CGM Alarms and Alerts Explained in Plain English
CGM alarms can be incredibly helpful, but the first few weeks with a sensor often feel like learning a new language. It might seem like the only options are responding to every notification or silencing them altogether to stay sane.
Luckily, a third option is possible with a little bit of device knowledge. When you understand what your CGM alerts and alarms are trying to tell you, you can respond calmly, spot patterns faster, and feel more in control.
CityDME helps people across Brooklyn and the NYC area get set up with continuous glucose monitoring (CGM), and we hear the same question all the time: “What does that alarm mean?” This guide breaks down CGM alarms in simple terms, so you know exactly what to expect.
How CGM Alerts Work
Most CGM alerts fall into a few main categories. Each one answers a different question:
- Safety alerts: “Is glucose dangerously low right now?”
- Everyday glucose alerts: “Am I outside my usual range?”
- Rate‑of‑change alerts: “Is glucose rising or falling quickly?”
- Signal or technical alerts: “Is my device working as expected?”
Understanding which category an alert belongs to makes it easier to decide how urgent it is and what to do next. A glucose alert is very different from a signal alert, even if both make your phone buzz.
This is why many CGM users find that alarms feel overwhelming at first. Once you recognize the type of alert you are getting, it becomes much easier to interpret.
Looking for help understanding your CGM alerts? Connect with CityDME’s CGM experts.
30‑Second Checklist: What to Do When Your CGM Alarms
Most CGM alerts are early warnings, not emergencies. This quick check can help you respond calmly and confidently.
1. Check the alert name
Is it Low, High, Urgent Low, or Signal Loss? Each means something different.
2. Check how you feels
If you’re a caregiver, take a second to ask (or ask yourself if you’re not):
- “Do you feel shaky, lightheaded, sweaty, confused, or off?”
- Or “Do you feel okay and normal?
Symptoms help you judge urgency.
3. Look at the number and the trend arrow
- Falling arrows = more urgency.
- Steady arrows = more time to think.
4. Take the clearest next step
- Low / Urgent Low: follow the care plan you’ve already been given.
- Signal Loss: You’re probably dealing with a connection issue—work to reconnect the device or reader.
5. If alarms keep happening
Make note of patterns (especially overnight). Repeated alerts can often be improved by adjusting settings or positioning.
Important safety note: if your readings or alarms do not match your symptoms or expectations, Abbott advises doing a fingerstick for real-time blood glucose value.
Common CGM Alert Categories
Below is a breakdown of common CGM alert types, including examples from the FreeStyle Libre systems CityDME provides.
Safety Alerts: When low blood sugar is the main concern
Safety alerts are designed to get your attention when glucose drops to a level that could become dangerous if ignored. These alerts are intentionally harder to miss, especially overnight.
What safety alerts generally mean:
- Your CGM detects that glucose has reached a very low level.
- The alert is focused on immediate awareness, not long-term trends.
- These alerts tend to repeat until acknowledged or until glucose rises.
Safety alerts are not meant to scare you, but they do signal that glucose should be addressed promptly according to the plan you already have in place.
FreeStyle Libre example:
The Urgent Low Glucose alert notifies users when glucose reaches a critically low level and is designed to reduce the risk of missing a severe low.
Everyday Low and High Alerts: Staying within your usual range
These are the most common CGM alerts people see daily.
What low and high alerts generally mean:
- Your glucose crossed a low or high threshold you or your care team set.
- These alerts are often adjustable so they match your routines and goals.
- They are meant to provide awareness, not judgment.
A high alert might show up after a meal. A low alert may appear hours later during activity or sleep. On their own, these alerts do not mean something went “wrong.” They are simply cues to notice patterns and timing.
How trend arrows fit in:
A low or high alert paired with a steady arrow often gives you more time to respond than the same alert paired with a fast‑moving arrow. This context becomes important later in the blog when readers reach the trend arrow glossary.
FreeStyle Libre example:
In the Libre app, Low Glucose and High Glucose alerts are customizable within set ranges. Many users adjust these alerts over time so they receive notifications they can actually act on, rather than constant reminders that add stress.
Rate‑of‑Change Alerts: When glucose is moving fast
Rate‑of‑change alerts are less about a specific number and more about speed.
What rate‑of‑change alerts generally mean:
- Glucose is rising or falling faster than usual.
- The direction and pace matter as much as the current reading.
- These alerts often explain why something “felt sudden.”
For example, glucose might still be in a normal range but dropping quickly. A rate‑of‑change alert can help you notice that momentum before a low happens.
Not all CGMs present rate‑of‑change alerts in the same way, but the concept is similar across devices: movement matters, not just the number.
