CGM vs BGM: Understanding Glucose Differences | Beyond Type 1 (2025)

Si alguna vez te has sentido frustrado cuando el número en tu medidor continuo de glucosa (MCG) no coincide con el número en tu medidor de glucosa (azúcar) en sangre, aquí hay algunos hechos no muy conocidos que debes saber.

Fact #1: Your CGM and your BGM are measuring two different things

Here’s the deal with CGM vs. BGM glucose differences—while we often call CGM readings “blood sugar levels,” a CGM doesn’t actually measure sugar in your blood.

Instead, it checks the glucose in the interstitial fluid found in your body fat (the subcutaneous tissue). This layer has no blood, but it does contain glucose—what we casually refer to as sugar.

Now, if you use a BGM to check your blood sugar, you might see slight differences in readings, like 99 m g/dL (5.5 mmol/L), 103 mg/dL (5.7 mmol/L), or 109 mg/dL (6.1 mmol/L).

That’s normal! At-home glucose measuring devices all have an allowable range of accuracy.

The FDA requires BGMs and CGMs to be within 15 percent of the true lab value at least 95 percent of the time—and within 20 percent at least 99 percent of the time. So, you can trust these devices to be reliable.

Fact #2: Food affects your BGM before your CGM

Glucose from food reaches your blood before the interstitial fluid where your CGM sensor sits—about 15 minutes earlier.

After meals, BGM and CGM readings may differ, especially when blood sugar is rapidly rising or falling.

If you’re managing insulin or treating lows, confirm with your BGM when needed to avoid overcorrecting or undertreating.

Fact #3: CGM and BGM differences can vary in certain situations

When your blood sugar changes quickly, expect bigger differences between CGM and BGM readings until things stabilize.

Here are some factors that can influence these differences:

• After eating,
• After a large insulin dose,
• During or right after exercise.
• After medications affecting blood sugar levels,

Keep in mind: Reacting too quickly to a CGM reading can lead to a blood sugar mishap. Patience is key—especially when correcting highs!

Before taking action, consider what might be influencing your CGM versus BGM numbers.

Waiting just 15 minutes could save you from unnecessary insulin or carbs. It might be frustrating, but it’s worth it!

The bottom line

Sometimes, you might run into an inaccurate CGM sensor. If your CGM readings are consistently different from your BGM throughout the day—and none of the usual factors apply—it’s a good idea to contact the manufacturer to report the issue.

In most cases, customer service will guide you to remove the faulty sensor and insert a new one. They can usually send you a replacement sensor free of charge.

It can be frustrating when the technology you rely on doesn’t seem perfect. But understanding how CGMs and BGMs measure differently can reassure you that they’re likely working as intended.

There’s so much to celebrate about how far diabetes tech has come—and the future is looking even brighter!

This content was made possible by Dexcom, an active partner of Beyond Type 1. Beyond Type 1 maintains editorial control over its content.

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