Impact of routine clinic measurement of serum C-peptide in people with a clinician-diagnosis of type 1 diabetes

Diabet Med. 2021 Jul;38(7):e14449. doi: 10.1111/dme.14449. Epub 2020 Nov 22.


Aims/hypothesis: The aim of this study was to determine the impact of the routine use of serum C-peptide in an out-patient clinic setting on individuals with a clinician-diagnosis of type 1 diabetes.

Methods: In this single-centre study, individuals with type 1 diabetes of at least 3 years duration were offered random serum C-peptide testing at routine clinic review. A C-peptide ≥200 pmol/L prompted further evaluation of the individual using a diagnostic algorithm that included measurement of islet cell antibodies and genetic testing. Where appropriate, a trial of anti-diabetic co-therapies was considered.

Results: Serum C-peptide testing was performed in 859 individuals (90% of the eligible cohort), of whom 114 (13.2%) had C-peptide ≥200 pmol/L. The cause of diabetes was reclassified in 58 individuals (6.8% of the tested cohort). The majority of reclassifications were to type 2 diabetes (44 individuals; 5.1%), with a smaller proportion of monogenic diabetes (14 individuals; 1.6%). Overall, 13 individuals (1.5%) successfully discontinued insulin, while a further 16 individuals (1.9%) had improved glycaemic control following the addition of co-therapies. The estimated total cost of the testing programme was £23,262 (~€26,053), that is, £27 (~€30) per individual tested. In current terms, the cost of prior insulin therapy in the individuals with monogenic diabetes who successfully stopped insulin was approximately £57,000 (~€64,000).

Conclusions/interpretation: Serum C-peptide testing can easily be incorporated into an out-patient clinic setting and could be a cost-effective intervention. C-peptide testing should be strongly considered in individuals with a clinician-diagnosis of type 1 diabetes of at least 3 years duration.

Keywords: C-peptide; misclassification; monogenic diabetes; reclassification; type 1 diabetes; type 1 diabetes genetic risk score; type 2 diabetes.

MeSH terms

  • Adolescent
  • Adult
  • Biomarkers / blood
  • C-Peptide / blood*
  • Diabetes Mellitus, Type 1 / blood*
  • Diabetes Mellitus, Type 2 / diagnosis
  • Humans
  • Young Adult


  • Biomarkers
  • C-Peptide