Measuring your blood sugar levels through finger sticks is always associated with pain and discomfort. Tracking your blood glucose levels with this method may require multiple tests throughout the day, which can be quite inconvenient.

Luckily, you can overcome these issues by using the FreeStyle Libre, which is a system that allows you to keep track of your blood sugar levels day and night.

However, the FreeStyle Libre isn’t without a flaw, and one of the common problems of the device is giving false low readings.

In this guide, we’ll walk you through 9 possible reasons behind FreeStyle Libre's false low readings and what you can do to avoid them. Let’s dive right in!

9 Factors Behind False Low Readings in FreeStyle Libre

Let’s start by having a closer look at some of the reasons that might cause inaccurate readings on your device.

1. Lag Time Between Blood and Interstitial Fluid Glucose

Let’s start with one of the main reasons why you might find your FreeStyle Libre giving false readings.

As previously established, FreeStyle Libre gives you a reading of interstitial glucose rather than blood glucose.

Since it takes some time before blood glucose enters the cell and reaches the interstitial fluid, your FreeStyle Libre might end up giving you a relatively low reading when compared to a traditional blood test.

This is commonly known as “lag time”, which is a natural part of the glucose pathway in the body, and it becomes more prominent in situations when blood sugar levels are increasing rapidly, such as:

  • Directly after meals
  • After intense physical activity and depleting carbohydrates reserves.

This is because it takes time for sugar to diffuse into the cells and catch up with blood sugar levels.

2. You’re Not Placing It on the Right Spot

The Freestyle Libre sensor is a reliable tool for tracking your blood sugar level, but it only works if you place it correctly. 

The ideal spot to apply the FreeStyle Libre sensor is at the back of one of your upper arms. 

This area of skin is perfect because it stays flat while it doesn’t bend, fold, or come in contact with surfaces while performing daily activities.

As a result, if you have any scars, moles, stretch marks, lumps, or other skin irregularities in these areas, you should avoid putting the sensor there.

3. The Sensor Isn’t Attached to the Skin

The Freestyle Libre sensors need to be firmly attached to the skin in order to maintain accurate measurements. 

If the sensor comes loose, it’ll give a false low result because it won’t be able to sense enough glucose in the interstitial fluid.

The sensor comes with an adhesive to ensure a firm attachment to the skin. Yet, applying the adhesive to oily or wet skin can compromise its strength.

For that reason, always make sure that you wash the application site with soap, use alcohol swabs, and let it air dry before application to remove any residue that can prevent the sensor from sticking.

4. Interference with Other Medications, Food, or Supplements

Another common reason for inadequate readings, whether false negative or positive, is drug interference.

Although FreeStyle Libre doesn’t operate systemically, some medications and dietary supplements might end up affecting its performance.

For instance, if you’re using exfoliating skin products that contain salicylic acid on the application site, you might notice false low sensor readings. 

This effect is proportional to the amount of active substance in contact with the sensor and isn’t only exclusive to topical salicylic acid, but to oral form as well (aspirin), according to the manufacturer .

Keep in mind that ascorbic acid (vitamin C) can cause a false positive reading for the same reason. 

Some users also claim that oral medications that contain paracetamol (acetaminophen) or ibuprofen also impacted the sensor’s readings, although this is more common among FreeStyle Libre competitors like Dexcom.

5. You’re Dehydrated

Glucose molecules diffuse into the cells using a natural phenomenon called “osmosis”, which depends greatly on the presence of water both inside and outside the barrier (the cell membrane).

In addition to having many negative health consequences, dehydration can prevent glucose from diffusing into cells, causing a false negative reading on your FreeStyle Libre.

6. Environmental Conditions

Similar to other glucose measuring devices, subjecting your FreeStyle Libe to extreme temperatures can have a detrimental effect on its performance and accuracy.

Ideally, while using the Freestyle Libre, you should avoid exposing it to direct sunlight and extreme temperatures and humidity, such as in saunas.

