Methodological Aspects on C‐peptide Measurements

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Abstract
It is rather difficult to draw conclusions from reported C‐peptide values, as the methods for determination differ, and C‐peptide may be measured in serum or in urine with the patient fasting or after stimulation. We have followed prospectively 49 children with IDDM with regular determinations of serum C‐peptide fasting and after a standardized breakfast. A subgroup of seven patients have been studied more thoroughly with 24‐hour‐profile of serum C‐peptide, C‐peptide excretion in urine, and stimulation by i.v. glucose + i.v. arginine. Our results indicate that the stimulation of the beta cells usually reaches a maximum around a blood glucose level of 10–12 mmol/l leading to a curve linear relationship between serum C‐peptide and blood glucose. Thus a simple quotient is not so useful but the degree of stimulation should be stated and actual blood glucose value noticed. Stimulation with a standardized breakfast gives roughly the same information as maximal stimulation with i.v. glucose + arginine, and little extra information is found by a 24‐hour‐profile. Urinary C‐peptide may give valuable information if it is related to the actual degree of metabolic balance. It can be of special interest in patients with very low serum C‐peptide levels.