A Continuous Glucose Monitor (CGM) is a wearable device that tracks glucose levels throughout the day and night. Hypoglycemia is a condition characterized by low blood sugar, which can cause symptoms like dizziness or confusion. A CGM is used to manage and prevent these hypoglycemic events by providing a constant stream of data. This helps users understand their glucose patterns in relation to meals, activity, and other factors.
How a CGM Identifies Hypoglycemia
A Continuous Glucose Monitor measures glucose within the interstitial fluid (the fluid surrounding your cells) via a tiny sensor filament inserted just beneath the skin. The sensor is often placed on the arm or abdomen and continuously samples this fluid. It then translates these measurements into glucose values every few minutes, sending the data wirelessly to a receiver or smartphone.
CGMs identify potential hypoglycemia through two primary types of alerts. The first is a threshold alert, which activates when your glucose level drops below a specific number set by you and your healthcare provider, such as 70 mg/dL. This alert signals that your glucose has entered a low range and requires attention.
The second method involves predictive alerts, which analyze the speed and direction of your glucose changes. Using an algorithm to interpret this trend data, the device can warn you of an impending low before it happens. This forecasting provides valuable time to take preventive action, such as consuming carbohydrates, to avoid a hypoglycemic event.
Interpreting and Responding to Low Glucose Alerts
When a low glucose alert sounds, look at both the number and the trend arrow for context. A reading of 80 mg/dL with a single arrow pointing down indicates a slow drop, while two arrows down signals a rapid fall requiring more urgent action. A stable or upward-pointing arrow next to a low number may suggest your glucose is already recovering.
First, assess how you feel. If you have symptoms of hypoglycemia, such as shakiness or sweating, treat the low immediately. If you feel fine or the reading seems unusual, confirm the CGM reading with a traditional fingerstick blood glucose meter (BGM) before consuming sugar. This step prevents over-treating a false low, which could cause your glucose to spike.
To treat a confirmed low, follow the “Rule of 15”: consume 15 grams of fast-acting carbohydrates, wait 15 minutes, and then recheck your glucose. You should also manage alert settings to avoid “alarm fatigue,” which occurs when frequent alarms cause you to ignore them. Customizing the low alert threshold and predictive warnings helps ensure the alerts you receive are meaningful and actionable.
Common Causes of False or Misleading Lows
Low glucose alerts are not always reflective of a true hypoglycemic event. A frequent cause of misleading readings is “compression hypoglycemia,” which occurs when you put pressure on the CGM sensor by sleeping or leaning on it. The pressure temporarily squeezes interstitial fluid away from the sensor, causing a falsely low reading that resolves once the pressure is removed.
Dehydration can affect sensor accuracy by altering glucose concentration in the interstitial fluid. Certain substances also interfere with sensor readings as well. High doses of vitamin C or acetaminophen may cause falsely high readings, while salicylic acid (in aspirin) can lead to falsely low readings with some sensors.
A new sensor requires a “warm-up” period of 30 minutes to a few hours after insertion. During this time, the sensor is calibrating to your body, and readings may be less reliable. Erratic highs and lows can occur during the first 24 hours until the sensor has fully settled.
CGM vs. Blood Glucose Meter for Hypoglycemia
The primary difference between a CGM and a blood glucose meter (BGM) is what they measure. A BGM analyzes glucose directly from a blood sample, providing the most current value. A CGM measures glucose in the interstitial fluid, which is the fluid that surrounds the cells in your tissues.
Because glucose moves from the bloodstream into the interstitial fluid, a natural delay, or “lag time,” exists between the two readings. This lag is most noticeable when glucose levels change rapidly, such as during a hypoglycemic event. A BGM will detect a drop in blood sugar before a CGM does, with studies showing a delay of around 6 to 12 minutes. Consequently, your CGM might read 85 mg/dL while your BGM shows 70 mg/dL.
The same effect occurs during recovery, as a BGM will show a rise in glucose sooner than a CGM after treatment. This lag time is why a fingerstick reading is the standard for confirming a suspected low. The BGM offers a precise snapshot of your current blood glucose, while the CGM provides the broader trend.