A patient asked a common question highlighting the confusion many face when comparing their CGM’s estimated A1c (GMI) with the result from a blood test. He mentioned that their CGM estimated their A1c at 7.2, but their actual A1c from blood work came back at 8.3—a significant difference. So, what went wrong?
The first thing to understand is that the estimated A1c, known as GMI (Glucose Management Indicator), from a CGM, is different from the blood test A1c that a doctor orders. The GMI estimates what the A1c could be based on the blood sugars that the CGM picks up. However, it’s important to note that the CGM measures glucose in interstitial fluid, not directly from blood. This is a key difference because interstitial fluid can sometimes provide readings that are slightly higher or lower than actual blood sugar levels.
Because the CGM measures glucose in interstitial fluid, the readings may not perfectly align with the blood test A1c, which is derived directly from blood. As a result, the GMI reading from the CGM could be slightly higher or lower than the actual A1c. In some cases, such as the one mentioned, this difference can be quite large—like the gap from 7.2 to 8.3.
It’s important to keep in mind that the GMI is only an estimate. It serves as a reference point, not a final measure of blood sugar control. Typically, the GMI is off by only 0.2-0.3%, but in some cases, the difference can be more significant.
For those who have been disappointed by a large discrepancy between their CGM’s estimated A1c and their actual A1c from blood work, it’s important to understand that this isn’t uncommon. The key takeaway is not to treat the GMI as absolute truth. While the GMI is a useful tool to give an idea of how blood sugars are behaving over time, it isn’t always perfectly accurate.
For anyone who has never been informed about the difference between their CGM’s GMI and their actual A1c, it’s a good idea to discuss this with a healthcare team. Diabetes educators and dietitians, especially those who specialize in Type 1 diabetes (T1D dietitians) or Type 2 diabetes (T2D dietitians), can help explain the nuances of CGM readings and how they fit into overall diabetes care.
Understanding this difference can help reduce confusion and frustration. While the GMI is a helpful tool, it’s just one part of the broader picture of managing diabetes. Consistent monitoring, a balanced diet, and a comprehensive treatment plan are what truly matter in maintaining good diabetes control.
So, for anyone wondering why their GMI and A1c might not match, now they have the answer. The goal is to focus on overall blood sugar management, and using both CGM data and A1c together can help achieve better results in the long run.
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