Continuous glucose monitoring (Libre Sensor)
DGRefHelp - NHS Dumfries & Galloway
Continuous Glucose Monitoring (CGM)
Prescribing note: Both CGM devices on the NHS D&G formulary should only be initiated or switched on the advice of the specialist diabetes team (DSN or secondary care). GPs prescribe the sensors on an ongoing basis following specialist recommendation.
Background
Two CGM sensors are available on the NHS D&G formulary. Both measure interstitial glucose via a small filament applied to the upper arm, providing real-time readings via a smartphone app or dedicated reader.
Device
FreeStyle Libre2 Plus ✓ First Choice
Dexcom One+ ✓ 2nd Line
Manufacturer
Abbott
Dexcom
Sensor wear
15 days (2/month)
10 days (3/month)
Alarms
Optional via LibreLink app (not on reader)
Continuous Bluetooth alarms via Dexcom app
Data access
LibreView portal / AGP report
Dexcom Clarity portal / AGP report
When used
All eligible patients — first line
2nd line where Libre not tolerated (e.g. adhesion failure, skin reaction) — on specialist advice
Important: CGM measures interstitial glucose, which lags capillary blood glucose by ~5–10 minutes. Finger-prick testing remains essential when the reading may be inaccurate — see below.
Eligibility Criteria
Funded CGM is indicated for:
Standard Eligibility
- Any person living with diabetes treated with insulin injections (Type 1 or Type 2)
- Pregnancy — Type 1 or Type 2 diabetes on insulin; gestational diabetes on insulin
- Situations where HbA1c is unreliable:
- Frequent blood transfusion or blood donation
- Dapsone treatment
- Haematological disorders affecting red cell turnover (e.g. haemolytic anaemia, haemoglobinopathies)
Case-by-Case Consideration
- Diabetes treated with sulphonylurea and either:
- Severe needle phobia preventing adequate SMBG, or
- Significant dexterity impairment preventing finger-prick testing
Discuss with local DSN or diabetes team before initiating in this group.
When finger-prick testing is still required even with CGM:
- Symptomatic hypoglycaemia (readings may lag)
- Rapidly changing glucose (trend arrow pointing sharply up/down)
- Readings inconsistent with symptoms
- During first 24 hours after sensor insertion
- Whilst following sick day rules for type 1 diabetes
Primary Care Management & Prescribing
How to Prescribe
Add sensors to the patient's repeat medication list following specialist recommendation. Do not initiate or switch devices without prior specialist advice.
CGM Prescription Reference
Item
FreeStyle Libre2 Plus ✓ First Choice
Dexcom One+ ✓ 2nd Line
Product name (GP10)
FreeStyle Libre2 Plus Sensor
Dexcom One+ Sensor
Quantity
2 per month
3 per month
Reader (if needed)
FreeStyle Libre 2 Reader or mobile app
Dexcom receiver or mobile app
Lancets & strips
Continue prescribing for confirmatory testing — do not stop self testing supplies
HbA1c: Continue to check HbA1c at annual review. In patients where HbA1c is unreliable, document this and rely on AGP reports instead.
What metrics does CGM produce?
While GPs/practice nurses are not expected to act on CGM metrics, it can be useful to understand what figures they produce and what they mean.
CGM reports use an Ambulatory Glucose Profile (AGP) rather than a single HbA1c value. Key metrics the specialist team will review include:
- Time in Range (TIR) — percentage of time glucose is within target range (typically 3.9–10 mmol/L). The primary measure of CGM control.
- Time Below Range (TBR) — percentage of time <3.9 mmol/L (hypoglycaemia). A critical safety metric.
- Time Above Range (TAR) — percentage of time >10 mmol/L (hyperglycaemia).
- GMI (Glucose Management Indicator) — an estimated HbA1c derived from CGM data. Not identical to lab HbA1c but useful for trend monitoring.
- Coefficient of Variation (CV%) — glucose variability. High CV (>36%) indicates unstable control even if TIR appears acceptable.
Metric
Usual adult target
Time in range 3.9–10 mmol/L
>70%
Time below 3.9 mmol/L
<4%