Dear Dr. Edelman,
My most recent lab A1c test was 7.6, but my 90-day CGM report (GMI) said my estimated A1c was 6.9. What’s the difference between the two, how can they be off by so much, and which one should I trust?
The A1c is the traditional laboratory test to determine what your average blood sugar is over the past 2-3 months. It has been known for quite some time that this lab test is commonly very inaccurate, and can be affected by many different situations such as pregnancy, ethnic background, kidney disease, liver disease, certain blood dyscrasias, etc. In fact, the HbA1c test can be off by .7-1%! On the other hand, the glucose management indicator or GMI (estimated A1c) is a number that represents the average blood sugar over the past three months because it is derived from the actual blood sugar from a CGM. My new favorite phrase is: “The best way to determine the average blood sugar over the past 2-3 months is to measure the average blood sugar over the past 2-3 months.” Which is exactly what these CGM devices do. Lastly, I believe the lab measurement of the A1c should be called an “estimated” value and the GMI the true value. In reality, for people who have a CGM, there is no need to get your blood drawn for the A1c except for insurance purposes, because they’re still living in the 1920s.