting plasma glucose (mg/dL)] x 100. CGR <2 indicates insulin deficiency (insulin therapy generally required); 2-5 indicates intermediate insulin secretion (insulin possible); >5 indicates adequate endogenous secretion (insulin generally avoidable).

Pearls and pitfalls

• Should be measured with a fasting plasma glucose below roughly 250 mg/dL; higher glucose can acutely (and reversibly) suppress C-peptide secretion via glucotoxicity and confound the result.
• Not valid when eGFR is below roughly 50 mL/min/1.73m^2 (C-peptide is renally cleared), or immediately after an acute hyperglycemic insult.

References

1. Fritsche A, Heni M, Peter A, et al. Considering insulin secretory capacity as measured by a fasting C-peptide/glucose ratio in selecting glucose-lowering medications. Exp Clin Endocrinol Diabetes. 2022;130(3):200-204.
2. Jones AG, Hattersley AT. The clinical utility of C-peptide measurement in the care of patients with diabetes. Diabet Med. 2013;30(7):803-817.