Time Below Range (TBR) in Continuous Glucose Monitoring – Evidence‑Based Guidelines
Definition
- TBR quantifies the proportion of continuous glucose monitoring (CGM) readings that fall below a predefined glucose threshold, thereby measuring exposure to hypoglycemia. 1
Classification of Hypoglycemia Severity
- Level 1 TBR (moderate hypoglycemia) is defined as glucose values between 54 mg/dL and 69 mg/dL (3.0–3.8 mmol/L). 1
- Level 2 TBR (severe/clinically significant hypoglycemia) is defined as glucose values below 54 mg/dL (<3.0 mmol/L). 1
Expression of TBR
- TBR may be reported as a percentage of total CGM readings or as absolute time per day; a value of <4 % corresponds to <1 hour of hypoglycemia daily. 1
Data Requirements for Reliable 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
Adults with Type 1 or Type 2 Diabetes
Older or High‑Risk Individuals
- Target TBR <70 mg/dL: <1 % of the day (≈ <15 minutes), reflecting the heightened risk of severe hypoglycemia. 1
Pregnancy
- The recommended glucose range is lower (63–140 mg/dL), with correspondingly stricter TBR thresholds to protect fetal health. 1
Therapeutic Priorities
- The primary management goal is to reduce TBR to the specified target before intensifying efforts to lower Time Above Range (TAR) or to increase Time in Range (TIR). 2
- Once TBR is controlled to <4 % (or <1 % in high‑risk groups), clinicians should aim for TIR >70 % and TAR <25 % in standard‑risk adults. 2
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][2]
- 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][2]