CGM Monitors Blood Sugar Accurately During Surgery

What This Study Found

A 2025 study published in the British Journal of Anaesthesia tested whether a continuous glucose monitor (CGM) — specifically the Dexcom G7 worn on the upper arm — could accurately track blood sugar levels during and after noncardiac surgery. Researchers compared CGM readings to direct blood glucose measurements taken from an artery at three key points: before surgery, at the end of surgery, and 24 hours after surgery began. More than 98% of CGM readings fell within clinically acceptable accuracy ranges.

Why Does This Matter?

High blood sugar after surgery — called hyperglycemia — is a real and common problem. Yet outside of intensive care units (ICUs), it often goes undetected. Uncontrolled glucose after surgery has been linked to higher rates of infections, heart injury, and other serious complications.

Right now, most hospitals only check blood sugar in surgical patients at set intervals, or when a problem is suspected. A CGM worn during surgery could provide continuous, real-time alerts when glucose climbs too high or drops too low — without requiring repeated blood draws.

What Did the Researchers Do?

The study enrolled 118 patients over age 50 who were having noncardiac surgery and expected to stay in the hospital for at least 24 hours. About 25% of participants had diabetes. Each patient wore a Dexcom G7 CGM sensor on their upper arm, and researchers compared those readings to arterial blood glucose measurements — considered the gold standard — at three scheduled timepoints, generating 340 paired readings total.

What Were the Key Results?

• Overall bias (difference from blood glucose): Just 0.38 mmol/L across all timepoints — a small and acceptable gap.
• Before surgery: The CGM read slightly higher than blood glucose (bias of 1.08 mmol/L), which is common when a sensor is newly placed and still warming up.
• End of surgery: Accuracy improved significantly, with bias dropping to just 0.15 mmol/L.
• Mean absolute relative difference: Ranged from 12% to 18.3%, which falls within ranges considered acceptable for clinical decision-making.
• Safety check: More than 98% of CGM readings landed in safe zones on the surveillance error grid — meaning very few readings were inaccurate enough to potentially cause harm.

What Are the Limitations?

It's worth noting that accuracy was lower right before surgery — likely because the sensor had just been placed and hadn't fully calibrated yet. This is a known feature of CGM technology, not a flaw unique to this study. The researchers used only one CGM model (Dexcom G7), so results may not apply to every device on the market.

What Does This Mean for People With Diabetes?

If you have diabetes and are planning surgery, this research supports asking your care team about glucose monitoring during your procedure. Wearing a CGM leading into and through your surgery could help your medical team catch blood sugar problems faster — potentially reducing your risk of post-surgical complications.

At MDS Diabetes, we carry a range of CGM supplies and accessories, including options compatible with the Dexcom G7. Staying equipped before and after a hospital stay can make a real difference in your recovery.

Citation: Janssen, Dias, Ahuja (2025). Accuracy of continuous glucose monitoring during noncardiac surgery: a prospective, blinded observational multicentre cohort study. British Journal of Anaesthesia. PMID: 40707283. DOI: 10.1016/j.bja.2025.05.057