If you’ve ever checked your blood sugar with a fingerstick and noticed that it’s not the same as the number on your CGM (continuous glucose monitor), you’re not alone. It can be confusing, but there’s a reason—and it’s normal.
Two Main Reasons Fingerstick and CGM Readings Differ
Reason #1: Your CGM Measures a Different Fluid
The biggest reason for the difference is that fingersticks measure sugar in your blood, while CGMs measure sugar in the fluid between your cells, called interstitial fluid. These numbers usually line up, but they don’t match perfectly—especially if your blood sugar is changing quickly.
Reason #2: Timing Is Everything
There’s a delay between when your blood sugar changes and when your CGM shows it. This is called a lag time, and it’s usually around 5 to 15 minutes. For example, if you just ate, your fingerstick might show a spike before your CGM catches up.
What Makes CGMs Less Accurate?
Sometimes your CGM can read falsely high or low. Here are a few common reasons why:
- Change your patch location—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 enough- drinking enough water (but not too much) will ensure glucose readings don’t read falsely high or low
- Use fingersticks for confirmation if a reading seems way off or doesn’t match how you feel.
- Let your sensor warm up fully (around 24 hours) before trusting the numbers.
When to Double-Check with a Fingerstick
It’s a good idea to do a fingerstick if:
- You feel symptoms of low or high blood sugar but your CGM doesn’t show it
- Your CGM is giving you unexpected numbers
- You’re treating a low and want to be sure before taking more sugar
Takeaways
It’s totally normal for CGM and fingerstick numbers to be a little different. What matters most is trends over time—whether your blood sugar is rising, falling, or staying steady. Trust your body, watch your patterns, and use fingersticks when needed to confirm.