Preparation, calibration and evaluation of the First International Standard for human C-peptide

Abstract

Background: Measurement of C-peptide by immunoassay contributes to the diagnosis of a number of disorders related to β cell function. Stocks of the current international reference reagent (IRR) for C-peptide, used to calibrate these immunoassays, are exhausted, and this report summarises the international study to establish a replacement World Health Organization (WHO) international standard (IS) to maintain the availability of a globally available reference material and support efforts to standardise C-peptide assays. Methods: The study was conducted in three phases; phase I involved the assignment of a value to a primary calibrant in mass units by amino acid analysis and phase II applied this value to the calibration of a candidate standard, 13/146, by reversed phase high-performance liquid chromatography (RP-HPLC) assay. In phase III, the candidate standard was compared to the first IRR by current immunoassays to assess its suitability to serve as an IS. Results: Calibration of the candidate standard by RP-HPLC gave a final estimated content of 8.64 μg/ampoule with expanded uncertainty of 8.21-9.07 μg/ampoule (95% confidence; k = 2.45). The candidate standard also appears sufficiently stable to serve as an IS, based on HPLC analysis of accelerated thermal degradation samples of 13/146, and was also shown to have appropriate immunological activity. A difference in bias approach was used to assess the commutability of 13/146 with human serum and urine samples. With the exception of two laboratories, the candidate standard demonstrated commutability with respect to the serum and urine samples included in this study. Conclusions:The candidate standard, 13/146, is suitable to serve as the First International Standard for human

Implementing a Reference Measurement System for C-Peptide: Successes and Lessons Learned

“…The candidate standard was also shown to have appropriate immunological activity in the immunoassay systems included in the study. C-peptide is measured clinically in serum/plasma and urine, and so, using a difference in bias approach (27)in ten of these immunoassay systems to assess the commutability of the candidate standard with serum and urine samples, it was additionally shown that with the exception of two of these assays the candidate standard demonstrated commutability with respect to the serum and urine samples included in the study (30). The First International Standard for C-peptide is now available from NIBSC.…”

“…Each ampoule of the material contains 8.64 μg of C-peptide with an expanded uncertainty of 0.43 μg. Since the material is solely intended to be used as a reference material for immunoassays, it was sent to manufacturers of C-peptide immunoassay methods and shown to be commutable with serum samples for 8 of 10 commercially available immunoassays (30). …”

Current Status of Serum Insulin and C-Peptide Measurement in Clinical Laboratories: Experience from 94 Laboratories in China

“…A likely explanation is that, as polypeptides, C-peptide and insulin may behave differently in pure solution than in a sample-matrix solution. The first international standard for human C-peptide (WHO ISR 13/146) and its international reference reagent (WHO IRR84/510) were reported to be incommutable for some measurement systems [ 12 , 16 ]. Comparability of C-peptide and insulin measurements between laboratories and measurement systems was effectively improved using one low-level and one high-level pooled sample to recalibrate clinical measurement systems, as reported previously [ 13 , 14 ].…”

Protective role of marine macroalgae extracts against STZ induced diabetic rats

“…C-peptide and insulin are the important products in proinsulin enzymatic cleavage that secreted into the circulation in equimolar concentrations [51]. C-peptide and insulin levels were decreased significantly in diabetes rats.…”