Seeing a significant difference between your CGM and a fingerstick reading can be alarming. Most discrepancies have explanations — here is how to interpret them and when to be concerned.
Expected Discrepancy: The Normal Range
A difference of up to 15-20% between CGM and fingerstick is within the normal accuracy range for both devices. At 100 mg/dL, a 15% difference means your CGM can legitimately read between 85 and 115 mg/dL while your fingerstick reads 100 mg/dL. Both measurements can be technically accurate.
The Lag Time Explanation
CGM measures interstitial fluid, which reflects blood glucose with a 5-15 minute delay. During rapid glucose changes, this lag creates apparent discrepancies that are not accuracy failures — they are physics:
- After eating: CGM reads 130, fingerstick reads 160 — your blood glucose rose faster than interstitial fluid caught up
- During hypoglycemia: CGM reads 65, fingerstick reads 50 — your blood dropped faster than the CGM tracked
Compression Artifact
Lying on a Dexcom or Libre sensor while sleeping compresses the tissue around the sensor filament, reducing glucose flow to the sensor. This can cause false low readings that normalize when pressure is relieved. If your CGM alarms low overnight but you feel fine, check whether you were lying on the sensor.
Sensor End-of-Life
In the last 24-48 hours of an approved sensor wear period, accuracy decreases as the enzyme coating degrades. More fingerstick confirmations are prudent at end of sensor life.
Cold Fingers (Fingerstick Error)
Cold peripheral circulation concentrates glucose — or reduces blood flow enough to affect fingerstick accuracy. Warm your hands before a fingerstick if you suspect environmental temperature is a factor. The middle and ring fingers typically give more consistent results than the pinky.
When to Be Concerned
Persistent discrepancies greater than 20% that are not explained by timing or pressure, occurring consistently over multiple days, warrant a call to your CGM manufacturer. Document the discrepancy with screenshots and the fingerstick value — replacements are typically provided for sensors that are consistently inaccurate.