CGM vs Fingerstick: Why Numbers Don’t Always Match
When the numbers don’t make sense
Continuous glucose monitors (CGMs) have changed the game for people managing diabetes. Instead of a single snapshot from a fingerstick, CGMs give you a moving picture of how your blood sugars shift throughout the day.
But here’s the part most people don’t talk about: sometimes the numbers don’t add up. And when your CGM is telling you one thing and your fingerstick another, it can leave you feeling frustrated, confused, and even a little defeated.
In this post, I’ll share my real-life story of when my CGM was consistently off and what I learned from it. We’ll also talk about why CGMs can sometimes be inaccurate, what the FDA says about CGM accuracy standards, and what you can do when the numbers don’t match.
When My CGM Sent Me Spiraling
I started a new CGM sensor, and for the first week everything was running smoothly. Then things took a weird turn.
One evening, I grabbed a couple of Oreo cookies. My CGM spiked up to 154 mg/dL. No surprise there. But after a brisk 45-minute walk (at about an 11–12 minute mile pace) and some weight lifting, my CGM still read 144. I doubled down: more water, more lifting. And it only dropped to 141.
The next morning, my CGM showed 114, but when I checked with a fingerstick, my blood sugar was actually 82.
To confirm, I ate breakfast with two sausages and a small pancake. Two hours later, my fingerstick was 92 while my CGM still read 121. At that point, I realized this wasn’t just lag. The sensor was consistently reading higher than reality.
Why Do CGMs and Fingersticks Sometimes Disagree?
Here are the most common reasons CGM readings can feel “off”:
1. Interstitial vs. Blood Glucose
Fingersticks measure glucose directly in your blood. CGMs measure glucose in interstitial fluid (the fluid under your skin). This naturally creates a lag of 5–15 minutes, especially when sugars are changing quickly.
2. Compression Lows
Sleeping on your sensor or putting pressure on it can cause false low readings.
3. Hydration
When you’re dehydrated, glucose movement in your interstitial fluid can slow down, making the CGM less reliable.
4. Circulation
CGMs rely on blood flow to deliver glucose into the tissue fluid where the sensor is measuring. If circulation is sluggish, which can happen when you’re cold, dehydrated, sitting still too long, or dealing with certain vascular issues, your CGM may lag behind your fingerstick.
5. Calibration Drift
At the end of the day, CGMs are tiny computers interpreting chemical signals. Sometimes they just drift off track and need to be replaced.
CGM Accuracy Standards
The FDA sets strict standards for CGM accuracy. Here’s what that looks like in plain English:
If your fingerstick is under 100 mg/dL, your CGM should usually be within ±15 points.
Example: If your meter says 90, your CGM should be between 75 and 105.If your fingerstick is over 100 mg/dL, your CGM should usually be within ±15%.
Example: If your meter says 200, your CGM should be between 170 and 230.
These standards aren’t guarantees for every scan, but they are the accuracy benchmarks CGMs must hit to be FDA cleared.
In my case, a fingerstick of 82 vs. CGM of 114 was way outside the acceptable ±15 mg/dL range. Same with 92 vs. 121post-meal. That’s how I knew this was more than a lag — it was a faulty sensor.
What To Do If Your CGM Is Off
If your CGM feels inaccurate, here’s what I recommend:
Trust your body. If your symptoms and numbers don’t match, check a fingerstick.
Look at patterns, not perfection. CGMs are most valuable for spotting trends over time, not one-off numbers.
Keep a meter handy. Always have strips available for double-checks.
Call for a replacement. If your CGM is consistently outside accuracy ranges, most manufacturers will replace the sensor.
Rotate and protect your site. Proper placement, hydration, and avoiding compression make a difference.
If you’ve ever felt like your CGM was gaslighting you, you’re not alone. Even diabetes educators like me get frustrated when the numbers don’t make sense.
Remember: CGMs are powerful tools, but they’re not perfect. Knowing their limits and when to double-check can help you use them with more confidence and less stress.
Work With Me
Want help making sense of your CGM data and learning how to focus on patterns instead of panic? That’s exactly what I do with my clients.
DM me on Instagram @balanced.bloodsugar.rd
Email: contact@russpowellnutrition.com
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