Why would a Dexcom G7 continuous glucose monitor give falsely low glucose readings?

Why Dexcom G7 Reports False Low Readings

The most common cause of falsely low readings on Dexcom G7 is "compression lows"—when sleeping position or pressure on the sensor site reduces blood flow to the subcutaneous tissue around the sensor, creating artifactually low glucose values that do not reflect actual blood glucose. 1

Primary Mechanism: Compression Lows

Compression of tissue around the sensor insertion site leads to falsely detected hypoglycemia due to decreased blood flow near the sensor tip in the subcutaneous tissue. 1 This occurs most commonly during sleep when body weight compresses the sensor site against a mattress or other surface, temporarily restricting local perfusion and causing the interstitial fluid glucose concentration to drop artificially.

Clinical Recognition

• These false lows typically occur during sleep and resolve when the patient changes position 1
• The readings often show rapid drops followed by equally rapid rises without corresponding symptoms of hypoglycemia 1
• Fingerstick blood glucose measurements during these episodes will show normal values, not hypoglycemia 1

Technical Limitations of CGM Systems

Suboptimal Accuracy in Low Glucose Range

CGM systems have inherently suboptimal analytical accuracy in the low glucose range, which is a recognized technological limitation. 1 The Dexcom system was specifically modified to operate at a high signal-to-noise ratio to improve performance at low glucose levels, but limitations persist. 1

Physiological Lag Time

There is an inherent lag time between sensor glucose values measured in interstitial fluid versus actual blood glucose, particularly during rapid glucose changes. 1 This physiological delay can contribute to apparent "false" lows when glucose is rising rapidly, as the sensor may still show lower values while blood glucose has already increased.

Other Potential Causes

Sensor Placement and Technical Issues

• Missing sensor glucose data due to transmission failures can occur 1
• Sensor warm-up periods may show inaccurate readings 1
• Environmental factors including temperature variation and electromagnetic fields can impact sensor accuracy 1

Medication Interference (Less Relevant for G7)

While older Dexcom versions (G4 Platinum) showed interference with acetaminophen, ethanol, albuterol, lisinopril, atenolol, and atorvastatin 1, 2, the Dexcom G6 and G7 incorporate a perm-selective membrane coating that significantly reduces pharmacologic interference. 1 Studies confirmed that 1 gram of acetaminophen caused only minimal interference (mean 3.1 mg/dL) in G6 sensors. 1

Clinical Management Algorithm

When False Lows Are Suspected:

1. Check for compression: Assess if the sensor is on a body site subject to pressure during sleep (arm pressed against body, abdomen compressed while lying prone) 1
2. Confirm with fingerstick: Perform capillary blood glucose measurement during the low reading 1
3. Evaluate timing patterns: Note if lows occur consistently during sleep or specific positions 1
4. Consider sensor relocation: Move sensor to alternate approved site (upper arm vs. abdomen) for next insertion 3, 4

Common Pitfalls to Avoid:

• Do not overtreating compression lows with carbohydrates, as this leads to actual hyperglycemia and creates a cycle of glucose variability 1
• Do not rely solely on CGM values for treatment decisions during suspected compression events—always confirm with fingerstick glucose 1
• Do not assume all nighttime lows are false—true nocturnal hypoglycemia remains a significant risk and must be distinguished from compression artifacts 1

Device-Specific Performance Data

The Dexcom G7 demonstrates overall excellent accuracy with mean absolute relative difference (MARD) of 8.2% for arm placement and 9.1% for abdomen placement in adults. 3 However, accuracy metrics show the system maintains good but not perfect performance across all glucose ranges, with %20/20 agreement rates of 95.3% (arm) and 93.2% (abdomen). 3 This means approximately 5-7% of readings may fall outside acceptable accuracy ranges, with false lows being one manifestation of this limitation.