What are compression lows? CGM sensor showing hypoglycemia (low blood sugar) without symptoms?

If you use a continuous glucose monitor (CGM), you’ve probably come across a low glucose alert that didn’t quite match how you were feeling, and a quick fingerstick check showed your blood sugar was actually fine. This confusing situation is often caused by what’s known as a compression low.

While compression lows aren’t dangerous themselves, they can be frustrating, especially if they wake you up with alarms. Understanding why they happen, how to recognize them, and how to prevent them can help you get more accurate readings and trust your CGM data with confidence.

In this post, we’ll cover:

• What are compression lows?

• Why do compression lows happen?

• How can you tell if it’s a compression low versus a real low blood sugar?

• What should you do if you suspect a compression low?

• How to prevent compression lows?

What are compression lows?

Compression lows refer to false hypoglycemia (low blood sugar) noted by continuous glucose monitors that occur when pressure is being applied onto the sensor (e.g. when lying on the sensor while sleeping).

Why do compression lows happen?

Compression lows happen when pressure is applied directly to your CGM sensor — for example, if you’re sleeping on it or leaning against it for a while. That pressure can restrict the flow of interstitial fluid around the sensor, causing the device to falsely read a drop in glucose.

How can you tell if it’s a compression low versus a real low blood sugar?

The most telling signs of a compression low are that it occurs while lying on the sensor, you are not experiencing any symptoms of low blood sugar, and testing with a fingerstick shows normal glucose levels.

If you are feeling symptoms of a hypo, you are likely having a real low, even if it occurs during your sleep. 

It’s worth mentioning that this is different from false hypos you may notice at the start of a new sensor. CGM sensors are more likely to read lower/show false hypos within the first 24 hours of a new sensor.

What should you do if you suspect a compression low?

If you notice a low on your FreeStyle Libre or Dexcom but you do not feel any symptoms, make it a habit to confirm those lows with a fingerprick. 

This helps to catch true lows from a safety perspective, and helps to avoid overtreating (resulting in unnecessary spikes to blood sugar) if it is a compression low. 

How to prevent compression lows?

The best strategy to prevent compression lows is proper sensor placement. Avoid applying the sensor to the side of your arms. When worn on the side of the upper arm, you are more likely to press on the sensor while lying down, especially if you are a side sleeper. 

Instead, try applying the sensor to the back of the upper arm. Other than reducing risk of compression lows, this also helps to reduce the risk of the sensor being knocked off early.