CGM Alarms and Alerts: Predictive Urgent Low and 5 Alert Types
5 CGM alert types — urgent low, low, high, rate of change, and predictive alerts. How customizable thresholds work and which devices offer the most advanced alarm systems.
How CGM Alerts Prevent Dangerous Glucose Events
CGM alarms and alerts are arguably the most life-saving feature of continuous glucose monitoring. Unlike fingerstick testing, which only tells you what your blood sugar is at the moment you test, a CGM tracks glucose trends in real time and can warn you before glucose reaches dangerous levels. Real-time CGMs like the Dexcom G7, FreeStyle Libre 3 Plus, and Eversense E3 offer multiple alert types: threshold alerts that trigger when glucose crosses a preset high or low boundary, rate-of-change alerts that warn when glucose is rising or falling rapidly, and predictive alerts that use trend algorithms to forecast where glucose will be in 10 to 20 minutes. The IN CONTROL trial demonstrated that CGM alerts reduced severe hypoglycemic events by 72% in people with type 1 diabetes and hypoglycemia unawareness — a population at highest risk for glucose-related emergencies.
Types of CGM Alerts
Modern CGMs offer 4 categories of alerts. High glucose alerts notify you when glucose rises above a customizable threshold (typically set between 180 and 250 mg/dL). Low glucose alerts trigger when glucose drops below a threshold (typically 70 or 80 mg/dL). Urgent low soon alerts are predictive — the Dexcom G7 triggers this alert when its algorithm predicts glucose will reach 55 mg/dL within 20 minutes, giving you time to consume carbohydrates before a crisis. Urgent low alarms at 55 mg/dL are non-customizable on Dexcom devices and cannot be silenced, because glucose below 54 mg/dL is classified as clinically significant hypoglycemia with risk of seizure or loss of consciousness. Signal loss alerts notify you if the CGM loses Bluetooth connection with your phone for more than 20 minutes, ensuring you are aware of data gaps. Each alert type can be configured for volume, vibration, and whether it overrides phone "Do Not Disturb" settings.
Customizing Alert Thresholds for Your Needs
The right alert settings depend on your medical situation and personal preferences. People on intensive insulin therapy typically set tighter thresholds: high alert at 180 mg/dL, low alert at 80 mg/dL, and urgent low at 55 mg/dL. Elderly patients or those with hypoglycemia unawareness may set a higher low alert at 90 mg/dL to provide more lead time. Non-diabetic wellness users often set a high alert at 140 mg/dL to catch postmeal spikes that indicate suboptimal metabolic health. Alert fatigue — the tendency to ignore or disable alerts that fire too frequently — is a real clinical concern. Studies show that users who receive more than 5 alerts per day are significantly more likely to disable their alerts entirely. To avoid alert fatigue, start with wider thresholds and gradually tighten them as your glucose control improves. The Dexcom G7 app allows you to set different alert profiles for different times of day, such as tighter thresholds during the day and wider thresholds at night to avoid sleep disruption.
Sharing Alerts with Caregivers
CGM data sharing extends alerts to family members, caregivers, and school nurses. The Dexcom Follow app allows up to 10 followers to receive real-time glucose data and customizable alerts on their own smartphones. If the CGM wearer is a child at school, their parent can receive an alert on their phone the moment glucose drops below 70 mg/dL — even though they are miles away. Abbott's LibreLinkUp app provides similar functionality for FreeStyle Libre users. For people with hypoglycemia unawareness who may lose consciousness during a severe low, caregiver alerts serve as a critical safety net. A 2023 study found that caregiver-received CGM alerts led to intervention within 10 minutes in 85% of hypoglycemic events, compared to 35% without remote monitoring. This capability is particularly valuable for pediatric diabetes management, where parents need visibility into their child's glucose during school hours.