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Evaluation of the commutability of control materials for C-peptide proficiency testing.

C-peptide testing is a backbone of diabetes research, but labs still face a headache: different assays, different results. A new study from Korea tackled a key piece of this puzzle—commutability. Translation: do the control samples used to check C-peptide test accuracy behave like real patient samples across different machines? If not, external quality assessment (EQA) programs risk comparing apples to oranges.

Clin Chim Acta

by Sooho Yu MD, Jung S, Yong Kwan Lim MD et al.

“Evaluation of the commutability of control materials for C-peptide proficiency testing. Sooho Yu MD(1), Jung S(2), Yong Kwan Lim MD(3), Soo Jin Yoo MD(4), Park HD(5). Author information: (1)Department of Laboratory Medicine and Genetics, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Republic of Korea. (2)Sungkyunkwan University School of Medicine, Seoul, Republic of Korea. (3)Department of Laboratory Medicine, Chung-Ang University Hospital, Chung-Ang University College of Medicine, Seoul, Republic of Korea. (4)Department of Laboratory medicine, Uijeongbu Eulji University Hospital, Uijeongbu-si, Republic of Korea. (5)Department of Laboratory Medicine and Genetics, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Republic of Korea; Department of Medical Device Management and Research, SAIHST, Sungkyunkwan University, Seoul, Republic of Korea. Electronic address: nayadoo@hanmail.net. BACKGROUND: Commutability refers to the consistency of measurement results between reference materials (RMs) and patient samples across different measurement procedures. C-peptide is essential for diabetes classification and management, however, inter-assay variability persists despite ongoing harmonization efforts. The use of commutable materials is therefore critical for calibration and external quality assessment (EQA). This study evaluated the commutability of pooled human serum and commercial control materials using CLSI- and IFCC-based approaches. METHODS: Commutability was evaluated using 30 clinical samples and six RMs, including three serum-based in-house prepared materials and three commercial materials. C-peptide was measured in triplicate using four major measurement procedures in Korea; Alinity i (Abbott Diagnostics), cobas e801 (Roche Diagnostics), UniCel DxI 800 (Beckman Coulter), and Atellica IM 1600 (Siemens Healthineers). Commutability was assessed using the CLSI EP14-ED4 Deming regression-based approach with 95% prediction intervals (PIs) and the IFCC difference-in-bias approach with expanded uncertainty, applying a total allowable error of 7.1% and a coverage factor of 1.9. RESULTS: Three clinical samples were excluded from the analyses after being identified as outliers in scatter plots. Using the CLSI approach with the remaining 27 clinical samples, all RMs fell within the 95% PIs across all method pairs and were classified as commutable. In contrast, the IFCC approach, with concentration partitioning at 1.0 ng/mL distinguished differences in commutability among the RMs. Serum-based in-house prepared materials were classified as commutable or indeterminate, whereas commercial materials showed multiple indeterminate results and one non-commutable result. CONCLUSION: Pooled human serum-based in-house prepared materials demonstrated more stable commutability than commercial materials, supporting their use in C-peptide EQA programs. Copyright © 2026. Published by Elsevier B.V. Conflict of interest statement: Declaration of competing interest The authors declare no conflict of interest.”

Researchers rounded up 30 clinical samples and six control materials—three homemade, three commercial. They sent them through four major C-peptide analyzers used in Korea. Two big-name commutability checks (CLSI and IFCC) were used, with strict error limits to keep things honest.

Key takeaway: Serum-based, in-house controls outperformed most commercial options. Using the CLSI method, all controls looked solid. But the IFCC approach—tougher and more granular—flagged several commercial materials as indeterminate or outright non-commutable at certain concentrations. In-house serum pools were consistently commutable or at worst indeterminate, showing an edge for labs that can make their own.

Why it matters:

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Reliable control materials mean less noise and more trust in C-peptide data across labs.

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Standardizing on commutable controls could tighten diabetes classification, research comparability, and even future diagnostics.

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Commercial controls aren’t always plug-and-play—homemade might beat store-bought for now.

For anyone deep in peptide measurement, this is a nudge to scrutinize your calibrators, not just your machines. The study also spotlights why rigorous EQA standards are vital for peptide research. For more on peptide analytics and assay challenges, check out the peptide research index.

Bottom line: In-house serum controls may be the gold standard for reliable C-peptide proficiency testing.

Back to News Read original on Clin Chim Acta

Evaluation of the commutability of control materials for C-peptide proficiency testing.

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