Guide series, Part 1 of 5
Is Your Time in Range What You Think It Is?
The international consensus says aim for 70% time in range. But that target was designed around one type of CGM accuracy. Some devices systematically read higher time in range than others for the same physiological glucose. Until you know which calibration zone your CGM sits in, your TIR number is harder to interpret against the international 70% target.
Why this matters before anything else in the guide
Time in range (TIR) is reported as a single number from your CGM. It looks objective. It is not. Your TIR depends on the calibration behaviour of the device measuring you, and different consumer CGMs do not all calibrate the same way.
Two people with identical underlying glucose patterns can record TIR numbers that differ by 5 percentage points or more, simply because they wear different sensors. If you do not know how your device behaves, comparing your TIR with the international 70% target is less reliable. That is the gap this Part exists to close.
The headline: the international consensus 70% TIR target (ATTD 2019, ADA 2024) was modelled on Zone P CGM behaviour. If your device sits in Zone B, your TIR reading is systematically higher than the consensus assumed. The same physiological glucose pattern that reads 70% TIR on a Libre would read roughly 75% on a Simplera.
The Zone A, P, B framework
The GNL CGM accuracy framework, built on the work of Pemberton (2023) and Eichenlaub (IFCC 2023), classifies every consumer CGM by how it calibrates against a reference glucose value. Three zones.
Zone A, Analytical
Performance approaches laboratory-grade reference glucose. No current consumer CGM operates in Zone A in routine real-world wear. Theoretical ceiling. No wearable sits there yet.
Zone P, Physiological
The device is calibrated against capillary or plasma-referenced venous glucose. Readings track underlying physiological glucose closely on average, with errors that do not consistently push in one direction. Devices placed in Zone P for the matrix in Part 4 (head-to-head dynamic accuracy data published): Dexcom G6, Dexcom G7, Abbott FreeStyle Libre 2, Abbott FreeStyle Libre 3 / Libre 3 Plus.
Zone B, Biased
The device shows a systematic calibration offset relative to capillary or plasma reference. The bias is not random noise; it is a consistent direction of error that produces higher TIR readings than a Zone P device would record for the same underlying glucose pattern. Devices placed in Zone B for the matrix in Part 4 (head-to-head dynamic accuracy data published): Medtronic Simplera and the MiniMed Guardian 4 sensor (MiniMed is the recently spun-off company carrying the Medtronic Diabetes business; the Guardian 4 sensor remains the workhorse on their pumps).
Why some CGMs aren’t placed on the matrix yet. Capillary or plasma alignment (Zone P architecture) is necessary but not sufficient for individual risk prediction. Zone placement on the matrix in Part 4 requires publicly available head-to-head accuracy data covering dynamic glucose conditions (rapid post-meal rises, exercise drops). The IFCC stress protocol (Pleus 2025) describes the test methodology; no manufacturer has yet completed formal certification under it.
- Roche SmartGuide is calibrated to capillary glucose (consistent with Zone P architecture) but does not yet have comparable head-to-head dynamic data published. It is not on the matrix until that data exists. The mHGI portion of the Part 4 framework still applies.
- Dexcom Stelo is calibrated to plasma reference (Zone P architecture) but the use case (T2D and prediabetes wellness, not T1D risk prediction) and head-to-head data scope mean it is not currently placed for the T1D matrix. Zone P architecture noted; matrix placement deferred.
- Eversense (implantable) is excluded pending sufficient comparative data.
The full device-by-device comparison sits in the CGM Guide. The GNL position is that formal IFCC certification under Pleus 2025 must become mandatory internationally for CGM regulatory clearance; until it is, the matrix can only name devices whose head-to-head data has been independently published. See Part 4 for the IFCC certification call.
What the bias actually does to your number
A Zone B device tends to read fewer values in the hyperglycaemic and hypoglycaemic tails than a Zone P device for the same physiological exposure. The result: the proportion of values falling between 3.9 and 10.0 mmol/L (the standard time-in-range window) goes up, even though nothing has changed physiologically. The number on the screen is higher; the underlying glucose is identical.
The approximate effect, for matched users with similar glucose patterns:
| Underlying physiological TIR | Zone P device reading | Zone B device reading |
|---|---|---|
| ~60% | ~60% | ~65% |
| ~70% | ~70% | ~75% |
| ~80% | ~80% | ~85% |
These are approximate, framework-level differences. Individual variation exists. The 5-percentage-point figure is the central estimate; head-to-head accuracy data (CGM Pack 1) places the upper bound of the Zone B / Zone P gap nearer 10 percentage points for some matched pairs, so the real-world spread between two users on different devices may be larger than the matrix suggests. The point is the direction and the order of magnitude, not a precise number.
Why this matters clinically
Comparing yourself to consensus targets
The 70% TIR target was set against the average behaviour of Zone P CGMs in trial cohorts. If you wear a Zone B device and you hit 70% TIR, your underlying physiological TIR is closer to 65%. You are further from the consensus than your number suggests. Conversely, hitting 75% on a Zone B device is roughly equivalent to 70% on Zone P.
Comparing yourself to a friend or family member
If two people compare TIR numbers and one wears a Libre while the other wears a Simplera, those numbers are not on the same scale. The Simplera number is around 5 percentage points higher for matched glucose. This is not a flaw in either device; it is a calibration design choice. But it matters when people use TIR to compare or compete.
Switching devices
If you move from a Zone P device to a Zone B device, your TIR will likely jump by around 5 percentage points overnight, with no actual change in your glucose pattern. The reverse is also true. Knowing this prevents you (and your care team) from over-interpreting a device-driven shift as a clinical change.
Setting your personalised target
Part 4 of this guide presents the personalised TIR matrix that combines your CGM zone with your glycator status. Your personal target depends on both. The CGM zone is the easy half: you find out which zone your device sits in (above or in the CGM Guide), and that sets one axis of the matrix.
What GNL research shows
Pemberton 2023 identified that CE Marking is not a quality standard for CGM.
Pemberton et al 2026 (DOM International Clinical Opinion) took the framework further and identified that the calibration alignment difference only appears under stress testing: on a still, fasted day the sensors agree, but under dynamic conditions (meals, exercise, overnight drift) the zones separate. Eichenlaub 2025 (three-CGM performance) provides the empirical head-to-head data that backs the stress-testing finding.
Until manufacturers disclose calibration architecture publicly, the GNL Zone framework is the working tool for interpreting any metric (including TIR) that is calculated from CGM data. The Via Negativa principle applies: remove the assumption that all CGMs read the same before adding any interpretation to the number on the screen.
Find your CGM’s zone
The full device-by-device classification, including UK availability, AID compatibility, and accuracy data, sits in the CGM Guide.
This guide is educational. The Zone A/P/B framework describes systematic average behaviour; individual sensor performance varies. It is not medical advice and cannot replace individual clinical guidance from your diabetes care team. CGM device assignments are kept current in the CGM Guide; if a manufacturer updates calibration architecture, the assignment may change.
Evidence cited: Pemberton 2023 (CE Marking is not a quality standard for CGM; originating Zone A/P/B application); Pemberton et al 2026 DOM (International Clinical Opinion, calibration alignment difference under stress testing); Eichenlaub 2025 (three-CGM head-to-head performance).