Q: I have had type 1 diabetes for 21 years and recently started to use a continuous glucose monitor (CGM). The average glucose on my device is close to 150 mg/dl, which corresponds with an A1C of roughly 7.0%. Last week, I had an A1C test, and my result was 8.1%. Why such a difference?
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A: This is something that many people wonder about. There are several possible explanations.
1. The A1C result generated by the blood test could be wrong. There are a number of things that can produce a false result, including iron deficiency or other types of anemia, low red blood cell count, kidney disease, a recent blood donation and slow recycling of red blood cells.
2. The CGM could be wrong. Overall, most CGM systems tend to underestimate glucose levels more than they overestimate. CGMs also tend to run low for extended periods of time after a person recovers from hypoglycemia (low blood glucose), even though their blood sugar may have returned to normal quickly. Even when using a system that does not require calibration, it is still important to perform finger-sticks on a regular basis to verify that your CGM is performing properly.
3. They could both be correct. Remember, the A1C reflects the average glucose over the past eight to 12 weeks. How many weeks of data did you look at when getting the A1C estimate from your device? Certain CGM software programs default to the last two weeks of data. If your average tightened up over the past two weeks, the estimated A1C would be lower than the actual A1C.
Want to learn more about A1C? Read “How to Lower A1C Levels Naturally,” “How to Lower Your A1C Levels: More Steps You Can Take” and “HbA1c: What It Is and Why It Matters.”