Can CGM GMI Replace HbA1c for Preoperative Clearance?
No, CGM-derived Glucose Management Indicator (GMI) should not be used instead of HbA1c for preoperative clearance, as current evidence-based guidelines specifically require HbA1c testing, and GMI performs poorly as an HbA1c estimate with significant discordance, particularly in patients with obesity and type 2 diabetes.
Guideline-Mandated Standard of Care
Current surgical guidelines explicitly require HbA1c testing, not GMI:
- The American College of Cardiology recommends preoperative HbA1c testing for patients with or at risk for diabetes undergoing elective noncardiac surgery if not performed within the preceding 3 months 1
- The Congress of Neurological Surgeons provides a Grade B recommendation that diabetic individuals undergoing spine surgery should have a preoperative HbA1c test and be counseled regarding increased risk of reoperation or infection if the level is >7.5 mg/dL 2
- The American Diabetes Association suggests a target HbA1c of <8% for elective surgeries to reduce surgical risk, mortality, infection, and length of stay 1
Why GMI Cannot Replace HbA1c for Preoperative Assessment
Poor Performance as HbA1c Estimator
The evidence demonstrates fundamental problems with GMI accuracy:
- GMI performs poorly as an estimate of HbA1c and should not be used as a substitute for laboratory HbA1c testing 3
- The absolute deviation between GMI and HbA1c ranges from 0.6% to 0.65% on average, which is clinically significant when surgical decisions hinge on specific thresholds like 7.5% or 8% 4
- GMI was originally proposed to simplify CGM data interpretation but has outlived its usefulness as a clinical tool 3
Greater Discordance in Surgical Patient Populations
The discordance is particularly problematic in the exact populations requiring surgery:
- Patients with increased BMI and type 2 diabetes show significantly greater discordance between GMI and HbA1c 4
- In a study of 278 patients using intermittent scanning CGM, the discordance was more pronounced with increased BMI, diagnosis of type 2 diabetes, and greater glucose variability 4
- This is critical because many surgical patients have obesity and type 2 diabetes, making GMI even less reliable in this context 4
Where CGM Technology Does Have Value Perioperatively
While GMI cannot replace HbA1c for clearance decisions, CGM has other perioperative applications:
- Real-time CGM can help with perioperative glycemic control in type 2 diabetes patients undergoing surgery 2
- CGM provides more frequent glycemic data and better characterizes glycemic trends compared to individual point-of-care glucose readings during surgery 5
- Both Dexcom G6 and Freestyle Libre 2.0 CGMs are feasible for intraoperative monitoring, though warm-up time (1-2 hours) and occasional sensor failure remain barriers 5
Critical Pitfalls to Avoid
- Never substitute GMI for HbA1c when making surgical clearance decisions, as guidelines specifically require laboratory HbA1c values and surgical risk thresholds are based on HbA1c, not GMI 2, 1
- Do not assume GMI and HbA1c are interchangeable—they measure different aspects of glycemic control, and leaving mean glucose in its raw form is preferable to converting it to GMI 3
- Be aware that GMI underestimates HbA1c in many patients, which could lead to false reassurance about surgical risk 4
Practical Algorithm for Preoperative Glycemic Assessment
- Obtain laboratory HbA1c within 3 months of elective surgery for all patients with or at risk for diabetes 1
- Use HbA1c thresholds to guide surgical timing: delay elective surgery if HbA1c >8% (or >7.5% for spine surgery) for glycemic optimization 2, 1
- Consider CGM for perioperative glucose management (not clearance), particularly for patients with type 1 diabetes or those on intensive insulin therapy 2
- If using CGM perioperatively, place the device during preoperative clinic evaluation the week prior to surgery to avoid warm-up time barriers on the day of surgery 5