How low is really low? Comparison of two C‐peptide assays to establish residual C‐peptide production in type 1 diabetes

Abstract

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.

Fasting and meal‐stimulated serum C‐peptide in long‐standing type 1 diabetes mellitus

“…no. IM3639, distributed by IMMUNOTECH s.r.o., Prague, Czech Republic) 4 . The limit of quantification of this ultrasensitive assay was 3.8 pmol/L, and interassay coefficient of variation (c.v.) was 9.1% at 6.5 pmol/L.…”

“… 5 Moreover, early studies showed a relation between C‐peptide reserve and fewer complications and severe hypoglycemia. 1 , 2 Using highly sensitive assays, 3 , 4 it became clear that in some type 1 diabetes individuals, even decades after diagnosis, residual insulin secretion persists. The presence or absence of residual insulin secretion is a biomarker of heterogeneity in type 1 diabetes disease (course).…”

Definition, Klassifikation, Diagnostik und Differenzialdiagnostik des Diabetes mellitus: Update 2022

“…Wie bei vielen anderen Messmethoden, hängt beim C-Peptid die Reproduzierbarkeit der Messung (VK), sowie die untere Nachweisgrenze von den verwendeten Assays ab 42 . Daher sollte die Messgüte bei diesen Messgrößen Labor-intern verifiziert werden, bevor sie bei der Differentialdiagnostik des Diabetes klinisch eingesetzt werden.…”

Which C-peptide assay do you use? Increasing need for describing C-peptide assay performance

“…This should include the WHO International Standard for human C-peptide (batch 13/146) [ 4 ]. We recently compared 2 commercially available assays and found that one of the assays did not meet the detection limit (limit of quantitation [LOQ]) claimed by the manufacturer [ 5 ], although both assays correctly measured the WHO standard for C-peptide.…”