Time-in-range (TiR) is now the ‘go-to’ metric for people with diabetes using continuous glucose monitoring (CGM) to manage their diabetes. Continuous glucose data have become much more mainstream in diabetes care, with increasing numbers of people with type 1 diabetes in the UK using the Freestyle Libre; overall numbers are already approaching the 20% anticipated by NHS England when their funding criteria were introduced earlier in the year, with many centres reporting much higher percentages than this. TiR becomes a much more meaningful metric with large amounts of continuous glucose data and the target of >70% TiR, where the range is 3.9–10.0 mmol/L, is increasingly familiar to healthcare professionals and sensor users. This equates to an HbA1c of 53 mmol/mol (7%) with each 10% increment in TiR representing a 5.5 mmol/mol (0.5%) change. TiR can therefore be used as an accurate surrogate for HbA1c, giving the user an idea of how their HbA1c might be changing as they adjust their insulin management in response to the CGM data. More importantly, the fact that changes in TiR can be observed over a much shorter timescale gives more frequent positive feedback to the user about improvements in glycaemic control, as all CGM systems provide a graphical illustration of time in various ranges, which can usually be pre-defined by the user.
At the recent EASD meeting in Barcelona, several presentations focussed on different aspects of TiR and related metrics (summarised):
- In people with type 2 diabetes, those with lower TiR were more likely to have advanced retinopathy, and there was a significant association with TiR and all stages of retinopathy even after controlling for HbA1c (Lu et al, 2018).
- The challenge of truly optimising blood glucose control was highlighted by a study of TiR in normal children, aged 6 or older, where 97% of CGM values were between 3.9 and 7.4 mmol/L.
- The importance of time below range (TbR) was highlighted, as minimising TbR can be as important to the user and their quality of life as TiR. Thomas Danne even suggested that TiR/TbR could be reported in the same way as blood pressure, expressed as systolic blood pressure/diastolic blood pressure, so 50% TiR and 15% TbR would be reported as 50/15. This is visually attractive, although it is questionable whether it will catch on! However, the recent consensus statement (Battelino et al, 2019) advising that the target TbR should be <5% for <3.9 mmol/L and <1% for <3.0 mmol/L provides important metrics for helping the user to reduce hypoglycaemic exposure.
TiR, TbR and time above range metrics are going to be increasingly important cornerstones of glucose management, particularly in people with type 1 diabetes using CGM. Furthermore, as closed-loop systems are commercialised this will be the metric that clinicians and users will focus on when trying to assess the effectiveness of different closed-loop algorithms.
Battelino T, Danne T, Bergenstal RM et al (2019) Clinical targets for continuous glucose monitoring data interpretation: recommendations from the International Consensus on Time in Range. Diabetes Care 42: 1593–603
Lu J, Ma X, Zhou J et al (2018) Association of time in range, as assessed by continuous glucose monitoring, with diabetic retinopathy in type 2 diabetes. Diabetes Care 41: 2370–76
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