CGM vs Lab A1c: Why They're Different and Which One Matters More
Your CGM says 5.8% but your lab A1c came back at 6.1%. Which one is right? Learn why these numbers differ and what to trust.
You've been wearing a continuous glucose monitor for the past two weeks. Your average glucose is 119 mg/dL, which converts to an estimated A1c of 5.8%. Great news, right? Then you get your lab A1c results back: 6.1%. Wait—what?
If your CGM-estimated A1c (officially called GMI, or Glucose Management Indicator) doesn't match your lab A1c, you're not imagining things. Here's why they differ, which one your doctor cares about more, and what you should actually pay attention to.
What's the Actual Difference Between CGM and Lab A1c?
Let's clear this up: a lab A1c and a CGM-estimated A1c (GMI) are measuring related but fundamentally different things.
Lab A1c (HbA1c)
What it measures: The percentage of hemoglobin proteins in your red blood cells that have glucose attached to them.
Timeframe: Reflects your average blood sugar over the past 2-3 months (the lifespan of red blood cells).
How it's done: Blood sample sent to a lab for biochemical analysis.
CGM-Estimated A1c (GMI)
What it measures: Your average glucose level over a specific period (usually 7-14 days), converted to an A1c equivalent using a mathematical formula.
Timeframe: Based on whatever period your CGM has been active.
How it's done: Continuous interstitial glucose readings converted using the formula: GMI = 3.31 + (0.02392 × average glucose in mg/dL).
The key takeaway: Lab A1c measures a protein; GMI estimates A1c from glucose readings. They're cousins, not twins.
Calculate Your GMI from CGM Data
Use our CGM to A1C converter to see what your average glucose translates to in A1c terms.
Convert Your CGM Data →Why Your CGM and Lab A1c Don't Match
Research shows that GMI and lab A1c typically differ by about 0.5%, but some people see discrepancies of 1% or more. Here's why:
Red Blood Cell Lifespan Varies
The A1c formula assumes your red blood cells live exactly 120 days. But if you have anemia, kidney disease, or certain genetic conditions, your cells might live shorter or longer. This throws off the math. Someone whose red blood cells turn over faster will show a lower A1c than their actual average glucose would predict.
Individual Glycation Rates
Some people's hemoglobin binds to glucose more aggressively than others. Two people with identical average blood sugars can have A1c values that differ by 0.5-1% simply because their biochemistry is different. Genetics matter here.
Recent Glucose Patterns
If your blood sugar has been improving or worsening recently, your CGM will reflect that immediately, but your lab A1c won't catch up for weeks. Your A1c is a lagging indicator; your CGM is real-time. If you started a new diet two weeks ago, your CGM will show improvement, but your A1c is still weighted toward the previous months.
Lab Measurement Error
A1c tests aren't perfect. The National Glycohemoglobin Standardization Program (NGSP) allows for a variability of ±0.5%. Run the same blood sample twice, and you might get slightly different results. Lab error is real.
So Which One Should You Trust?
The honest answer: it depends on what you're trying to measure.
For Diagnosis: Lab A1c Wins
Doctors use lab A1c for official diabetes diagnosis because it's the standardized, validated test recognized by the American Diabetes Association. If you're borderline prediabetic or trying to confirm a diagnosis, the lab test is what counts. Insurance companies, clinical trials, and treatment guidelines all rely on lab A1c.
For Daily Management: CGM Wins
If you're actively managing diabetes or prediabetes, your CGM data is far more actionable. It shows real-time trends, meal responses, and time-in-range—metrics that matter way more than a single A1c number for day-to-day decisions. You can see exactly how that pasta dinner spiked your glucose at 9 PM, something an A1c test three months from now won't tell you.
Here's the practical advice: Use lab A1c as your official benchmark, but manage your diabetes using CGM data. Think of A1c as your quarterly report card and CGM as your daily homework feedback.
What Your Doctor Actually Cares About
Most endocrinologists now look at both metrics, but they weight them differently:
- Lab A1c: Used for treatment thresholds (e.g., starting medication, adjusting insulin doses) and tracking long-term trends.
- CGM metrics: Used to understand variability, hypoglycemia risk, and quality of control. Time in Range (TIR) is increasingly seen as more important than A1c alone.
If your lab A1c is 7.0% but your CGM shows 85% time in range with minimal variability, your doctor will be much happier than if your A1c is 6.5% but your CGM shows wild swings with frequent lows. Stability matters more than the average.
When to Worry About the Discrepancy
A small difference (0.3-0.5%) is normal and expected. But here's when you should dig deeper:
Red Flags:
- Difference greater than 1% consistently
- Lab A1c is much higher than GMI (could indicate red blood cell issues)
- You have anemia, kidney disease, or take iron supplements
- Recent blood transfusion or major blood loss
- Hemoglobin variants (common in certain ethnic groups)
If you fall into any of these categories, ask your doctor about alternative tests like fructosamine or glycated albumin, which aren't affected by red blood cell lifespan.
Compare Your Lab A1c to Average Glucose
See how your lab A1c converts to average glucose and compare it with your CGM data.
Convert Your A1c →The Bottom Line
Your CGM-estimated A1c and your lab A1c won't perfectly match, and that's okay. They're measuring different things, over different timeframes, using different methods. What matters is the trend, not the exact number.
If your GMI is consistently 0.3% lower than your lab A1c, that's your personal offset—just track both and watch for changes. If you see a sudden divergence, investigate. Otherwise, use your lab A1c for official benchmarks and your CGM for daily management.
The goal isn't perfect agreement between the two numbers. The goal is stable glucose, minimal variability, and avoiding complications. Whether that shows up as a 5.8% GMI or a 6.1% lab A1c, what matters is that you're moving in the right direction.
References
- Bergenstal, R. M., et al. (2018). Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. Diabetes Care, 41(11), 2275-2280.
- Beck, R. W., et al. (2017). The Relationships Between Time in Range, Hyperglycemia Metrics, and HbA1c. Journal of Diabetes Science and Technology, 13(4), 614-626.
- Nathan, D. M., et al. (2008). Translating the A1C assay into estimated average glucose values. Diabetes Care, 31(8), 1473-1478.
- Vigersky, R. A., & McMahon, C. (2019). The Relationship of Hemoglobin A1C to Time-in-Range in Patients with Diabetes. Diabetes Technology & Therapeutics, 21(2), 81-85.
Medical Disclaimer
This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider before making changes to your diabetes management plan. Individual results may vary.