As a passionate advocate for metabolic health, I've long emphasized the importance of glucose control and the transformative potential of continuous glucose monitoring (CGM) technology. These small wearable devices have revolutionized our ability to understand blood sugar patterns in real-time, providing valuable insights that can guide our nutrition and lifestyle choices.
However, like any technology, CGMs aren't perfect. They can sometimes provide readings that don't accurately reflect your true blood glucose levels. Understanding when and why these false readings occur is crucial for anyone using a CGM to optimize their metabolic health.
The Science Behind CGM Technology
Before diving into what can go wrong, let's briefly understand how CGMs work. Most modern CGMs use a tiny sensor inserted just under the skin that measures glucose in the interstitial fluid—the fluid that surrounds your cells—rather than directly from your bloodstream.
The sensor contains an enzyme (typically glucose oxidase) that reacts with glucose to generate an electrical current. This current is proportional to the glucose concentration and is converted into a glucose reading that's transmitted to your smartphone or receiver.
This indirect measurement approach is why CGM readings typically lag behind finger-stick measurements by about 5-15 minutes. It's also why certain substances and conditions can interfere with readings.
Common Culprits Behind False High Readings
When your CGM shows an unexpectedly high glucose spike, before panicking about that seemingly innocent sweet potato you ate, consider these potential causes:
1. Acetaminophen (Tylenol) Interference
One of the most well-documented CGM interferences comes from acetaminophen. The molecular structure of acetaminophen can be misinterpreted by some CGM sensors as glucose, resulting in artificially elevated readings that persist for several hours after taking the medication.
This effect varies significantly between CGM brands and models. For example, earlier generations of Dexcom sensors were quite susceptible to this interference, while newer versions have improved considerably. If you must take acetaminophen, it's worth consulting your device's documentation for specific guidance.
2. Vitamin C in High Doses
While vitamin C (ascorbic acid) is essential for health, high doses—typically above 500mg—can create electrochemical interference with the glucose oxidase reaction in your CGM sensor. The resulting oxidation reaction can mimic glucose detection, causing false high readings.
This is particularly relevant for those who take high-dose vitamin C supplements for immune support or follow certain intravenous vitamin C protocols. If you're monitoring a puzzling glucose pattern, consider whether your vitamin regimen might be a factor.
When Your CGM Reads Falsely Low
False low readings can be equally problematic, potentially leading to unnecessary carbohydrate consumption or medication adjustments. Here's what might cause them:
1. The "Compression Low" Phenomenon
This is perhaps the most common cause of false low readings and occurs when pressure is applied to the area where your sensor is located. If you sleep on your sensor or press it against something firmly, the reduced local blood flow can cause an artificial drop in the glucose reading.
The telltale sign of a compression low is a sudden drop in glucose that rapidly returns to normal once pressure is removed. If you notice a concerning low reading in the night or after leaning against something, try changing positions and rechecking the reading before responding with food.
2. Salicylic Acid Effects
Compounds containing salicylic acid—found in aspirin and many topical skincare products—can potentially interfere with CGM readings by either promoting insulin secretion (which genuinely lowers blood sugar) or by directly interfering with the sensor's electrochemical reactions.
This interference can be particularly confusing since aspirin is commonly used by many adults for cardiovascular protection. If you notice consistent discrepancies between your CGM and finger-stick readings after using aspirin or applying salicylic acid-containing skincare products near your sensor, this could be the culprit.
Environmental and Temporal Factors Affecting CGM Accuracy
Beyond specific substances, several other factors can impact the reliability of your CGM readings:
1. Temperature and Humidity Extremes
CGM sensors are designed to function within specific environmental parameters. Extreme heat, cold, or humidity can affect the sensor's chemistry and physical integrity, leading to decreased accuracy.
This becomes particularly relevant during:
- Hot summer days or sauna sessions when increased sweating and heat can affect sensor performance
- Winter activities in very cold conditions
- Exercise that generates significant body heat and perspiration
If you're experiencing unusual readings during extreme weather or activities, consider this potential factor before making significant changes to your treatment approach.
2. The "Break-In" Period and End-of-Life Inaccuracies
Most CGM manufacturers acknowledge that the first 24-48 hours after sensor insertion often yield less reliable data as the sensor equilibrates to your interstitial environment. Similarly, as sensors approach or exceed their rated lifespan (typically 10-14 days, depending on the model), accuracy tends to decline.
I've observed in both my personal experience and clinical practice that:
- Day 1 readings often require verification with finger sticks
- The sweet spot for most accurate readings is typically days 3-10
- Beyond the recommended wear time, expect increasing deviation from true values
3. Medication Interactions
Beyond the well-known acetaminophen and aspirin interferences, various other medications have been identified as potentially affecting CGM accuracy:
- Hydroxyurea (used for certain blood disorders)
- Lisinopril and other ACE inhibitors
- Albuterol (commonly used in asthma inhalers)
- Tetracycline antibiotics
- Certain antidepressants and antipsychotics
The mechanisms behind these interferences vary—some directly affect the sensor's chemistry, while others may actually alter glucose metabolism.
Practical Tips for CGM Users
Given these potential inaccuracies, how can you maximize the value of your CGM while minimizing misleading information? Here are my recommendations:
-
Keep a log of medications and supplements you take regularly, especially noting when you start or stop something new that coincides with unusual CGM patterns.
-
Be strategic about sensor placement to avoid compression lows—rotating sites not only reduces skin irritation but can help you find optimal locations that don't get compressed during your typical sleep positions.
-
Have a backup finger-stick meter for verification when readings seem suspect or don't match how you feel.
-
Don't overreact to single data points. The true value of CGMs lies in identifying patterns and trends over time, not in isolated spikes or drops that may be artifacts.
-
Consider environmental context when interpreting unusual readings—did you just come in from very hot or cold conditions? Is your sensor nearing the end of its lifespan?
The Bottom Line
CGMs remain incredible tools for understanding and optimizing metabolic health, despite their occasional inaccuracies. By understanding the common causes of false readings, you can interpret your data more intelligently and avoid unnecessary anxiety or interventions based on misleading information.
Remember that the goal of continuous glucose monitoring isn't perfection, but progress. Even with occasional inaccuracies, the longitudinal data and pattern recognition CGMs provide vastly outweigh the limitations of periodic finger-stick testing.
The key is developing a nuanced relationship with your data—trusting it enough to learn from it, but maintaining healthy skepticism when readings don't align with your symptoms or circumstances.
References
-
Fokkert MJ, van Dijk PR, Edens MA, et al. Performance of the FreeStyle Libre Flash glucose monitoring system in patients with type 1 and 2 diabetes mellitus. BMJ Open Diabetes Res Care. 2017;5(1):e000320. doi:10.1136/bmjdrc-2016-000320
-
Calhoun P, Johnson TK, Hughes J, Price D, Balo AK. Resistance to acetaminophen interference in a novel continuous glucose monitoring system. J Diabetes Sci Technol. 2018;12(2):393-396. doi:10.1177/1932296818755797
