BG (Blood Glucose) is the fingerstick meter reading — a direct capillary blood measurement, treated as the reference value. SG (Sensor Glucose) is the CGM's reading, taken from interstitial fluid. The two normally differ a little — interstitial fluid lags blood by roughly 5–15 minutes — so some gap is expected, especially right after a meal, dosing, or exercise when glucose is changing quickly.
A widely used clinical accuracy benchmark for glucose-monitoring devices: a device reading is considered accurate if it falls within ±20% of the reference value at higher glucose levels, or within ±20 mg/dL at lower glucose levels where a percentage would be too tight to be meaningful. Regulatory bodies including the FDA have used thresholds like this when evaluating and clearing home blood-glucose and CGM devices.
| BG reading | Rule applied |
|---|---|
| Below 80 mg/dL (4.4 mmol/L) | SG must be within ±20 mg/dL of BG |
| 80 mg/dL and above | SG must be within ±20% of BG |
This calculator uses 80 mg/dL as the cutover point. Some references use 70 or 75 mg/dL instead — the underlying idea is the same: below that point, a fixed mg/dL window is used instead of a percentage.
Ask the patient for their current fingerstick (BG) and what the sensor is showing (SG) at the same moment, enter both here in the unit they're using, and tap Check accuracy. A PASS means the sensor is within the expected accuracy range for its current level — some gap is normal. A FAIL means the two readings are farther apart than the 20/20 rule allows, which may warrant a sensor check, recalibration, or escalation per your usual troubleshooting flow.