If you’ve ever watched your CGM show a stubborn low after you already treated it, or a sudden drop that makes no sense, you’re not alone. CGM accuracy can feel confusing because a CGM is not measuring blood glucose directly. It’s measuring glucose in the fluid between your cells, and that changes on a slightly different schedule.
This post is CGM accuracy explained in plain language, with the three biggest sources of confusion: interstitial fluid lag, sensor errors, and false lows (especially compression lows). Once you see what’s normal physiology and what’s a real device issue, the graph stops feeling like a verdict and starts feeling like a useful signal.
One sentence takeaway: use CGM arrows and patterns to guide decisions, and confirm with a fingerstick when symptoms and numbers don’t match or when readings look off.
What a CGM is actually measuring, and why that causes a built in time lag
A CGM sensor sits under your skin and measures glucose in interstitial fluid, the watery fluid that surrounds cells. Your fingerstick meter (and lab tests) measure glucose in blood. Those are connected, but they’re not identical in the moment.
A simple way to picture it: blood is the highway, interstitial fluid is the neighborhood street. Glucose moves from the highway into the neighborhood. When traffic changes fast, the neighborhood doesn’t update instantly.
That’s the built in reason CGMs can “trail” behind what your blood is doing. This time delay is often called interstitial fluid lag or “lag time.” Many times, what looks like a mismatch is not a device failure at all, it’s timing plus fast-changing glucose. If you want a straightforward description of lag time from a CGM education page, see lag time in CGM explained.
This matters because most CGM decisions aren’t about the exact number at one second. They’re about where you’re headed. A CGM shines at showing direction, speed, and patterns, like “I’m dropping quickly after my workout” or “I spike after that breakfast every time.”
Lag also varies by situation. When glucose is steady, blood and interstitial fluid are closer. When glucose changes rapidly, the gap can widen. That’s why CGMs can feel “less accurate” during chaos, and pretty spot-on during calm stretches.
Interstitial fluid lag, explained with a real life example
Let’s use a common scenario: you treat a low with 15 grams of fast carbs. Your blood glucose may start rising within minutes. Your CGM, though, might keep showing the low for a bit, sometimes long enough to make you doubt yourself.
What’s happening is not that the carbs “didn’t work.” It’s that the sensor is reading the glucose level in interstitial fluid, which is still catching up to the new blood level. For many people, lag is roughly 5 to 15 minutes, and it can be longer when glucose is moving fast.
The same thing happens after a meal. Blood glucose can climb quickly, especially with a high glycemic meal. Interstitial glucose follows behind. So your CGM number might look lower than you expect right after eating, then “suddenly” rise, even though the rise was already underway in the bloodstream.
This is where trend arrows earn their keep. Even if the number feels “wrong,” the arrow can still be telling the truth about direction. A 75 mg/dL with a steady arrow is different from a 75 mg/dL with a double-down arrow. One is a pause, the other is a slide.
If you remember only one thing about lag, make it this: CGMs are best at showing trends, not acting like a real-time blood test.
Why rapid changes make CGM readings look less accurate
CGMs tend to look their best when glucose is flat and calm. They tend to look messy when glucose is swinging up or down hard. That’s expected.
Rapid changes can come from everyday stuff, including:
- High carb meals (or sugary drinks)
- Insulin doses and corrections
- Sprints or intense intervals (adrenaline can push glucose up, then activity can pull it down)
- Alcohol (especially later, when liver glucose output drops)
- Hot showers or hot tubs (blood flow changes can shift readings)
- Dehydration (less fluid can mean noisier interstitial readings)
During these moments, the CGM may trail behind, overshoot, or bounce. It can also show “urgent low” briefly and then recover, especially if pressure or circulation changes are part of the picture.
What to do when it’s moving fast: give it a little time, recheck in 10 to 15 minutes, and focus on the arrow and recent events. If you’re about to treat a low aggressively or take a large correction, confirm with a fingerstick if anything seems off.
The most common reasons CGMs give wrong numbers (not just lag)
Lag explains a lot, but not everything. Sometimes the sensor really is giving a wrong value, or at least one that shouldn’t be acted on without a reality check.
True errors usually have a pattern. They show up at certain times (like at night), in certain positions (like lying on the sensor), or at certain sites (like near scar tissue). Once you learn your “usual suspects,” the weird readings become easier to spot.
Two big buckets cause most real-world problems:
- Pressure and circulation issues around the sensor, which can create false lows.
- Site and sensor issues, which can add noise, gaps, or persistent bias high or low.
If night-time lows are a frequent stress point, the University of Michigan has a helpful overview of causes and fixes in CGM low readings at night. It lines up with what many users experience: you’re not “doing diabetes wrong,” the sensor is sometimes reacting to your sleep position and tissue pressure.
Compression lows, the classic “false low” while sleeping or leaning on the sensor
A compression low is a false low caused by pressure on the sensor site. Think sleeping directly on an arm sensor, leaning into a couch armrest, or lying with your body weight on the device.
Pressure can reduce local fluid flow around the sensor. Less fluid movement can change what the sensor detects, and it may read lower than reality. Then, when you roll over, readings often rebound quickly, sometimes like a checkmark on the graph.
Common signs it’s a compression low:
- A sudden, steep drop without a clear reason
- A low alarm that doesn’t match how you feel
- A quick bounce back after you change position
- No similar drop on past nights unless you slept on that side
It’s tempting to “just eat something” and go back to sleep, but that can lead to over-treating and waking up high. If it’s safe for you to do so, a fingerstick can confirm before you take carbs you don’t need.
For a brand-specific explanation many people find relatable, Dexcom addresses this directly in why CGM says low at night.
