Sensitivity and Reproducibility of Urinary C‐peptide as Estimate of Islet B‐cell Function in Insulin‐treated Diabetes

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Abstract
The aims of the present study were to evaluate the ability of urinary C‐peptide determination to demonstrate presence of residual insulin secretion, and to evaluate the reproducibility of urinary C‐peptide excretion in 125 insulin‐treated diabetic patients. C‐peptide was determined in two consecutive 24‐h urine specimens and related to plasma C‐peptide 6 min after the intravenous injection of 1 mg glucagon. The detection limit of C‐peptide in plasma was defined analytically (≤0.02 nmol 1−1) and from pancreatectomized patients (≤0.06 nmol I−1), and in urine only analytically (≤0.1 nmol I−1). If the analytical detection limit of plasma C‐peptide was used as indicator of residual insulin secretion, islet B‐cell function was preserved in all patients. In patients with stimulated plasma C‐peptide levels from 0.02‐−1 no increase was found in plasma C‐peptide values after stimulation with glucagon. This unresponsiveness of islet B‐cells is in good agreement with the existence of a biological detection limit of C‐peptide in plasma of 0.06 nmol I‐1. Using this biological plasma C‐peptide detection limit, 49 of 125 patients were without residual insulin secretion. In contrast to this, only 7 patients were diagnosed as C‐peptide non‐secretors using the analytical detection limit of urinary C‐peptide. Eighty‐four per cent of patients considered to have Type 1 (insulin‐dependent) diabetes with a duration of diabetes of more than 15 years had detectable C‐peptide in the urine. Only 16% of such patients had stimulated plasma C‐peptide levels above the biological detection limit. The biological detection limit of plasma C‐peptide of 0.06 nmol I−1 corresponded to a urinary C‐peptide concentration of 0.3 nmol I‐1. This value may represent a biological detection limit of C‐peptide in urine. The coefficients of variation of urinary C‐peptide expressed as nmol I−1, nmol mmol‐urinary‐creatinine−1 24 h−1 and nmol 24 h−1 were 39 %, 41 %, and 45 %, respectively. In conclusion, determination of C‐peptide in 24 h urine samples is much more sensitive as an indicator of residual insulin secretion than determination of C‐peptide in plasma using the analytical detection limit of C‐peptide in urine and the biological detection limit of C‐peptide in plasma. The final destruction of islet B‐cells may proceed more slowly than previously anticipated in Type 1 diabetes. The reproducibility of urinary C‐peptide is low.