A Continuous Glucose Monitor (CGM) is a small, wearable device that provides real-time measurements of glucose levels throughout the day and night. Unlike traditional fingerstick testing, which offers a single snapshot of blood sugar, the CGM measures glucose in the interstitial fluid found between cells. Hypoglycemia, or low blood sugar, is typically defined as a glucose level below 70 milligrams per deciliter (mg/dL) and becomes dangerous if it drops below 54 mg/dL. Monitoring these drops is important for safety, as severe hypoglycemia can lead to confusion, loss of consciousness, and requires immediate intervention.
How CGM Detects Low Glucose Events
The detection process begins with a tiny sensor placed just under the skin, measuring the concentration of glucose in the interstitial fluid every few minutes. This sensor uses an enzymatic reaction to convert glucose into an electrical signal, which is translated into a numerical reading displayed on a receiver or smartphone app. Data is transmitted every one to five minutes, allowing the device to report the current level and calculate the rate of change.
Users configure customized alert thresholds, typically setting a low glucose warning at 80 mg/dL or 70 mg/dL for early notification. An urgent low alert, usually set at 55 mg/dL, sounds a distinct tone to wake a sleeping user or demand immediate attention. Beyond static thresholds, the CGM generates predictive alerts based on the calculated rate of glucose change.
These predictive alerts notify the user when glucose is dropping rapidly, even if the current reading is still within the target range. For instance, if the trend arrow indicates a steep decline predicting a low glucose event within the next 20 minutes, the device issues a warning. This proactive capability allows for intervention before the glucose level hits the low threshold, reducing the duration and severity of the event. The real-time display of glucose trend arrows—indicating stable, slow, or rapid change—is central to this early warning system.
Responding to CGM Hypoglycemia Alerts
When a low glucose alert sounds, the first step is to assess the current glucose reading alongside the trend arrow displayed on the monitor. The trend arrow provides context, showing if the glucose is falling quickly or slowly, which dictates the urgency of treatment. A reading with a double-down arrow indicating a rapid drop demands a preemptive response.
Standardized protocols, such as the “15-15 Rule,” guide the response to a confirmed low reading, typically below 70 mg/dL. This rule involves consuming 15 grams of fast-acting carbohydrates, such as glucose tablets or juice, to raise the glucose level quickly. The user then waits 15 minutes before rechecking the CGM reading and reassessing the trend arrow to confirm the glucose is rising.
If the glucose level remains below 70 mg/dL or the trend arrow still points down after the initial 15-minute wait, the user repeats the process of consuming another 15 grams of carbohydrates. A fingerstick blood glucose measurement is recommended if the CGM reading does not align with the user’s physical symptoms or if the user is treating an extremely low reading. Modern CGM systems are increasingly approved for non-adjunctive use, meaning treatment decisions can often be made based on the sensor reading without confirmation.
Understanding CGM Accuracy During Lows
The CGM measures glucose in the interstitial fluid, creating a physiological difference compared to glucose measured directly in the blood. This difference results in “lag time,” where the interstitial fluid reading trails the blood glucose reading. This delay typically ranges from 5 to 15 minutes.
Lag time is most noticeable during periods of rapid glucose change, particularly when the level is dropping quickly toward hypoglycemia. Since blood glucose drops first, the CGM reading may be artificially higher during this decline, potentially delaying the low alert. This emphasizes the importance of treating the trend arrow proactively rather than waiting for the number to hit a low threshold.
Another consideration is the “compression low,” a false low reading that often occurs during sleep. This happens when direct pressure is applied to the sensor, restricting interstitial fluid flow and resulting in a falsely low signal. Recognizing the characteristic steep, sudden drop on the graph without corresponding physical symptoms helps users differentiate a true low from this reading error.
CGM’s Role in Addressing Hypoglycemia Unawareness
Hypoglycemia unawareness is a condition where a person no longer experiences the common physical symptoms of low blood sugar, such as shakiness, sweating, or hunger. Repeated episodes of low glucose can blunt the body’s counter-regulatory hormone response, silencing the physical warning signals that would normally prompt the individual to treat the low. This lack of physiological warning places individuals at a much higher risk for severe, life-threatening hypoglycemia.
The CGM provides a critical technological substitute for these missing physical symptoms, acting as an artificial alarm system. By setting customized low and urgent low alerts, the device ensures that a warning is issued even when the body fails to send its own signals. This continuous vigilance is especially important overnight when the user is asleep and unable to monitor their own condition.
Studies have shown that the consistent use of CGM can significantly reduce the frequency and severity of hypoglycemic episodes in people with unawareness. By providing constant data and predictive alerts, the technology enables individuals to successfully preempt lows. The ability to avoid hypoglycemia over time can, in some cases, help retrain the body’s warning system and partially restore a person’s natural awareness.