Practical ways to prevent compression lows:
- Choose a site you don’t sleep on often.
- If you’re a side sleeper, consider which arm faces up most nights.
- Use a pillow “barrier” so your sensor side doesn’t bear full weight.
- Avoid tight sleeves or bra straps pressing the sensor during sleep.
Site issues and sensor problems: placement, hydration, and sensor error messages
Where you wear a sensor matters more than most people expect. A site can be technically “allowed” and still be a bad fit for your body or routine.
Sites that can cause more problems include areas with scar tissue, spots that get bumped, places near waistbands, or areas with a lot of muscle movement. If the sensor gets jostled, compressed by clothing, or tugged by tape, the readings can get jumpy.
Hydration and temperature can also play a role. When you’re dehydrated, interstitial fluid dynamics can change and readings may look noisier. Cold can reduce circulation near the skin for some people, which can also affect readings. None of this means your CGM is broken, it means your tissue environment isn’t stable.
Then there are true device issues: signal loss, “sensor error,” or repeated gaps. Those alerts can mean different things depending on the system, but in plain terms they often point to a communication problem, unstable sensor data, or a warm-up hiccup.
If you’re using a Medtronic system, their support page is a good example of what “error” can cover and what steps they recommend in sensor troubleshooting guidance.
A quick sanity checklist when a reading seems wrong (or errors repeat):
- Wash and dry hands before any fingerstick comparison (food residue can fake a high).
- Wait 15 to 20 minutes and recheck if the arrow shows fast change.
- Look for pressure on the sensor, tight clothing, or recent impact.
- Follow your brand’s instructions for any restart steps (only if recommended).
- Contact support if the sensor repeatedly fails or drops signal.
How to sanity check your CGM so you can trust it more day to day
Trust doesn’t come from believing every dot on the graph. It comes from knowing when the graph is strong evidence, and when it’s more like a blurry photo.
A good rule: trust your CGM more when glucose is steady, and be more cautious when glucose is changing fast or when the reading doesn’t match your body.
It also helps to separate two kinds of decisions:
- Small, reversible choices, like taking a walk, having a small snack, or waiting 10 minutes to recheck.
- High-stakes choices, like driving, taking a large correction dose, or treating a possible low at night when you’re half asleep.
For the first category, trends and arrows often give enough guidance. For the second category, confirm when you can.
This matters for people using CGMs for wellness too. If you don’t take glucose-lowering meds, a scary “low” alert can be a false low from compression or sensor noise. That can cause stress, late-night snacking, and a lot of second-guessing. If you’re experimenting with a CGM for wellness, treat the data as feedback, not a diagnosis, and talk to a clinician if you’re seeing repeated lows.
When to confirm with a fingerstick (and how to compare the right way)
A fingerstick is most useful when it changes what you’ll do next. These are common times to confirm:
- Symptoms don’t match the CGM number (you feel low but CGM says normal, or the reverse).
- You suspect a compression low, especially at night.
- Before driving if you feel “off,” or your CGM is dropping quickly.
- During a fast drop, when the arrow suggests you’re falling faster than the number shows.
- After treating a low, if CGM still reads low and you’re deciding whether to treat again.
- Before taking a big correction dose, especially if the CGM reading seems out of character.
Comparing the “right way” prevents a lot of confusion. Wash and dry your hands, use a fresh strip, and compare when glucose is not changing rapidly. If your CGM has a steep arrow, you’re comparing two different moments in time, so the gap may look larger than it really is.
Also remember that a small difference is normal. Two devices, two sample types (blood vs interstitial fluid), and two time points won’t always match perfectly.
Calibration basics, without breaking your sensor
CGM calibration sounds simple: enter a fingerstick value so the sensor lines up better. In practice, calibration can help in some systems, and cause headaches in others if it’s done at the wrong time.
Some modern CGMs are factory-calibrated and don’t need routine calibrations. Others ask for scheduled calibrations. Your best guide is your device instructions, since “best practice” depends on the model. For a clear definition of calibration and how it’s used, see the ADCES reference on CGM calibration meaning.
A few calibration principles tend to hold up across systems:
Calibrate only when glucose is stable (a flat or gently changing arrow). If you calibrate during a sharp rise or drop, you can teach the sensor the wrong relationship, and accuracy may get worse for hours.
Don’t “force” a sensor to match a bad fingerstick. If your hands weren’t clean, or the strip is old, the meter value might be the one that’s wrong.
If calibrations are optional in your system, use them sparingly and strategically. For example, if the sensor has been consistently off in one direction during steady periods, a calibration done during a calm moment may help. If readings are jumping around due to compression, dehydration, or a bad site, calibration usually won’t fix the root issue.
Conclusion
CGMs are powerful tools, but they’re not instant blood tests. Lag is normal, compression lows are common, and real sensor problems tend to repeat in recognizable ways. Once you know what to look for, a strange reading becomes a clue, not a crisis.
Keep a simple three-step rule in mind: check the arrow, think about what just happened (food, insulin, exercise, pressure, heat), and confirm with a fingerstick when it matters. If you’re using a CGM for wellness, remember that false lows can create unnecessary stress, so don’t panic and don’t treat every alert like an emergency.
The aim isn’t perfect numbers, it’s making better choices over time, based on patterns you can trust in your glucose hub.
Gas S. is a health writer who covers metabolic health, longevity science, and functional physiology. He breaks down research into clear, usable takeaways for long-term health and recovery. His work focuses on how the body works, progress tracking, and changes you can stick with. Every article is reviewed independently for accuracy and readability.
- Medical Disclaimer: This content is for education only. It doesn’t diagnose, treat, or replace medical care from a licensed professional. Read our full Medical Disclaimer here.