Pediatric Diabetes 2012: 13: 45 – 50 doi: 10.1111/j.1399-5448.2011.00807.x All rights reserved  2011 John Wiley & Sons A/S Pediatric Diabetes Original Article C-peptide in the classification of diabetes in children and adolescents Ludvigsson J, Carlsson A, Forsander G, Ivarsson S, Kockum I, Lernmark ˚ A, Lindblad B, Marcus C, Samuelsson U. C-peptide in the classification of diabetes in children and adolescents. Pediatric Diabetes 2012: 13: 45 – 50. Aim: To report C-peptide results in newly diagnosed patients and the relation to clinical diagnosis of diabetes. Methods: A nation-wide cohort, the Better Diabetes Diagnosis study was used to determine serum C-peptide at diagnosis in 2734 children and adolescents. Clinical data were collected at diagnosis and follow-up. C-peptide was determined in a validated and controlled time-resolved fluoroimmunoassay. Results: The clinical classification of diabetes, before any information on human leukocyte antigen, islet autoantibodies, or C-peptide was received, was type 1 diabetes (T1D) in 93%, type 2 diabetes (T2D) in 1.9%, maturity onset diabetes of the young (MODY) in 0.8%, secondary diabetes (0.6%), while 3.3% could not be classified. In a random, non-fasting serum sample at diagnosis, 56% of the patients had a C-peptide value >0.2 nmol/L. Children classified as T2D had the highest mean C-peptide (1.83 + 1.23 nmol/L) followed by MODY (1.04 ± 0.71 nmol/L) and T1D (0.28 ± 0.25 nmol/L). Only 1/1037 children who had C-peptide <0.2 nmol/L at diagnosis was classified with a type of diabetes other than T1D. Predictive value of C-peptide >1.0 nmol/L for the classification of either T2D or MODY was 0.46 [confidence interval 0.37 – 0.58]. Conclusions: More than half of children with newly diagnosed diabetes have clinically important residual beta-cell function. As the clinical diagnosis is not always straightforward, a random C-peptide taken at diagnosis may help to classify diabetes. There is an obvious use for C-peptide determinations to evaluate beta-cell function in children with diabetes. J Ludvigsson a , A Carlsson b , G Forsander c , S Ivarsson d , I Kockum e , ˚ A Lernmark f , B Lindblad c , C Marcus g and U Samuelsson a a Division of Pediatrics and Diabetes Research Center, Department of Clinical and Experimental Medicine, Link ¨ oping University Hospital, Link ¨ oping, Sweden; b Department of Pediatrics, Lund University Hospital, Lund, Sweden; c Department of Pediatrics, The Queen Silvia Children’s Hospital, G ¨ oteborg, Sweden; d Department of Pediatrics, University Hospital MAS, Malm ¨ o, Sweden; e Department of Molecular Medicine, Karolinska Institute, Stockholm, Sweden; f Department of Clinical Sciences, Lund University, Malm ¨ o, Sweden; and g Department of Pediatrics, Karolinska University Hospital, Huddinge, Sweden Key words: beta-cell function – children – C-peptide – type 1 diabetes – type 2 diabetes Corresponding author: Johnny Ludvigsson, Division of Pediatrics and Diabetes Research Center, Department of Clinical and Experimental Medicine, Link ¨ oping University Hospital, S-581 85 Link ¨ oping, Sweden. Tel: +46101030000; fax: +4613148265; e-mail: johnny.ludvigsson@lio.se Submitted 5 April 2011. Accepted for publication 11 July 2011 It has been claimed that at the time of type 1 diabetes (T1D) diagnosis 80–90% of the pancreatic islet beta cells have been destroyed. However, it cannot be excluded that the main problem is a deterioration of the beta-cell function. Decades ago, C-peptide determination (1) was used to better understand the course of T1D. It was shown that young children with classical ketosis prone insulin-dependent 45