As hypothesised, we demonstrated significant associations between residual C-peptide secretion and lower glucose variability and low-glucose events in flash glucose monitoring users. These associations were independent of prevailing HbA1c and diabetes duration, suggesting routine evaluation of C-peptide may have clinical utility in the management of type 1 diabetes. These data also highlight the limitations of HbA1c, which did not differ between groups, as a means of assessing optimal glycaemic management. In the modern era, it is conceivable that HbA1c will be supplanted by CGM metrics to optimise glycaemic management , particularly in efforts to minimise glucose variability, which has been posited as an independent risk factor for diabetes complications .
Our flash glucose monitoring findings are accompanied by significant differences in self-reported hypoglycaemia, consistent with findings from several previous investigations [9,10,11], although significant differences were limited to asymptomatic hypoglycaemia in our study. Marren et al  describe significant differences in self-reported symptomatic and asymptomatic hypoglycaemia in those with preserved C-peptide, although the median C-peptide concentration in this group (114 pmol/l) was substantially higher than in our study (32 pmol/l). While we report random plasma C-peptide and Marren et al  used values after a standard mixed-meal tolerance test, the correlation between random and post-mixed-meal C-peptide is known to be very strong (R = 0.91) and is unlikely to substantially limit the comparability of these studies . Other important contrasts with this study are the higher HbA1c (67–69 mmol/mol [8.3–8.5%] vs 57–58 mmol/mol [7.4–7.5%] in our cohort) and significant difference in age between C-peptide groups, which was not different in our study (which was limited to adults) despite differences in diabetes duration and age at diagnosis. Kuhtreiber et al  demonstrated similar associations between hypoglycaemia and fasting C-peptide, although the median concentration of C-peptide associated with mild and moderate hypoglycaemia (42.4 pmol/l) was higher than the median in both our preserved and micro-secretor categories. The novelty of our self-reported hypoglycaemia data is the demonstration of lower rates of asymptomatic hypoglycaemia at an even lower C-peptide threshold than previously demonstrated [9,10,11].
The fact that C-peptide status is strongly associated with reduced low-glucose events and glucose variability, but not average glucose, time in range or time above range, offers a mechanistic insight into the consequences of preserved C-peptide. If this were simply a ‘buffering’ effect of ongoing endogenous insulin secretion, smoothing out the peaks and troughs in glucose in individuals receiving exogenous insulin, we might expect to see this reflected in a lower HbA1c. However, this was not the case in either our cohort or that of Marren et al . Loss of the glucagon counter-regulatory response to hypoglycaemia occurs in many individuals with type 1 diabetes within 5 years of diagnosis and is linked, in part, to the loss of a paracrine effect of endogenous insulin on alpha cells in pancreatic islets . Adults with type 1 diabetes and preserved C-peptide (>99 pmol/l after a mixed-meal tolerance test) have relative preservation of the glucagon response to hypoglycaemia . Therefore, the role of intra-islet insulin signalling offers a compelling mechanism to explain the association of preserved C-peptide with reduced low-glucose events [21, 22], although this phenomenon was not observed in young individuals within 1 year of diagnosis . The only cohorts where preserved C-peptide was associated with lower HbA1c were the intensive arm of DCCT  and the study by Kuhtreiber et al , both of which had comparatively low HbA1c levels. Cohorts with no difference in HbA1c, in relation to C-peptide status (including this study), may reflect a failure to intensify glycaemic management in individuals who would be at lower risk of hypoglycaemia .
The key strength of this study is its novelty in assessing the relationship between C-peptide and flash glucose monitoring variables in a ‘real-world’ clinical context. Where associations of CGM with C-peptide were reported previously, this was in a largely paediatric population, within 2 years of diagnosis and with relatively higher levels of C-peptide . Our cohort also benefits from being balanced in terms of current age and HbA1c between low and preserved C-peptide groups. As a ‘real-world’ assessment, the various measures obtained in this study (questionnaire data, HbA1c, C-peptide and flash monitoring data) were not captured simultaneously, although we would envisage this increasing the likelihood of a type II error rather than producing false positive associations with C-peptide. Random C-peptide appears to be as robust a measure of C-peptide status as values obtained after a mixed-meal tolerance test , and indeed we found no significant correlation between C-peptide and concomitant plasma glucose. It would have been preferable to have access to different low-glucose thresholds (e.g. <3 mmol/l); however, the nature of the data capture process did not permit this. It would also have been useful to have reported insulin dose data, but unfortunately these were not consistently available. The decision to exclude individuals with C-peptide >200 pmol/l was pragmatic, to limit the likelihood of including misclassified cases, but also because the specific research question related to the effect of relatively low levels of C-peptide secretion. We did not measure diabetes antibodies as a matter of routine and so it is possible that our cohort contained a very small proportion of individuals with a cause of insulin deficiency other than type 1 diabetes, e.g. hepatocyte nuclear factor 1-β monogenic diabetes. However, this does not affect the central tenet of our study regarding the relationship between C-peptide and low-glucose events. While the ‘real-world’ design is a strength in terms of generalisability, our cohort is skewed towards people with lower HbA1c and greater CSII usage than our centre’s total type 1 population.
These findings suggest that routine clinical measurement of C-peptide in type 1 diabetes may be important not only in confirming the correct diagnosis of diabetes, but also in informing the risk of low glucose and glycaemic instability. Given that we have shown an effect in the C-peptide range 10–50 pmol/l, we suggest that wider availability of higher-sensitivity C-peptide assays may be of value, as many currently available clinical assays report 50 pmol/l as their lower limit of quantification. Our findings support previous conclusions, drawn from self-reported hypoglycaemia , that there appears to be a failure of intensification in glucose-lowering therapy in people who are at lower risk of hypoglycaemia and glucose variability. These data also raise the possibility of stratified glycaemic targets which acknowledge the influence of residual C-peptide secretion. Given the apparent importance of persistent C-peptide secretion, every effort should be made to ensure early intensification of glycaemic control at diagnosis, as this is currently the only available intervention shown to preserve C-peptide secretion . Moreover, studies of novel strategies to preserve C-peptide secretion, such as immunotherapies, should be supported and funded .