Use of a common standard for comparison of insulin C-peptide measurements by different laboratories.

A synthetic human C-peptide analogue has been used as a common standard for the comparison of insulin C-peptide measurements in seven assay systems in six laboratories. Even in terms of this common standard there was statistically significant numerical heterogeneity between laboratories for estimates of the C-peptide content of the same plasma samples. However, the consistency in ranking order of estimates of C-peptide in the plasma samples between laboratories suggests that laboratories are in most cases measuring at least similar immunoreactive constituents and that a reference plasma might prove useful in comparing results between laboratories. Until a more suitable reference material is available, the synthetic analogue, 64 formyllysine C-peptide, in ampoules coded 76/561, will be made available for research purposes.