Glucose Monitoring: SMBG, CGM & Time in Range
Key Takeaways
- The ADA-endorsed Time in Range target for most adults with diabetes is greater than 70% of readings between 70-180 mg/dL, roughly 17 hours per day.
- Time below range under 54 mg/dL (Level 2 hypoglycemia) should stay under 1% of the day, about 15 minutes, per ADA Standards of Care.
- The Glucose Management Indicator formula is GMI (%) = 3.31 + (0.02392 x mean CGM glucose in mg/dL).
- GMI requires a minimum of 14 days of CGM data with the sensor active at least 70% of the time to be considered valid.
- The Ambulatory Glucose Profile displays median glucose plus interquartile and interdecile bands over a single 24-hour modal day.
Self-Monitoring of Blood Glucose (SMBG)
Self-monitoring of blood glucose (SMBG) with a fingerstick meter remains a core self-care skill even as continuous glucose monitoring (CGM) becomes the CDCES-preferred technology for insulin-treated diabetes. Device selection should weigh accuracy (meters must meet ISO 15197 accuracy standards, within 15% of a laboratory reference for at least 95% of results), cost and insurance coverage, screen size and audio output for visual impairment, and the person's dexterity and health literacy. Correct technique matters as much as device choice: wash and dry hands before testing (residual food or sugar on fingers can falsely elevate results), warm the hand to improve blood flow, use the side of the fingertip rather than the pad to reduce pain, and apply an adequate first drop rather than "milking" the finger, which dilutes the sample with tissue fluid.
Testing frequency is individualized to regimen. People using multiple daily injections (MDI) or insulin pumps typically test 4 or more times per day — before meals, at bedtime, and before driving or exercise — while people managing type 2 diabetes with non-insulin therapies may test far less often. The CDCES teaches structured SMBG (paired pre- and post-meal testing on selected days) to reveal patterns that guide therapy adjustments without an unsustainable daily testing burden.
Continuous Glucose Monitoring (CGM)
CGM measures interstitial glucose every 1-5 minutes through a subcutaneous sensor and is now recommended for most people using insulin, and increasingly for people with type 2 diabetes not on insulin. Two categories exist: real-time CGM (rtCGM), which streams data continuously to a receiver or smartphone with customizable high/low alarms, and intermittently scanned CGM (isCGM), or "flash" monitoring, which stores data that the user retrieves by scanning the sensor. Many current systems integrate with automated insulin delivery (AID/hybrid closed-loop) pumps that adjust basal insulin delivery from CGM trends in real time.
Device selection factors include sensor wear duration (most current sensors run 10-15 days), whether factory calibration eliminates fingerstick calibration, alarm customization (alarm fatigue is a common reason for CGM discontinuation), insertion-site tolerance and skin reactions, sensor warm-up time, and the person's smartphone access and digital literacy. The CDCES role is matching device features to the individual's lifestyle, insurance formulary, and self-management capacity — not recommending a single "best" device for everyone.
Time in Range: The Modern Glycemic Framework
CGM's biggest clinical contribution is Time in Range (TIR), the metric structure endorsed by the ADA Standards of Care and built on the International Consensus on CGM Data Interpretation. TIR reframes glycemic assessment around the percentage of a 24-hour day spent in, below, and above target glucose ranges, giving the person and care team an actionable, real-time picture that A1C alone cannot provide — A1C is a 2-3 month average and cannot reveal hypoglycemia, variability, or time course.
| Metric | Target Range | Goal (most adults, T1D/T2D) |
|---|---|---|
| Time in Range (TIR) | 70-180 mg/dL | Greater than 70% (~17 hr/day) |
| Time Below Range, Level 1 | Below 70 mg/dL | Less than 4% (~1 hr/day) |
| Time Below Range, Level 2 | Below 54 mg/dL | Less than 1% (~15 min/day) |
| Time Above Range, Level 1 | Above 180 mg/dL | Less than 25% (~6 hr/day) |
| Time Above Range, Level 2 | Above 250 mg/dL | Less than 5% (~1 hr 12 min/day) |
| Glycemic variability | — | Coefficient of variation ≤36% |
These targets are relaxed for populations at higher hypoglycemia risk: older or high-risk adults target TIR greater than 50% and Level 1 time below range under 1%, while pregnancy with type 1 diabetes uses a narrower 63-140 mg/dL range with TIR greater than 70%, time below 63 mg/dL under 4%, time below 54 mg/dL under 1%, and time above 140 mg/dL under 25% (covered further in Chapter 12). The CDCES individualizes targets rather than applying the general-adult table universally.
Glucose Management Indicator (GMI) and the Ambulatory Glucose Profile (AGP)
The Glucose Management Indicator (GMI) converts a CGM's mean glucose into an A1C-like percentage: GMI (%) = 3.31 + (0.02392 × mean CGM glucose in mg/dL). GMI requires a minimum of 14 days of CGM data with the sensor active at least 70% of the time to be considered representative. GMI and laboratory A1C frequently diverge — sometimes by more than a full percentage point — because GMI reflects only the CGM-worn period while A1C reflects glycation over the full red blood cell lifespan; the CDCES should present both values together and explain the difference rather than treating them as interchangeable.
The Ambulatory Glucose Profile (AGP) is the standardized, one-page CGM report recommended by ADA and international consensus for use across CGM brands. It overlays glucose readings from the reporting period onto a single 24-hour "modal day," displaying the median glucose line plus shaded interquartile (25th-75th percentile) and interdecile (10th-90th percentile) bands. This visualization exposes patterns invisible in a single log — dawn phenomenon, consistent post-breakfast spikes, recurrent nocturnal hypoglycemia — that the CDCES uses collaboratively with the person to prioritize the next intervention, whether a meal-timing change, a basal-rate adjustment, or referral back to the prescriber.
According to the ADA Standards of Care Time in Range framework, what is the goal for time spent below 54 mg/dL (Level 2 hypoglycemia) for most adults with diabetes?
A CGM report shows a Glucose Management Indicator (GMI) calculation. What minimum data requirement must be met for the GMI to be considered representative?