Human proinsulin connecting peptide (C-peptide) was measured by immunoassay in urine from 25 normal subjects, 18 patients with diabetes mellitus, and 34 patients with various degrees of renal insufficiency. Assay validation studies showed that pancreatic C-peptide was quantitatively recovered when added to urine. Fractionation of urine by gel filtration indicated that most endogenous C-peptide eluted in fractions that corresponded to the C-peptide standard.

In 34 nondiabetic subjects with normal kidney function or various renal diseases, C-peptide clearance was independent of creatinine clearance over a range of 6 to 190 ml./min. Urine C-peptide clearance (5.1 ± 0.6 ml./min.) is greater than that of insulin (1.1 ± 0.2 ml./min.), and the total quantity of C-peptide excreted in the urine per day represents 5 per cent of pancreatic secretion, as against only 0.1 per cent of secreted insulin.

Healthy subjects excreted 36 ± 4 μg. C-peptide per 24 hours, while this value in juvenile-onset diabetics was only 1.1 ± 0.5 μg. Adult-onset diabetics excreted 24 ± 7 μg./24 hr., the range overlapping the excretory rates of both normal subjects and juvenileonset diabetics. Two insulin-requiring adult-onset diabetics showed significant beta-cell reserve during the course of acute infections.

These results suggest that urine C-peptide provides a useful means of assessing beta-cell secretory capacity over a period of time and is especially advantageous when frequent blood sampling is not feasible.

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