ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Endocrine Abstracts (2019) 66 P33 | DOI: 10.1530/endoabs.66.P33

C-peptide and antibody testing to aid diabetes diagnosis in children and young people

Kathryn Cox & Louise Bath

Royal Hospital for Sick Children, Edinburgh, UK

Introduction: Recent experience of C-peptide testing in adults with longstanding diabetes has revealed misdiagnosis of monogenic diabetes as either type 1 or type 2 with important implications for quality of life and healthcare costs. Correct diagnosis earlier in life may be possible with the use of routine antibody testing at diagnosis and c-peptide measurement 3 years after initial diagnosis. Negative or low titres for all 3 antibodies suggest non-type 1 diabetes. Persistence of detectable c-peptide beyond 3 years from diagnosis raises the possibility of non-type 1 diabetes

Methods: All children and young people with type 1 diabetes known to the Royal Hospital for Sick Children diabetes team underwent testing. C-peptide levels were measured at the time of annual review with paired serum glucose, and considered positive if >50 pmol/l. Glucose >8 mmol/l required at time of testing to avoid false negative. Antibodies were measured at diagnosis, or at annual review appointment for any patients in whom antibodies had not previously been checked. Cut-off values for positive results were GAD>5, IA2 >7.5 and ZnT8 >15.

Results: 214 individuals aged <18y had c-peptide levels requested. Results were available for 203 patients. Results were categorised as 1–3 years or >3 years since diagnosis. 7 patients had persistent c-peptide at >3 years, of which 1 was >200 pmol/l. 14 patients had persistent c-peptide at >3 years, of which 2 were >200 pmol/l. 230 patients aged <16y had antibody testing. 72 tested for GAD & IA2, 153 tested for GAD, IA2 & ZnT8, 5 tested for a single antibody. 45 patients were single antibody positive. 12 patients were, of whom 2 had been diagnosed less than 2 years. 1 triple antibody negative patient had undetectable c-peptide. The other triple antibody negative patient had c-peptide of 170 pmol/l at 13 months since diagnosis, raising possibility of non type 1.

Conclusion: This study has identified a number of young people with persistent c-peptide beyond 3 years from diagnosis. Negative antibody testing can guide future investigation. We recommend measuring GAD, IA2 and ZnT8 antibodies in all patients at diagnosis, and C-peptide at 3 years.

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