Introduction
C‐peptide is an important marker to assess residual insulin production in individuals with type 1 diabetes (T1D). The accuracy and detection limits of C‐peptide assays are important to detect C‐peptide microsecretion and to reliably observe changes over time in these people. We compared and verified two commercially available assays able to measure C‐peptide in the picomolar range.
Methods
The ultrasensitive Mercodia enzyme‐linked immunosorbent C‐peptide assay (ELISA) was compared with the Beckman immunoradiometric assay (IRMA) for C‐peptide, assessing reproducibility (coefficient of variation [CV]), limit of blank (LoB), limit of detection (LoD) and limit of quantitation (LoQ).
Results
For both assays within‐run and between‐run variation were high at the low (around the detection limit) C‐peptide concentration range, with CVs of around 40%. LoB values for the ultrasensitive ELISA and the IRMA were 1.3 and 0.16 pmol/L respectively. LoD values were 2.4 and 0.54 pmol/L respectively. LoQ values were 9.7 and 3.8 pmol/L respectively. Only the IRMA met the specifications claimed by the manufacturer.
Conclusions
The IRMA provided the lowest threshold for quantification of serum C‐peptide. LoQ of commercially available assays should be established in‐house before applying them in research studies and clinical trials in which low C‐peptide levels have clinical or scientific relevance.
Many C-peptide assays are commercially available for research and routine use. However, not all assays yield consistent results, especially in the low concentration ranges. We searched the literature describing C-peptide measurements to assess which assays are mainly used in the diabetes research field and if they are specified. Percentages of publications on C-peptide measurements in type 1 diabetes (T1D), type 2 diabetes (T2D) and other forms of diabetes were 32%, 54% and 14%, respectively. In only 54% of the publications the used assay was specified. Information on detection limit, measurement range and variation was provided in 12%, 2% and 11% of publications, respectively. In 22% of all publications no C-peptides concentrations were mentioned. This may be a problem especially for T1D research, where measuring very low levels of C-peptide is becoming increasingly important and concordance between assays is low.
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