Time in Range (TIR) — CGM Assessment
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Continuous glucose monitoring time-in-range (CGM TIR) analysis is the quantification of the proportion of time a person's blood glucose remains within defined target ranges, derived from continuous glucose monitoring sensor data. CGM sensors measure interstitial glucose every 5-15 minutes throughout the day and night, generating hundreds of data points per day compared to the 4-7 finger-stick readings in traditional self-monitoring of blood glucose. The Advanced Technologies and Treatments for Diabetes (ATTD) International Consensus Group published standardised CGM metrics in 2019, establishing the primary glucose targets that should be evaluated in every CGM report. The five core metrics are: Time in Range (TIR) — the percentage of time glucose is between 3.9 and 10.0 mmol/L (70-180 mg/dL), with a target of above 70% for most adults with type 1 or type 2 diabetes; Time Above Range (TAR) — above 10.0 mmol/L (180 mg/dL), target below 25%; Time Below Range (TBR) — below 3.9 mmol/L (70 mg/dL), target below 4%; TBR Level 2 — below 3.0 mmol/L (54 mg/dL), target below 1%; and TAR Level 2 — above 13.9 mmol/L (250 mg/dL), target below 5%. Studies show a strong correlation between TIR and HbA1c (each 10% TIR increase corresponds to approximately 0.5% HbA1c reduction), as well as with diabetes complications — higher TIR is associated with lower rates of retinopathy progression, microalbuminuria, and neuropathy. In pregnancy, the TIR target is more stringent at above 70% within the tighter range of 3.5-7.8 mmol/L (63-140 mg/dL).
TIR (%) = (Time with glucose 3.9-10.0 mmol/L / Total monitoring time) x 100; TAR (%) = (Time with glucose >10.0 mmol/L / Total time) x 100; TBR (%) = (Time with glucose <3.9 mmol/L / Total time) x 100; Target: TIR ≥70%, TAR <25%, TBR <4%, TBR Level 2 <1%
- 1Wear a CGM sensor continuously for a minimum of 14 days (the standard recommended period for clinically meaningful TIR analysis — more data improves reliability).
- 2Ensure sensor wear time exceeds 70% of the analysis period for the TIR data to be considered statistically robust.
- 3Download CGM data to the manufacturer's reporting platform (Dexcom Clarity, LibreView, Medtronic CareLink) or a universal platform (Tidepool, Diasend).
- 4Examine the Ambulatory Glucose Profile (AGP): the standardised CGM report showing median glucose, interquartile range, and 5th-95th percentile over a 24-hour period.
- 5Record TIR, TAR, TBR, TBR Level 2, and TAR Level 2 percentages.
- 6Compare to targets: TIR above 70% (>70% for most adults; >50% for elderly or high-risk; >70% for T1D and T2D on insulin); TBR below 4%; TBR Level 2 below 1%; TAR below 25%.
- 7Identify actionable patterns: high TBR (especially nocturnal) suggests over-insulinisation; high TAR after meals suggests inadequate mealtime insulin or carb coverage; high overnight TAR suggests basal insulin inadequacy or dawn phenomenon.
Overall good control; review cause of 4% TBR to reduce hypoglycaemia frequency
This patient meets TIR target (74%) and TAR target. The TBR of 4% is at the boundary — some guidelines now recommend aiming for below 3% TBR. The 4% may be contributed by post-exercise lows or overnight hypoglycaemia. AGP pattern analysis will identify timing and context.
Predominantly hyperglycaemic — increase basal insulin dose or add mealtime coverage
The high TAR (45%) with low TBR (3%) indicates suboptimal glycaemic control driven by hyperglycaemia rather than hypoglycaemia. This patient has room to safely intensify insulin therapy. The AGP will show whether the high glucose is primarily fasting (basal inadequacy) or postprandial (mealtime coverage needed).
Patient at high risk of severe hypoglycaemia and hypoglycaemia unawareness — immediate action required
A TBR Level 2 of 4% means the patient spends approximately 58 minutes per day below 3.0 mmol/L — critically elevated severe hypoglycaemia burden. This creates risk of seizures, cardiac arrhythmias, and hypoglycaemia unawareness (where the normal warning symptoms are blunted). Immediate insulin dose reduction across all components (basal and bolus) is urgently needed.
Pregnancy requires the most stringent CGM targets; consult obstetric diabetes team urgently
In pregnancy, the glucose target range is tighter (3.5-7.8 mmol/L) and TIR target is above 70%. Both TAR (postprandial hyperglycaemia) and TBR (hypoglycaemia — risky in pregnancy) are elevated. Immediate adjustment of both meal boluses and basal insulin is needed. Nocturnal TBR in pregnancy is particularly concerning and may indicate basal insulin over-dosing.
