Six methods for the determination of C-peptide evaluated

Six assays for C-peptide were evaluated. Five assays were radioimmunoassays ; one was a chemiluminescence assay. The within-run coefficient of variatio n ranged between 2.1 and 22.9 %. The methods correlated well with each othe r (r = 0.925 to 0.978). There were, however, proportional biases between th e assays, in particular in the higher concentration range. All assays were linear up to at least 4.5 ng/ml C-peptide concentration. All assays were ea sy to perform. However, the systematic bias between the assays demonstrates that further effort is needed to standardize the measurement of C-peptide. In the meantime, we recommend that laboratories conducting C-peptide measu rements establish local, method-specific reference ranges.