Getting more from CGM and closed-loop systems

Sensors and closed-loop systems give far more information than finger-prick tests, and how well they work depends a great deal on settings and day-to-day use. This guide covers the numbers worth looking at and the commonest reasons a system underperforms.

The numbers that matter

HbA1c is a three-month average and says nothing about highs, lows or variation. A continuous glucose monitor (CGM) does. International consensus targets for most adults with type 1 diabetes:

• Time in range (3.9 to 10.0 mmol/L): more than 70%
• Time below 3.9 mmol/L: less than 4%
• Time below 3.0 mmol/L: less than 1%
• Time above 10.0 mmol/L: less than 25%

These are targets for most people. Older adults and people at high risk of hypos are usually given gentler ones.

NICE recommends CGM for all adults with type 1 diabetes.

How hybrid closed-loop works

A hybrid closed-loop system links a CGM to an insulin pump through an algorithm that adjusts insulin every few minutes. "Hybrid" means you still tell it about meals. NICE recommends these systems for adults whose HbA1c remains 58 mmol/mol or above, or who have disabling hypoglycaemia, despite best use of a pump or CGM, and for children and young people and for pregnancy. NHS England is rolling them out over several years.

Why systems underperform

• Meal insulin given late, or missed: the algorithm corrects afterwards, but slowly
• Carbohydrate estimates that are consistently off
• Cannula sites in areas of lumpy, overused skin (lipohypertrophy), where insulin absorbs unpredictably
• Settings that no longer suit: targets, carbohydrate ratios, active insulin time
• Exercise, alcohol, illness and the menstrual cycle, each of which changes insulin needs
• Alarm fatigue, which leads people to switch alerts off

What a specialist review involves

You can share your data through the platform your device uses. I go through the patterns with you, we agree any changes together, and with your agreement a written summary goes to you, your GP and your NHS diabetes team.

Working with your NHS team

A private review adds to your NHS care. Sensors, pumps and supplies continue to come through your NHS team, and device training is provided by the manufacturer or your NHS team. What I can offer is time to go through your data in detail and agree changes you can try before your next NHS appointment.