Continuous Glucose Monitors (CGMs) are great tools for people with diabetes. They help you see your glucose trends and make smarter health choices. But like any technology, they aren’t perfect all the time. Let’s break down how accurate CGMs are—and what can make them a little “off.”
First: How Accurate Is a CGM?
Most CGMs today are very good. They usually stay within about 15% of your actual blood glucose when compared to a fingerstick. But they don’t always give the exact same number—and that’s okay. What matters most is the trend (going up, down, or steady), not just the number.
When Might CGMs Read Too High?
Sometimes your CGM says your sugar is high when it really isn’t. This can happen if:
- You’re dehydrated: Less water in your body can make glucose in tissue fluid look more concentrated.
- You’re sleeping or lying on the sensor: Pressure on the sensor can cause it to show false highs or highs that stick longer.
- Your blood sugar is changing very quickly: CGMs measure sugar in the fluid under your skin, which lags behind blood sugar by about 10–15 minutes. So, if you just ate, the sensor may “catch up” later.
- Sensor is new: The first 24 hours of a new sensor can be less accurate while it’s warming up.
When Might CGMs Read Too Low?
A sensor might report a low that isn’t really low when:
- Your blood sugar is dropping quickly: The CGM may show a low before your blood sugar actually hits that level.
- You’re sleeping on the sensor: Just like with high readings, pressure can cause false lows too.
- You're anemic: Low red blood cell levels can sometimes confuse the readings.
- It’s near the end of the sensor’s life: Some sensors get less accurate near the end of their wear time. Check the time in range data to see how it's fluctuating.
Takeaways and Top Tips to Improve Accuracy
CGMs are very helpful, but they’re not perfect. Here are the 4 basic tips summed up to have readings you can trust:
- Check your sensor placement—make sure it’s in a spot with good blood flow, not near scar tissue, and not on the side that you sleep on.
- Stay hydrated- drinking too little water may cause readings to be falsely high, while overhydratinon can make readings temporarily low.
- Use fingersticks for confirmation if a reading seems way off or doesn’t match how you feel.
- Let your sensor warm up fully before trusting the numbers.
Most importantly, use the glucose readings as a guide, not as the only answer. Watching the trends over time—and noticing how your body feels—is just as important as the number on the screen. If you’re ever unsure, it’s okay to do a finger stick to double-check!