What’s Hiding in Your CGM Download?

This case is extremely important because we rely on CGM downloads to get an idea of how patients are doing. If you have a time below range of 0%, it would appear that you’re not having any lows. In reality, you could have had a severe low, but it didn’t register on your download because you didn’t have it long enough (depending on the length of the download) and your healthcare provider will miss picking up on the low blood sugar. This is extremely important because questions need to be asked regarding the hypoglycemic event (e.g., what time of day did it happen, was it a compression low, was it due to too much fast-acting insulin, was the basal rate of your pump or your injection too high because you lost weight, etc.). You and your provider really need to be aware that you can miss a hypoglycemic event when looking at these downloads.

The Patient:

José is an 85-year-old Army veteran who has had type 1 diabetes for over 20 years. He’s currently on a multiple daily injection (MDI) regimen, and he has a degree of hypoglycemia unawareness. He came into clinic recently with the 14-day download from his FreeStyle Libre 3  CGM, and at first glance it looked extremely good:

As you can see from the report:

• His time in range (target range) was 78%
• His lows/very lows were 0%
• His average glucose was 146 mg/dL
• His GMI (sensor-derived A1c) was 6.8%
• His glucose variability was 31% (goal is less than 36%)

In the AGP chart below, you can see that he had a couple of ups and downs, but with those metrics at 85 years old (and with zero lows) I was very happy.

So…If Everything Looks So Good, What’s the Problem?

After looking at the first-page summary of José’s download, I told him it looked like he was doing great, especially because he wasn’t having any lows. He said, “What do you mean? I have had a couple of bad lows.” I dug deeper into the download and sure enough, he did have a few low blood sugars that were well below 55 mg/dL. You can see the tiny red dips in the glucose pattern chart below.

Since José’s lows lasted less than 15 minutes in total, they weren’t long enough to register as even 1% on the download (1% represents 15 minutes on average for each day of the download). If your report says you were low 1% of the time, that means you were below 70 mg/dL 15 minutes per day on average for the duration of the download.

The key point of this case is that when you look at the general summary data for a CGM, when it says 0% low or 0% very low, it’s an average of the amount of time you’re below 70 mg/dL and 55 mg/dL respectively for the length of the download.

The Solution:

As you can see from the chart below, the time in range goal for a person with type 1 diabetes who is older with comorbidities is greater than 50% with minimal lows. Although José’s time in range was excellent, as an 85-year-old man, he does not need to be so aggressive with his goals. He also shouldn’t be experiencing any lows at all. When you’re 85 years old, 65 mg/dL is dangerous. I reduced José’s basal insulin by 10%, and I asked him to check in with me in a few weeks.

The Takeaway:

The key point in this case is that José had some pretty bad lows, but a provider wouldn’t have known that from looking at the download summary.

You have to remember that these summaries average out all the data (GMI, glycemic variability, lows, etc.), and if you’re not low for a long enough period of time, the download will read 0%.

As a patient, if your download summary doesn’t reflect your blood sugar history accurately, let your provider know. If you are a provider, look for trouble spots in the entire download, and check in with your patients about any concerning hypoglycemic and hyperglycemic episodes they have experienced.