Glucose sensors: how to read your time in range
What a glucose sensor measures and the international consensus targets: time in range, hypoglycaemia and variability.
Glucose sensors have changed life with diabetes. But hundreds of readings a day are only useful if you know how to read them. These are the numbers that really matter.
Key points
- A sensor measures glucose in interstitial fluid every few minutes, not in blood.
- Time in range (70–180 mg/dL) is the main metric: the general goal is above 70%.
- Limiting time in hypoglycaemia matters just as much: less than 4% below 70 mg/dL.
What a sensor actually measures
Continuous glucose monitoring uses a very thin filament under the skin that measures glucose in interstitial fluid. That value closely follows blood glucose, but a few minutes behind. So when glucose changes quickly (after meals, during exercise or when treating a hypo), the sensor and a finger-prick test may not match exactly.
Its great advantage is that it shows a film rather than a snapshot: trends, rising or falling arrows, and what happens overnight when nobody is checking.
The international consensus metrics
In 2019 an international consensus defined how to interpret sensor data. For most people with type 1 and type 2 diabetes, the targets are:
| Metric | General target |
|---|---|
| Time in range (70–180 mg/dL) | Above 70% |
| Time below 70 mg/dL | Below 4% |
| Time below 54 mg/dL | Below 1% |
| Time above 180 mg/dL | Below 25% |
| Time above 250 mg/dL | Below 5% |
| Coefficient of variation | 36% or less |
For older people or those at high risk of hypoglycaemia, targets are more cautious: a slightly lower time in range is accepted in exchange for keeping lows to a minimum.
How to review your data
- Look at at least 14 days of data, with the sensor active more than 70% of the time.
- Start with hypos: fixing them always comes first.
- Then look for patterns: does glucose always rise after breakfast? Does it drop in the early hours?
- Change one thing at a time and watch the effect over the next two weeks.
During the online consultation we review your report together (LibreView, Dexcom Clarity, CareLink, Glooko…) and agree on adjustments.
When to seek advice
If you spend more than 4% of the time below 70 mg/dL, have night-time or severe hypos, or your time in range does not improve despite changes, review your treatment with your endocrinologist.
References
- Battelino T, Danne T, Bergenstal RM, et al. Clinical Targets for Continuous Glucose Monitoring Data Interpretation: Recommendations From the International Consensus on Time in Range. Diabetes Care. 2019;42(8):1593-1603.
- American Diabetes Association Professional Practice Committee. 7. Diabetes Technology: Standards of Care in Diabetes—2025. Diabetes Care. 2025;48(Suppl 1).
This article is for information only and does not replace medical advice. If you have questions about your case, talk to your doctor.
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