Monitoring glycaemic control between clinic visits, enabling dose adjustments based on objective data rather than patient recall, enabling practitioners to make well-informed quantitative decisions based on validated computational methods and industry-standard approaches
Identifying the timing and pattern of hypoglycaemia (nocturnal, post-exercise, post-meal) to guide targeted insulin adjustments, helping analysts produce accurate results that support strategic planning, resource allocation, and performance benchmarking across organizations
Demonstrating the effectiveness of new therapies (GLP-1 RAs, SGLT2 inhibitors, closed-loop systems) in clinical trials using TIR as a primary endpoint, allowing professionals to quantify outcomes systematically and compare scenarios using reliable mathematical frameworks and established formulas
Guiding meal composition and timing decisions by showing individualised postprandial glucose responses to specific foods, supporting data-driven evaluation processes where numerical precision is essential for compliance, reporting, and optimization objectives
Motivating and empowering patients with real-time glucose visibility and pattern feedback, improving treatment adherence and self-management, which requires precise quantitative analysis to support evidence-based decisions, strategic resource allocation, and performance optimization across diverse organizational contexts and professional disciplines
Hypoglycaemia unawareness and TBR
Hypoglycaemia unawareness (impaired awareness of hypoglycaemia, IAH) develops when frequent hypoglycaemic episodes blunt the normal warning symptoms (sweating, tremor, palpitations). IAH is directly related to high TBR — CGM studies show that patients with IAH spend significantly more time below 3.9 mmol/L and below 3.0 mmol/L than those with intact awareness. CGM-guided strategies to reduce TBR (insulin dose reduction, revised targets, CGM alerts) can partially restore hypoglycaemia awareness over 2-3 months.
Type 1 diabetes in pregnancy
Pregnant women with type 1 diabetes have the most stringent CGM targets because of the critical importance of both avoiding hyperglycaemia (macrosomia, congenital anomalies at high glucose) and preventing maternal hypoglycaemia (seizures, falls). The recommended TIR range narrows to 3.5-7.8 mmol/L (instead of 3.9-10.0 mmol/L), with TBR below 4% at below 3.5 mmol/L. Real-time CGM with low-glucose alerts is strongly recommended throughout pregnancy.
Post-exercise hypoglycaemia and CGM
Aerobic exercise causes immediate insulin sensitivity increase and delayed hypoglycaemia risk for 12-24 hours post-exercise in type 1 diabetes. CGM is uniquely able to identify the timing of post-exercise TBR — whether hypoglycaemia occurs during exercise (adjust pre-exercise basal reduction), immediately post-exercise (consume carbohydrate), or overnight (reduce nocturnal basal). These patterns guide precise management that finger-stick monitoring cannot provide.
CGM in non-diabetes populations
CGM use is expanding beyond diabetes into metabolic research, weight management, athletic performance optimisation, and personalised nutrition. In non-diabetic individuals, normal glucose profiles show a TIR near 100% within very tight limits. Post-meal excursions rarely exceed 7.8 mmol/L. CGM is increasingly used by health-conscious individuals to understand their personal glycaemic responses to specific foods, meal timing, sleep, and stress.
| Metric | Target — Adults (T1D & T2D) | Target — Pregnancy | Target — Elderly/High-risk |
|---|---|---|---|
| TIR 3.9-10.0 mmol/L (70-180 mg/dL) | ≥ 70% | ≥70% (3.5-7.8 mmol/L range) | ≥ 50% |
| TAR Level 1 (>10.0 mmol/L) | < 25% | < 25% (>7.8 mmol/L) | < 50% |
| TAR Level 2 (>13.9 mmol/L) | < 5% | < 5% | < 10% |
| TBR Level 1 (<3.9 mmol/L) | < 4% | < 4% (<3.5 mmol/L) | < 1% |
| TBR Level 2 (<3.0 mmol/L) | < 1% | < 1% | < 1% |
| Glucose variability (CV%) | < 36% | < 36% | < 36% |
| Sensor wear time | ≥ 70% over 14 days | ≥ 70% | ≥ 70% |
What CGM metrics are included in the ATTD 2019 consensus?
The ATTD consensus identified 10 standardised CGM metrics: mean glucose; glucose management indicator (GMI — a formula converting mean CGM glucose to estimated HbA1c); glucose variability (coefficient of variation, CV%); time in range (TIR 3.9-10.0 mmol/L); time below range Level 1 (below 3.9 mmol/L); time below range Level 2 (below 3.0 mmol/L); time above range Level 1 (above 10.0 mmol/L); time above range Level 2 (above 13.9 mmol/L); sensor wear time; and number of hypoglycaemic events.
