The Dexcom G7 is one of the most accurate continuous glucose monitors available, with an overall mean absolute relative difference (MARD) of 8.2% in adults and 8.1% in children when worn on the back of the upper arm. In practical terms, that means if your actual blood glucose is 100 mg/dL, the G7 will typically read somewhere between about 92 and 108 mg/dL.
What MARD Means in Practice
MARD is the standard way researchers measure CGM accuracy. It represents the average percentage gap between what the sensor reads and what a lab-grade blood glucose measurement shows at the same moment. A lower MARD is better. Anything under 10% is generally considered accurate enough to guide insulin dosing decisions, and the G7 clears that bar comfortably.
The G7 also holds up well across different glucose ranges, not just in the “easy” middle zone. A study of 130 adults with type 1 or type 2 diabetes found a MARD of 8.3% in the normal range (70 to 180 mg/dL), 8.2% in the mild low range (54 to 69 mg/dL), 7.8% in the high range (181 to 250 mg/dL), and 7.4% for readings above 250 mg/dL. That last number is notable because many older sensors struggled with accuracy at very high glucose levels. The G7 actually gets slightly more accurate there.
How It Compares to the FreeStyle Libre 3
The Dexcom G7’s main competitor is Abbott’s FreeStyle Libre 3. According to a comparison published by the Association of Diabetes Care & Education Specialists, the Libre 3 has a MARD of 8.9% in adults and 9.4% in children. Both devices are accurate enough for treatment decisions, but the G7 holds a small edge in published clinical data, particularly in pediatric use where the gap widens to over a full percentage point.
One caveat: Abbott also published a Libre 3 study conducted without deliberate glucose challenges (meaning participants weren’t asked to eat large meals or exercise to force rapid swings), which produced a MARD of 7.9%. Study design matters when comparing these numbers, so the difference between the two systems in real-world use may be smaller than the headline figures suggest.
When Accuracy Drops
No CGM reads perfectly all the time. There are a few predictable situations where the G7’s readings may drift further from your actual blood glucose.
The First 24 Hours
CGM sensors tend to be least accurate right after insertion, during the “warm-up” period when the sensor is settling into the interstitial fluid beneath your skin. The G7 has a 30-minute warm-up, but many users report that readings stabilize further over the first day. If a number looks off in the first few hours, a fingerstick can confirm.
Rapid Glucose Changes
All CGMs measure glucose in the fluid between your cells, not directly in your blood. That interstitial fluid lags behind blood glucose by roughly 5 to 15 minutes. During rapid swings, like after a high-carb meal or intense exercise, the sensor may show you’re still rising when a fingerstick would show you’ve already started to drop. Dexcom states the G7’s accuracy across different rates of change supports its use during rapid fluctuations, but the physics of the lag can’t be fully eliminated.
Compression Lows
If you sleep on the arm where the sensor is placed, the pressure can restrict blood flow to the area and cause a false low reading. You might see a dramatic dip overnight that corrects itself when you roll over. These “compression lows” aren’t unique to the G7, but they’re worth knowing about so you don’t treat a low that isn’t real.
Acetaminophen and Other Interferences
Earlier Dexcom models were notorious for producing falsely high readings when users took acetaminophen (Tylenol). The G7 handles this much better. You can take up to 1 gram (1,000 mg) every 6 hours, which is the standard maximum dose for adults, and still trust your sensor readings for treatment decisions. Only doses exceeding that threshold may cause the sensor to read higher than your actual glucose.
Other substances and medications can also interfere. Dexcom maintains a list on their website, but for most people, standard over-the-counter medications at normal doses won’t cause problems.
Sensor-to-Sensor Variation
Even with strong overall MARD numbers, individual sensors can vary. You might have one sensor that tracks almost perfectly against fingersticks for its entire wear period, then the next one consistently reads 10 to 15 points high. This is normal and falls within the expected accuracy range. A MARD of 8.2% is an average across thousands of paired readings, not a guarantee for every single data point.
If a sensor seems consistently off by a wide margin, Dexcom’s support team will typically replace it. Many users find it helpful to do a few fingerstick checks in the first day or two of a new sensor to get a sense of how that particular sensor is tracking before relying on it fully for dosing.
What This Accuracy Means for Daily Decisions
For most people using the G7, an 8% MARD translates to readings that are close enough to guide meal boluses, correct highs, and catch lows before they become dangerous. The sensor is FDA-cleared for making treatment decisions without confirmatory fingersticks, which is a regulatory threshold that not all CGMs meet.
Where you’ll want to be more cautious is at the extremes. If the G7 shows you at 65 mg/dL and you feel fine, a fingerstick is worth the few seconds. The same goes if you see a reading above 300 that doesn’t match how you feel. At those critical thresholds, the difference between the sensor’s reading and reality matters most, and a quick blood check removes the guesswork.