Interpreting Continuous Glucose Monitoring (CGM) Data

Moving away from 4-6 fingerstick blood glucose values a day, capturing only points in time, to capturing upwards of 288 glucose values a day, the retrospective pattern analysis of CGM data is one of the best ways to help someone with diabetes to benefit from CGM use. This requires the HCP to have access to¹:

1. Standardised CGM metrics
2. Graphical visualisation of the glucose data and daily CGM profile
3. Clear clinical targets

Standardised CGM Metrics

In February 2017, the Advanced Technologies and Treatment for Diabetes (ATTD) Congress convened an international panel of expert diabetes clinicians, researchers and individuals from all over the world to define core metrics for assessing CGM data¹. The list of CGM metrics has now been streamlined for use in clinical practice. The expert panel selected ten metrics that may be most useful in clinical practice (see Table 1). Fundamental to accurate and meaningful interpretation of CGM is ensuring that adequate glucose data is available for evaluation. Having the CGM attached for at least 70% of the time over the most recent 14 days provides data which correlates strongly with three months of mean glucose, time in ranges and hyperglycaemia metrics. Of note, longer CGM data collection points may be required for individuals with greater glycaemic variability, particularly to assess hypoglycaemia exposure¹.

Table 1: Standardised CGM metrics for clinical care

CV, coefficient of variation

* Some studies suggest that lower %CV targets (<33%) provided additional protection against hypoglycaemia for those receiving insulin or sulfonylureas

Adapted from:  Diabetes Care 2019:42:1593-1603

Time in Ranges

Time spent within blood glucose ranges as a metric of glycaemic management provides more actionable information than HbA1c alone. The expert panel agreed that establishing target percentages of time in the various glycaemic ranges with the ability to adjust the percentage cut points to address the specific needs of people living with diabetes, would allow for effective therapeutic decision-making.

The time in ranges metric includes five key CGM measurements:

• Percentage of readings and time per day spent within the target glucose range- Time in range (TIR) (3.9- 10.0 mmol/L)
• Time below target glucose range (TBR) level 1 (3.0-3.8mmol/L)
• Time below target range (TBR) level 2 (<3.0 mmol/L)
• Time above range (TAR) level1 (10.1-13.9mmol/L)
• Time above range (TAR) level 2 (>13.9 mmol/L)

CGM metrics and individualising care

The time in range metrics can be applied to different diabetes populations including those who are older/higher risk, pregnant women with pre-gestational diabetes and women with gestational diabetes (GDM) (Figure 1).

(Diabetes Care 2019; 42:1593-1603)

Older and/or high- risk refers to people who are at notably at higher risk for severe hypoglycaemia due to their age, duration of diabetes, duration of insulin therapy and/or presence of hypoglycaemia unawareness¹. For older people living with diabetes we also need to consider cognitive and physical impairments, living alone and other comorbidities.  In high risk individuals, a conservative approach is recommended, with a strong focus on reducing the percentage of TBR and preventing excessive hyperglycaemia¹.

During pregnancy, the goal is to safely increase TIR as quickly as possible, while reducing TAR and glycaemic variability¹. Research has found that increasing TIR during the second and third trimesters

is associated with decreased risk of LGA and neonatal outcomes including macrosomia, shoulder dystocia, neonatal hypoglycaemia, and neonatal intensive care admissions¹.  More research is needed to define clinical CGM targets for women with GDM or type 2 diabetes in pregnancy, but there is some evidence that more stringent targets are needed to optimise outcomes¹.

Standardisation of CGM data presentation

There is a general consensus that a useful CGM report is one that can be understood by HCP and people with diabetes. A single page report that the clinical team can review and file in the medical record and can be used as a shared decision-making tool with people with diabetes was considered to be of value¹.  A CGM report that allows  the HCP to openly discuss  in the presence of the person, reflecting on real time data is of most value.  Allowing the person with diabetes, and/or their family or carer, to view their CGM report can help to  enhance their understanding of diabetes, through identifying areas of concern. This, in turn, supports the provision of more personalised therapy through shared decision-making and goal setting.

 If software could be standardised in line with the recommendations of the consensus committee, again it would be much easier to ensure collaboration of care. A one-page Ambulatory Glucose Profile (AGP) Report such as the example in Figure 2, meets the recommendations of the international panel of HCPs, researchers and individuals with diabetes¹. It is colourful and visual and provides a concise summary that is engaging and easy for the HCP to discuss with the person with diabetes and/or their family/carers.

(Diabetes Care 2019; 42:1593-1603)

Using the AGP report in clinical practice

Several factors can be considered when reviewing the AGP with an individual with diabetes:

• Are glucose levels within target on waking? If not, consider adjusting basal insulin.
• Do levels spike above 10.0mmol/L two hours after meals? If so, consider whether are they taking insulin before or after eating or whether mealtime insulin doses need adjusting.
• Evaluate the day to day variability – if variability is high then is the cause of the variability understood?
• Does the percentage of hypoglycaemia exceed the recommendation? In my opinion this is the most important range to review because the practical implications for a person driving, working, and living with diabetes who has reduced hypoglycaemic awareness can be extremely dangerous.

As time is of the essence in most public and private practices it is important to find a concise way of obtaining and analysing the data to look at the most important issues.  The ideal AGP report shows Time in Ranges, Glucose Statistics and Targets, at least 14 days of data to analyse as well as Daily Glucose Profiles. The HCP can use the glucose metrics above to analyse the data efficiently.

After reviewing the data, the following could be considered:

• The first question to ask a person with diabetes in a consultation should be “Is there anything concerning you about your diabetes?”
• Discuss the CGM report with the person with diabetes, along with recommended CGM-based targets and determine their personal goals.
• Help the person with diabetes and/or their family/carer to understand the limitations of the HbA1c compared to the estimated HbA1c or glucose management indicator seen on the CGM reports which allow for direct observation of glycaemic excursions and immediate feedback.
• Do glycaemic targets require revision?
• Develop a plan to increase TIR.
• Do glucose lowering medicines need adjustment as a result of the data? If the person with diabetes is self-adjusting glucose lowering medications (GLMs), is that being done in a way that indicates they understand the data, or are interventions such as insulin stacking occurring?
• Check the postprandial excursions and consider the need to strengthen insulin to carbohydrate ratios or refer onto an Accredited Practising Dietitian experienced in diabetes management².
• Establish a therapeutic action plan via summary of the consultation’s main points and why changes need to take place.

Copy the clinical report letter to the person with diabetes as well as the GP so goals are summarised and clear in writing.