Random non‐fasting C–peptide: bringing robust assessment of endogenous insulin secretion to the clinic

Abstract

Background: Measuring endogenous insulin secretion using C–peptide can assist diabetes management, but standard stimulation tests are impractical for clinical use. Random non‐fasting C–peptide assessment would allow testing when a patient is seen in clinic. Methods: We compared C–peptide at 90 min in the mixed meal tolerance test (sCP) with random non‐fasting blood C–peptide (rCP) and random non‐fasting urine C–peptide creatinine ratio (rUCPCR) in 41 participants with insulin‐treated diabetes [median age 72 (interquartile range 68–78); diabetes duration 21 (14–31) years]. We assessed sensitivity and specificity for previously reported optimal mixed meal test thresholds for severe insulin deficiency (sCP < 200 pmol//l) and Type 1 diabetes/inability to withdraw insulin (< 600 pmol//l), and assessed the impact of concurrent glucose. Results: rCP and sCP levels were similar (median 546 and 487 pmol//l, P = 0.92). rCP was highly correlated with sCP, r = 0.91, P < 0.0001, improving to r = 0.96 when excluding samples with concurrent glucose < 8 mmol//l. An rCP cut‐off of 200 pmol//l gave 100% sensitivity and 93% specificity for detecting severe insulin deficiency, with area under the receiver operating characteristic curve of 0.99. rCP < 600 pmol//l gave 87% sensitivity and 83% specificity to detect sCP < 600 pmol//l. Specificity improved to 100% when excluding samples with concurrent glucose < 8 mmol//l. rUCPCR (0.52 nmol/mmol) was also well‐correlated with sCP, r = 0.82, P < 0.0001. A rUCPCR cut‐off of < 0.2 nmol/ mmol gave sensitivity and specificity of 83% and 93% to detect severe insulin deficiency, with area under the receiver operating characteristic curve of 0.98. Conclusions: Random non‐fasting C–peptide measures are strongly correlated with mixed meal C–peptide, and have high sensitivity and specificity for identifying clinically relevant thresholds. These tests allow assessment of C–peptide at the point patients are seen for clinical care.