FreeStyle Libre example:
While Libre systems emphasize visual trend arrows more than separate rate alarms, those arrows serve the same purpose. A sharply downward arrow indicates glucose is falling quickly, which can add urgency to a low alert or help explain why an alert appeared earlier than expected.
Signal and Technical Alerts: Device issues, not glucose issues
Signal and technical alerts are easy to misinterpret at first. These alarms are about communication, not your blood sugar.
What signal or technical alerts generally mean:
- Your sensor and app or reader are not communicating.
- Glucose data or alarms may not update until the issue is resolved.
- These alerts do not indicate high or low glucose.
Common causes include distance from your phone, Bluetooth being turned off, or the app being closed or restricted in the background.
Why these alerts matter:
If the signal is lost, you may temporarily lose glucose alerts as well. That is why CGMs notify you when the connection drops instead of silently failing.
FreeStyle Libre example:
On FreeStyle Libre systems, this appears as a Signal Loss alert. It means the sensor has not communicated with the device for a set period of time. In most cases, bringing the phone closer and allowing the app to reconnect resolves the issue.
Why You Might Get Alerts When You Feel Fine
Sometimes an alert is accurate, but sometimes there is a reason it does not match how you feel in the moment.
Here are a few common causes:
- Compression lows at night: Lying on the sensor can temporarily reduce fluid around it and cause falsely low readings while sleeping.
- Temporary data gaps: If you have signal loss, the CGM may reconnect and then catch up, which can feel like alerts are “suddenly happening” after a quiet period.
If something feels off, treat symptoms seriously and use the backup method your care team recommends.
How to Make Alerts More Helpful (And Less Exhausting)
A CGM is most useful when alerts are actionable, not constant background noise.
1. Keep alarms tied to a clear “next step”
Your CGM alerts are intended to tell you when to take action:
If an alarm goes off and you don’t know what you would do differently, it is worth discussing your settings with your clinician. Libre high and low thresholds are designed to be customizable for that reason.
2. Reduce repeats that do not add value
Libre guidance notes that if you do not dismiss a glucose alarm notification, you may receive it repeatedly while you remain above or below the threshold. Once you’ve noted an alert and taken what action you can, make sure you dismiss it in the app.
3. Treat “Signal Loss” as a setup issue, not a health event
If Signal Loss is common for you, the fix is usually practical: keep your phone closer, check Bluetooth, and make sure the app can run in the background. Libre defines Signal Loss as a communication gap rather than a glucose reading problem.
Customize Your Alerts with a CityDME Expert
For Caregivers: How to Support Someone Using CGM
If you help a family member or patient in Brooklyn or NYC, your role is often “calm translator.”
When an alarm happens, three questions help you triage quickly:
- Which alert is it? (Low, High, Urgent Low, Signal Loss)
- How do you feel right now?
- What does the arrow show?
That is usually enough to decide whether you are troubleshooting a connection issue or responding to a glucose change.
CityDME and FreeStyle Libre in NYC: Getting Set Up Without the Runaround
Understanding alerts is one piece of the puzzle. The other is actually getting the right supplies consistently.
CityDME specializes in FreeStyle Libre’s top-performing CGM systems and local support for patients across Brooklyn and the NYC area. We’re passionate about providing CGM access and reliable ongoing supplies, and we can help streamline the steps that often slow people down.
If you are considering (or already using) FreeStyle Libre 3, or Libre 3+, our team can help you figure out what you need to get started and keep refills on track.
FAQ: CGM Alarms and Alerts
What does “Signal Loss” mean on a FreeStyle Libre?
It means your sensor has not communicated with the Libre app for about 20 minutes, often because the phone is too far away or there is a connection issue.
Are CGM alarms always an emergency?
Not always. Many alarms are early warnings based on thresholds you set, and others (like Signal Loss) are connection alerts.
Why does my CGM alarm at night when I feel fine?
One common cause is a “compression low,” which can happen if you sleep on the sensor and it reads artificially low.
Can I change my FreeStyle Libre high and low alerts?
Yes. Libre high and low glucose alarms are designed to be customizable in the app, within allowable ranges.
What should I do if my CGM reading does not match how I feel?
Abbott advises using a fingerstick blood glucose value for treatment decisions if your readings or alarms do not match symptoms or expectations.
Why does my Libre alarm keep repeating?
Libre guidance indicates that if you do not dismiss an alarm notification, you may receive it repeatedly while glucose remains above or below the set threshold.
Do I need to scan to see my glucose when I get a Libre alarm?
No. When alarms are enabled, the Libre app can display your current glucose and trend arrow in notifications without needing to scan in that moment.
