C-peptide testing gives a measure of endogenous insulin production. It can be tested in blood (ideally) or urine. C-peptide should be measured alongside a blood glucose level and is only valid if blood glucose is >4mmol/l.
It is not recommended to measure c-peptide as a matter of routine if duration of diabetes is <3 years, but should be measured if a trial of insulin withdrawal is being considered, to aid with the diagnosis of LADA/ insulin deficiency, or if a monogenic or syndromic form of insulin resistance is suspected. C-peptide testing is recommended in all individuals with T1D at 3 years, even if a robust diagnosis has been made, as individuals with significant detectable C-peptide have reduced glucose variability, reduced frequency of hypoglycaemia and better long term outcomes.

Does serum C-peptide need repeated after an interval?

Providing serum C-peptide was measured after 3 or more year’s duration of diabetes, there should not be any need for this to be repeated as a matter of routine.

Urine C-peptide

Tables to aid interpretation of urine C-peptide results (expressed as a urine C-peptide to creatinine ratio; UCPCR) are also available on the ‘Diabetes Diagnostics’ App from the University of Exeter and on the Exeter MODY website (www.diabetesgenes.org). See: https://www.exeterlaboratory.com/test/c-peptide-urine/


How do I interpret pancreatic islet cell antibody results?

Elevated islet cell antibodies are a marker of autoimmunity and thus support a diagnosis of autoimmune diabetes (e.g. Type 1 Diabetes (T1DM) or Latent Autoimmune Diabetes of Adulthood (LADA)). However, it is important to remember that true false positive antibody results can occur with any immunoassay, while positive islet cell antibodies may occur in the general population and not cause diabetes. On the other hand, approximately 5% of people with true T1D have negative islet cell antibodies, when three antibodies are tested (GAD, IA-2, ZnT8). Broadly speaking, higher antibody titres and/or multiple different positive antibodies are more likely to be ‘real’ results, indicative of a diagnosis of T1D. islet antibody titres >99th centile are considered ‘strongly’ positive, while antibody titres between the 97.5th and 99th centiles are considered ‘weakly’ positive. Specific antibody titre cut-offs are shown in Appendix 2. As with the C-peptide cut-offs, it is important that these cut-offs are not applied rigidly and that clinical judgement is shown. For the purposes of the algorithm, a diagnosis of T1D is considered robust if one or more antibody is ‘strongly’ positive or there is more than one antibody ‘weakly’ positive. Note, in people with long standing diabetes, antibody levels can wane/ reduce with time.

UK Lab Antibody Titres Thresholds

‘Weakly Positive’ 

‘Strongly Positive’ 


97.5th centile 

99th centile 

GAD U/ml 



IA-2 U/ml 



ZnT8 U/ml 

Age < 30  



ZnT8 U/ml 

Age  30