What is time‑below‑range (TBR) in continuous glucose monitoring (CGM)?

What is Time Below Range (TBR) in Continuous Glucose Monitoring?

Time Below Range (TBR) quantifies the proportion of continuous glucose monitoring readings that fall below predefined glucose thresholds, thereby measuring exposure to hypoglycemia. 1

Two-Level Classification System

TBR is stratified into two severity levels to guide clinical decision-making:

• Level 1 TBR (moderate hypoglycemia): glucose values between 54–69 mg/dL (3.0–3.8 mmol/L). 1
• Level 2 TBR (severe/clinically significant hypoglycemia): glucose values below 54 mg/dL (<3.0 mmol/L). 1

How TBR is Expressed

• TBR can be reported as a percentage of total CGM readings or as absolute time per day; for example, a TBR of <4% corresponds to <1 hour of hypoglycemia daily. 1
• Reporting TBR as an integer percentage has limitations due to rounding errors that may be misleading when stratifying severe hypoglycemic risk, particularly at very low percentages. 2

Data Requirements for Accurate Assessment

• A minimum of 14 days of CGM data with ≥70% sensor wear time is required to obtain a reliable estimate of TBR, especially in individuals with variable glycemic control. 1

Population-Specific Clinical Targets

Standard-Risk Adults with Type 1 or Type 2 Diabetes

• Target TBR <70 mg/dL: <4% of the day (approximately <1 hour). 3, 1
• Target TBR <54 mg/dL: <1% of the day. 1

Older or High-Risk Individuals

• Target TBR <70 mg/dL: <1% of the day (approximately <15 minutes), reflecting the heightened risk of severe hypoglycemia in this population. 1

Pregnancy

• The recommended glucose range is lower (63–140 mg/dL), with correspondingly stricter TBR thresholds to protect fetal health. 1

Pediatric Considerations

• For children and adolescents not using automated insulin delivery systems, reducing TBR to <5% (rather than just targeting TIR >70%) should be the cardinal glycemic goal, given the risk of lasting neurological damage from frequent severe hypoglycemia. 4

Critical Management Principle: TBR Takes Priority

The primary therapeutic priority is to reduce TBR to the specified target before intensifying efforts to lower Time Above Range (TAR) or to increase Time in Range (TIR). 1 This hierarchy exists because:

• Changes in TIR are not associated with changes in TBR; thus, TBR should be addressed independently of TIR, particularly in older automated insulin delivery users. 5
• Once TBR is controlled to <4% (or <1% in high-risk groups), clinicians should then aim for TIR >70% and TAR <25% in standard-risk adults. 1

Integration with Other CGM Metrics

• TBR should always be interpreted together with TIR, TAR, and glycemic variability; a coefficient of variation ≤36% is recommended for acceptable variability. 1
• The Ambulatory Glucose Profile (AGP) is the preferred tool to visualize TBR patterns over a 14-day period, helping to pinpoint times of day when hypoglycemia occurs and to guide targeted interventions. 1
• TBR remains <4% across most personal glucose targets in hybrid closed-loop systems, except at the lowest targets (80–89 mg/dL), where TBR may exceed 4% in very young children. 6

Common Pitfalls

• Focusing solely on improving TIR or reducing TAR without first addressing TBR can increase hypoglycemia risk, particularly in vulnerable populations. 1
• Rounding TBR to integer percentages may mask clinically important differences in hypoglycemia exposure, especially when values are very low (<1%). 2
• Assuming that achieving TIR goals automatically ensures adequate TBR control is incorrect; these metrics must be evaluated independently. 5