Introduction
Glucose alerts—high and low—are among the most powerful features of continuous glucose monitors (CGMs). When configured well, they prevent emergencies, improve sleep, and increase time-in-range by prompting timely action before numbers drift too far. When configured poorly, they can cause alarm fatigue, unnecessary stress, and even premature sensor removals. This guide explains each alert type, where to start with thresholds, how to adapt for night, school, and exercise, and how to keep alerts meaningful without overwhelming your day.
How CGM alerts work
Threshold alerts
Trigger when sensor glucose crosses a set high or low value. Good for baseline safety and clear actions (treat a low, correct a high).
Predictive alerts
Warn that glucose will likely cross a threshold soon based on trend and velocity, giving time to prevent a low or blunt a spike.
Rate-of-change alerts
Trigger when glucose rises or falls quickly (e.g., fast drop), regardless of absolute value—useful for catching rapid shifts.
Urgent/urgent-low alerts
Non-silenceable safety alerts for severe lows (and sometimes “low soon” on select systems) to protect against dangerous hypoglycemia.
Technical alerts
Signal issues like sensor failure, lost signal, or calibration prompts so you can restore data continuity.
Choosing starting thresholds
Note: Always tailor to your clinician’s plan and personal risk. The ranges below are practical starting points many users refine over time:
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Low alert (day): 80–85 mg/dL if you act quickly, 90–100 mg/dL if you need more lead time or have hypoglycemia unawareness.
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Low alert (night): Consider +10 mg/dL above your daytime setting to allow earlier interventions while sleeping.
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High alert (day): 180–200 mg/dL for general use; adjust lower if aiming for tight control and you can act without stress, or higher if frequent non-actionable pings.
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High alert (night): Often relaxed by ~20 mg/dL compared with day to reduce sleep interruptions unless a clinical reason suggests tighter control.
Predictive and rate-of-change tuning
Predictive lows: Enable if available; set a short lead time (e.g., 15–30 minutes) to act before a symptomatic dip. If you get too many false prompts, shorten the horizon or raise the low threshold slightly instead.
Rate-of-change: Turn on one or two high-value rates (e.g., “falling fast”) that genuinely change behavior. If an RoC alert doesn’t trigger a specific action, disable it.
Alarm fatigue: prevention and fixes
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Start conservative: Use fewer alerts at first—low threshold, one high threshold, and one predictive or RoC alert that truly helps.
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Require an action: Every alert should map to a specific play (snack, confirm with meter, take insulin correction, hydrate, pause activity).
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Use schedules: Quiet or gentler alerts during meetings and study time; more protective settings overnight or when alone.
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Prune duplicates: If predictive low is on, you may not need an aggressive low threshold at the same time; avoid “double-notifying” for the same event.
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Review weekly: Look at when alerts triggered and whether you acted. Raise or lower thresholds by small steps (5–10 mg/dL) based on actual behavior.
Night safety settings
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Slightly higher low: Night low at 90–100 mg/dL gives earlier warning.
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Predictive enabled: Turn on predictive low; consider disabling non-essential highs at night to protect sleep.
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Site and compression: Choose a site less likely to be slept on; a soft sleeve can reduce compression lows that cause false alarms.
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Bedtime routine: Stabilize with a balanced snack if recommended; verify site comfort and app volume before sleep.
Exercise and “activity modes”
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Pre-exercise: Temporarily increase the low alert (e.g., +10–15 mg/dL) or enable exercise mode if your device supports it.
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During activity: Keep rate-of-change “falling fast” on; consider pausing strict high alerts if they don’t change your in-session actions.
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Post-exercise: Delayed lows can occur. Keep predictive low on for 6–8 hours after strenuous sessions; hydrate and plan a recovery snack if advised.
School and work strategies
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Minimal disruption: Use vibration or gentle tones with actionable alerts only.
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Caregiver sharing: Enable sharing so a parent, nurse, or colleague can help if alerts are missed.
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Plan of action: Agree on “who does what” for each alert during exams, field trips, and shifts to avoid confusion.
Pediatrics: confidence without chaos
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Fewer, clearer alerts: Start with one low, one predictive low, and a moderate high that genuinely requires action.
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Receiver vs. phone: If phones are restricted, a receiver provides consistent alerts; make sure staff knows how to respond.
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Review cadence: Reassess thresholds after new sports seasons, growth spurts, or schedule changes.
Adults: daily life and stress management
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Meetings/driving: Use scheduled quiet periods and automotive modes to prevent unsafe distraction; rely on vibration and predictive low when driving.
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Travel: Insert a sensor a day before trips; check roaming/notifications; pack spare patches and fast carbs.
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Sleep quality: Balance safety and rest; aim to reduce non-actionable highs at night if they don’t change therapy.
Accuracy and confirmation
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Interstitial lag: During rapid change (sprints, treating lows) CGM values can lag behind blood glucose. If readings and symptoms disagree, confirm with a meter before big dose decisions.
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Compression lows: Avoid sleeping directly on the sensor; change site if false alerts persist.
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Warm-up hours: Expect more variability in the first 12–24 hours on some systems; avoid over-reacting to early fluctuations.
Building your alert playbook
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Low alert play: Treat per plan (e.g., fast carbs), re-check in 15 minutes, avoid stacking extra carbs.
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Predictive low: Preempt with a small snack if appropriate, reduce or pause intense activity, hydrate.
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High alert play: Take a correction if indicated by plan, walk/hydrate, review meal timing and carb quality.
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RoC alerts: For “falling fast,” act sooner and re-check. For “rising fast,” inspect meal timing, correction timing, and active insulin.
Weekly review checklist
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Did alerts trigger clear actions? If not, adjust or disable.
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Were there false lows at night? Revisit site, compression, and night thresholds.
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Did highs prompt useful corrections? If not, raise the high threshold or rely on predictive tools instead.
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Any repeat patterns by time of day? Shift thresholds or routines for those windows.
Quick setup template
Day: Low 80–90 mg/dL; High 180–200 mg/dL; Predictive low ON; “Falling fast” ON if actionable.
Night: Low +10 mg/dL vs. day; Predictive low ON; High relaxed or OFF if it doesn’t change actions; vibration strong enough to wake.
Exercise: Raise low alert +10–15 mg/dL or enable exercise mode; keep “falling fast” ON; maintain predictive low for 6–8 hours post-workout.
FAQs
What alert types should I enable first?
Start with a daytime low, a nighttime low (slightly higher), and either predictive low or a “falling fast” alert—only if each alert triggers a specific action.
How do I stop alert fatigue?
Reduce to essential alerts, schedule quiet periods, eliminate duplicates, and review weekly. Every alert should imply one clear next step.
Should I tighten high alerts at night?
Only if you will take action without harming sleep. Otherwise, relax high alerts at night and keep protective low/predictive alerts on.
Do I still need fingersticks?
Yes, for safety in edge cases. Confirm with a meter when symptoms and CGM readings don’t match or during rapid changes.
How often should I adjust thresholds?
Review weekly at first, then monthly. Change by small increments (5–10 mg/dL) based on behavior and outcomes, and update for seasons, sports, or schedule shifts.
