A Continuous Glucose Monitor (CGM) provides real-time glucose data by measuring the sugar level in the interstitial fluid (ISF). While CGMs offer a comprehensive view of glucose trends, users often find the reading does not match a traditional fingerstick blood glucose meter (BGM) reading. These discrepancies are common and are usually due to predictable physical, biological, or chemical factors, not a faulty device. Understanding these non-device-failure reasons is important for correctly interpreting the data and ensuring effective glucose management. The difference between the CGM and BGM can be significant, especially during periods of rapid change.
Sensor Site and Physical Placement Errors
The physical location and integrity of the sensor significantly influence the reliability of a CGM reading. The sensor filament, inserted just beneath the skin, must be fully in contact with the interstitial fluid for accurate electrochemical measurement. If the adhesive patch is not firmly secured or the sensor is not fully seated upon insertion, fluid dynamics can be disrupted, leading to unreliable readings.
One of the most frequent causes of false readings is a “compression low,” which typically happens during sleep. This occurs when sustained pressure is placed directly on the sensor, such as when lying on the device. The pressure temporarily restricts the flow of glucose-rich interstitial fluid to the sensor, causing it to register a falsely low glucose value. A compression low often presents as a sudden, sharp drop in the glucose trend line and is a common reason for alarms at night.
The site chosen also affects initial accuracy, particularly during the first 12 to 24 hours after application, known as the “bedding-in” phase. During this period, the body reacts to the foreign object, and readings may be erratic until the site stabilizes. Placing the sensor in areas with more subcutaneous fat and less muscle movement often promotes better stability and adherence.
Sensor accuracy can naturally degrade toward the end of its prescribed life, typically 10 to 14 days. This degradation is often due to the slow breakdown of the glucose-measuring enzyme or the gradual weakening of the adhesive. Weakening adhesive can lead to minor dislodgement or kinking of the filament. Manufacturers advise against extending wear time due to the risk of unreliable data.
Biological Factors Causing Reading Lag
The primary biological reason for a difference between a CGM and BGM reading is the physiological lag between glucose in the blood and glucose in the interstitial fluid (ISF). Traditional BGMs measure glucose directly in the capillary blood, while a CGM measures the glucose that has diffused into the ISF. This diffusion process takes time, resulting in a physiological delay that can range from five to fifteen minutes.
This time delay is most pronounced when blood glucose levels are changing rapidly, such as following a meal or during intense physical activity. For example, after eating, the blood glucose level may spike rapidly, but the CGM reading will trail behind. If a low blood sugar event is treated, the BGM will show a quicker rise than the CGM, which catches up as glucose equilibrates into the ISF.
Internal body states also influence glucose movement. Severe dehydration, for instance, can thicken the ISF, slowing the rate at which glucose diffuses to the sensor. This reduced fluid exchange can cause CGM readings to become erratic or falsely elevated. Staying adequately hydrated is important for maintaining optimal sensor performance.
Rapid physiological shifts, like the release of adrenaline from stress or illness, also affect the lag effect. Adrenaline can cause a surge in blood glucose. Because the CGM reading is delayed, the sensor may appear inaccurately low or high relative to the immediate blood glucose value.
External Substances and Environmental Interference
The electrochemical mechanism CGMs use is sensitive to certain external compounds in the interstitial fluid. Some medications contain molecules that interfere with the sensor’s chemical reaction, leading to false high or low readings. High-dose acetaminophen, for example, can be mistakenly detected by some sensors as glucose, resulting in a falsely elevated reading.
Ascorbic acid (Vitamin C), especially in doses exceeding 500 mg per day, is another common interferer that can skew readings. Other medications, such as salicylates (aspirin) or the cancer drug hydroxyurea, have also been shown to impact the sensor’s ability to measure glucose accurately. Users should check manufacturer guidelines for a comprehensive list of known interfering substances specific to their device.
Topical products applied to the skin can also compromise the sensor site. Creams, lotions, or sunscreens applied near the adhesive patch can seep underneath and interact with the sensor filament or degrade the adhesive. This contamination can lead to inaccurate measurements or premature detachment. Cleaning the application site with alcohol and allowing it to dry completely before insertion helps prevent this interference.
Environmental factors, specifically temperature extremes, temporarily affect sensor function. Very hot conditions, like those in a sauna or hot tub, can increase local blood flow and metabolism, altering the diffusion rate of glucose into the ISF. Conversely, very cold temperatures constrict blood vessels, slowing fluid exchange and potentially causing unreliable readings until the body stabilizes.
Steps to Validate and Resolve Inaccurate Readings
When a CGM reading seems suspicious or does not align with how a person feels, the first step is to validate the number with a traditional fingerstick blood glucose meter (BGM). The BGM measurement remains the standard for immediate, actionable glucose values, especially when the CGM is trending low or high. Use a BGM to confirm any CGM reading that falls below 70 mg/dL or above 180 mg/dL, or whenever physical symptoms conflict with the display.
Before immediately resorting to calibration, troubleshoot common issues like compression lows. If a low reading occurs overnight, rolling off the sensor and waiting 15 to 30 minutes before rechecking often resolves the false alarm. Checking hydration status and drinking water can address potential ISF viscosity issues. If calibration is required, perform it only when glucose levels are stable, as calibrating during rapid change can worsen accuracy due to physiological lag.
If the discrepancy persists after troubleshooting, and the BGM consistently shows a wide difference, the issue may be sensor failure. A persistent, unresolvable discrepancy or a sudden loss of signal indicates contacting the device manufacturer’s technical support. They can help determine if the sensor needs to be replaced under warranty due to a manufacturing or performance defect.