Aims/hypothesis: Our objectives were to explore whether the phenomenon of HbA1c 'tracking' occurs in individuals with type 1 diabetes, how long after diagnosis does tracking take to stabilise, and whether there is an effect of sex and age at diagnosis on tracking.
Methods: A total of 4525 individuals diagnosed with type 1 diabetes between 1 January 1995 and 1 May 2015 were identified from The Health Improvement Network (THIN) database. Mixed models were applied to assess the variability of HbA1c levels over time with random effects on general practices (primary care units) and individuals within practices.
Results: 4525 individuals diagnosed with type 1 diabetes were identified in THIN over the study period. The greatest difference in mean HbA1c measurement (-7.0 [95% CI -8.0, -6.1] mmol/mol [0.6%]) was seen when comparing measurements made immediately after diagnosis (0-1 year since diagnosis) with those at 10 or more years (the reference category). The mean difference in HbA1c for the successive periods compared with 10 or more years after diagnosis declined and was no longer statistically significant after 5 years. In the stratified analysis using sex and age group there was considerable heterogeneity with adult onset type 1 diabetes appearing to track earlier and at a lower mean HbA1c.
Conclusions/interpretation: In individuals with type 1 diabetes, glycaemic control measured by HbA1c settles onto a long-term 'track' and this occurs on average by 5 years following diagnosis. Age at diagnosis modifies both the rate at which individuals settle into their track and the absolute HbA1c tracking level for the next 10 years.
Keywords: Glycated haemoglobin; HbA1c; Tracking; Type 1 diabetes mellitus.