How does TIR correlate with HbA1c?
Multiple studies have established a strong linear correlation between TIR and HbA1c: a TIR of 70% corresponds approximately to an HbA1c of 7.0%; each 10% increase in TIR corresponds to approximately 0.5% reduction in HbA1c (or approximately 14 mg/dL reduction in eAG). However, TIR provides additional information that HbA1c does not — specifically the time spent in hypoglycaemia, which HbA1c cannot detect.
What TIR targets are recommended for older adults or those at high hypoglycaemia risk?
For elderly individuals or those with high hypoglycaemia risk (frequent hypoglycaemia unawareness, severe comorbidities, limited ability to treat lows), the ATTD consensus recommends a more lenient TIR target of above 50% with TBR below 1% (rather than the standard below 4%). The priority in this group is minimising hypoglycaemia even if TIR is lower and TAR is modestly higher.
What is glucose variability and why does it matter?
Glucose variability is the fluctuation in blood glucose throughout the day, measured as the coefficient of variation (CV%) of CGM readings. A CV below 36% is considered stable; above 36% is highly variable. High glucose variability is independently associated with hypoglycaemia risk, oxidative stress, and possibly with cardiovascular outcomes, even when mean glucose and HbA1c are at target. TIR does not fully capture variability — CV% complements TIR as a quality metric.
What is the Glucose Management Indicator (GMI)?
The Glucose Management Indicator (GMI) is a formula that converts the mean CGM glucose to an estimated HbA1c equivalent: GMI (%) = 3.31 + 0.02392 x mean glucose (mg/dL). It is different from eAG derived from HbA1c and may differ substantially from the actual HbA1c in patients with haemoglobin variants or altered red cell lifespans. GMI represents what HbA1c 'should' be based on CGM data alone.
Why is at least 14 days of CGM data recommended for TIR analysis?
With fewer than 14 days of data, TIR estimates are unreliable due to day-to-day variability in eating patterns, activity, stress, and illness. Studies show that 14 days of CGM data captured with at least 70% sensor wear captures the representative glycaemic behaviour with sufficient accuracy for clinical decision-making. Longer wear periods (30 days) give even more stable estimates.
Does TIR improve with closed-loop insulin delivery?
Yes, significantly. Randomised controlled trials of hybrid closed-loop (artificial pancreas) systems consistently show TIR improvements of 10-15 percentage points compared to open-loop pump therapy or multiple daily injections. The largest improvements are typically in overnight TIR (reduced by the system's automated response to rising nocturnal glucose) and in TBR (reduced by automatic insulin cessation during predicted hypoglycaemia).
Can TIR be used to guide diabetes management in type 2 diabetes?
Yes, increasingly. While CGM was originally developed for type 1 diabetes, its use in type 2 diabetes — particularly in those on insulin — is growing. CGM-derived TIR identifies postprandial hyperglycaemia missed by HbA1c and fasting glucose monitoring, guides medication and dietary adjustments, and improves patient engagement. The ATTD TIR targets (above 70%, TBR below 4%) apply to both type 1 and type 2 diabetes.
プロのヒント
When reviewing a CGM report, prioritise TBR before TIR. A patient with a 65% TIR and 1% TBR is safer than one with 75% TIR and 6% TBR. The international consensus places hypoglycaemia avoidance first — each percentage of TBR Level 2 (below 3.0 mmol/L) represents approximately 14 minutes per day in severe hypoglycaemia, which carries real risk of seizure, unconsciousness, and sudden cardiac death.
ご存知でしたか？
The ATTD 2019 TIR targets — particularly the '70/25/4/1' rule (TIR ≥70%, TAR <25%, TBR <4%, TBR Level 2 <1%) — were established by an international panel of 43 experts from 20 countries through a Delphi consensus process. Despite intense debate, the group aligned within three rounds of voting, creating the first universal standardised CGM outcome metrics — a remarkable achievement given that HbA1c-based targets took decades to achieve similar consensus.
参考文献
- ›Battelino T et al. Clinical Targets for Continuous Glucose Monitoring Data Interpretation: Recommendations from ATTD 2019. Diabetes Care 2019
- ›Beck RW et al. Validation of Time in Range as an Outcome Measure for Diabetes Clinical Trials. Diabetes Care 2019
- ›Danne T et al. International Consensus on Use of Continuous Glucose Monitoring. Diabetes Care 2017
- ›NICE NG17 — Type 1 Diabetes in Adults: Diagnosis and Management 2015 (updated 2022)