Time in Range explained

If you’re using continuous glucose monitoring, understanding how to use your Time in Range is important knowledge.

Whether you use continuous glucose monitoring (CGM) in combination with an insulin pump or with multiple daily injections, research has shown that there are significant clinical benefits of CGM use in people with diabetes.

If you use CGM you can improve the management of your diabetes, while reducing the risk of developing diabetes related complications, by monitoring your Time in Range.

What is Time in Range?

The Time in Range is the percentage of time a person with diabetes spends with their sensor glucose (SG) levels in a particular target range.

What this range is will vary from person to person. Generally, it’s recommended to aim for SG levels between 3.9-10.0 mmol/L, while reducing time spent in hypoglycaemia (very low glucose levels).

Time in Range includes three key CGM measurements:

  1. Percentage of readings and time per day within the target glucose range (TIR)
  2. Time spent below the target glucose range (TBR)
  3. Time spent above target glucose range (TAR)

The main goal for effective and safe diabetes management is to increase the TIR while reducing the TBR.

TIR can also be reflected as the hours per day spent in range.

What is the optimal Time in Range target?

Most diabetes experts agree that a target range of 3.9-10.0 mmol/L is ideal. However special considerations, such as pregnancy or other health conditions, will need to be considered.

It is recommended to spend:

  • Less than 4% of the day in TBR (<3.9 mmol/L)
    • And less than 1% of the day <3.0 mmol/L
  • At least 70% of the day TIR (3.9-10.0 mmol/L)
  • Less time in TAR (>10.0 mmol/L)

Is TIR the same as the HbA1c?

The short answer is: no. However, there are some connections.

Unfortunately, HbA1c does not give any indication as to how much time you spent in hypoglycaemia. You can have an HbA1c of 7.0% with blood glucose levels between 4.0 and 8.0 mmol/L, but you can also have an HbA1c of 7.0% with BGLs between say 2.0 and 22.0 mmol/L.

Similarly, the TIR alone does not give you any information about how much time you spent in hypoglycaemia. Therefore, it is important to also look at the TBR. Generally though, you can assume that if your TIR is high, your HbA1c is more likely to be in the target range.

Research showed that a TIR of 70% matched with an HbA1c of 7% (53 mmol/mol), whereas a TIR of 50% corresponded with an HbA1c of 8% (64 mmol/mol).

Similarly, an increase of TIR of 10% (which is equivalent to 2.4 hours per day more time spent in range) corresponds to a decrease in HbA1c of around 0.5% (5.0 mmol/mol).

Higher percentages of TIR, similar to lower HbA1c results, are associated with a decreased risk of the development of diabetes-related complications.

The limitations of using HbA1c alone

Your HbA1c level reflects your average glucose over the past three months. But your HbA1c does not show exactly how much hypo- and hyperglycaemia you had. Also, it does not show how much and how often your glucose levels varied from one day to another or within a particular day.

HbA1c is a “weighted average of glucose levels”; this means that glucose levels in the past 30 days contribute considerably more to the level of HbA1c than do glucose levels from 90-120 days earlier. If you had big changes in your BGLs in the past month (such as often happens during holidays or around Christmas or Easter time) this may have a significant impact on your HbA1c.

Sometimes the HbA1c does not accurately reflect your average, or mean glucose, as some medical conditions can affect the HbA1c measure. This includes conditions such as anaemia, iron deficiency, haemoglobinopathies (a range of conditions that affect haemoglobin) and even pregnancy.

Limitations of only using TIR

CGM is not a “set and forget” system; it requires you (and your diabetes healthcare professional) to interpret the data and act upon this appropriately.

You need to actively use CGM in order for it to be effective and this can be costly.

CGM lets you observe changes in your glucose levels and daily profiles, including patterns of hypo- or hyperglycaemia. This can greatly assist you (and your diabetes healthcare professional) in making therapy decisions and any lifestyle changes.

However, when glucose levels are changing rapidly there may be a delay in the CGM registering SG changes.

How can the TIR be improved?

In most cases it is recommended to first address any hypos you may be having. By avoiding hypoglycaemia you will be able to reduce your TBR. Often you will minimise rebound hyperglycaemia at the same time, which will reduce your TAR and hence improve your TIR. Talk to your diabetes healthcare professional for advice on how you can achieve this.

If you have type 1 diabetes you can improve your TIR by making sure you count carbohydrates actively and accurately, by adjusting your bolus (pre-meal) insulin doses based on your current SG or BG level, your insulin to carbohydrate ratio (ICR), insulin sensitivity factor (ISF), any insulin left on board (IOB) and your insulin action/duration.

Physical activity and meal composition (the balance between carbohydrates, protein, fat, vitamins, minerals and fibre) also need to be considered.

When it comes to making insulin dose adjustments, slow and steady wins the race.

Always follow the advice of your diabetes healthcare professional (endocrinologist or credentialled diabetes educator).

Click here to read about eligibility for subsidized CGM and access to subsidised CGM products through the NDSS.

By Carolien Koreneff

Credentialled Diabetes Educator, Registered Nurse, FADEA

 

 

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