In fact, FreeStyle Libre recommends storing the sensor in temperatures between 39 to 77 °F (3.8 to 25 °C) and non-condensing humidity of 10 to 90% for optimal results.

7. The Sensor Is Nearing the End of Its Lifespan

According to FreeStyle Libre, each sensor has a lifespan of around 14 days. However, many users noticed that the sensor’s accuracy starts to decline after around 10 to 12 days of use. 

If your FreeStyle Libre is approaching the end of its lifespan and it starts to give false negative results, you simply need to swap it for a new one.

8. Other Inaccuracy Factors

Although FreeStyle Libre is one of the most stable and reliable CGM systems out there, its reliability isn’t established for everyone.

According to FreeStyle Libre’s Safety Information Page, FreeStyle Libre is not approved for individuals under 18 years old, so if you’re under 18, it might give inaccurate results. This also applies to other groups, such as:

  • Pregnant women
  • Critically ill patients
  • Patients on dialysis, pacemakers, and other devices that impact blood flow directly or indirectly.

9. Malfunctioning, Damaged, or Expired Sensor

If all else fails or doesn’t apply to your case, there’s a chance that you have a faulty sensor. This is generally rare, as it’s usually due to problems in the manufacturing, transportation, and/or storage process, but it can still happen.

In that case, your only option is to simply opt for a new sensor and contact FreeStyle Libre Support to seek further help if necessary.

How Does Freestyle Libre Work?

In order to troubleshoot the FreeStyle Libre, you need to understand how it works. FreeStyle Libre is an FDA-approved continuous glucose monitoring (CGM) system that allows diabetic patients, whether type I or type II, to keep track of their glucose levels.

The main advantage of the device is that it eliminates the need for traditional finger sticking to test blood sugar levels. 

The device is made of a sensor and reader embedded under the skin (subcutaneous) to detect glucose and its levels in the interstitial fluid.

The system automatically measures the glucose level throughout the day and stores the reading in 15-minute intervals through the device that you stick to your skin. 

You can then keep up with the data through the FreeStyle Libre app for easy diabetes management.

Frequently Asked Questions

What is the difference between sensor and blood glucose readings?

If you perform a finger stick blood glucose test and compare its results to the readings of the FreeStyle Libre, you’re most likely going to find different results between the two.
This is because the two systems use different methods to test for blood glucose levels. Traditional testing obtains its results directly from the blood, while sensor glucose readings rely on the glucose level in the interstitial fluid that surrounds the cells.

Is interstitial glucose as reliable as blood glucose?

Interstitial glucose is proportional to blood glucose, as the glucose first enters the blood before it reaches the cells and remains in the interstitial fluid.

As such, the results of the two tests will usually be related but not exactly the same, as it takes some time for the glucose to complete the cycle and move to the cells.

For that reason, blood glucose meters are more reliable than sensor glucose readings, while the latter is only more convenient and spares you the continuous lancing.

Do I need to calibrate the freestyle libre before use?

Some continuous glucose measuring systems may require a calibration process before application to adjust the readings of the sensor.

However, this isn’t the issue with FreeStyle Libre because it comes precalibrated during manufacturing with no option for user calibration.

You only need to make sure that you’ve applied it as directed to avoid any issues moving forward.

Conclusion

There you have it! 9 common reasons why your FreeStyle Libre is giving you false readings. As you can see, several factors can play a role in inaccurate sensor readings, so you must ensure you use the system as directed.

Remember to use the Libre on flat, clean, dry skin, preferably a site that doesn’t fold or wrinkle while carrying out daily activities, such as the back of your upper arm. 

If the Libre is giving inaccurate results, consider changing it for a new one. If the problem persists, consider contacting FreeStyle Libre’s customer support by phone or online.

Sources

To ensure that we give you correct, accurate, and relevant information, all articles on Diabetic & Me are backed by verified information from academic research papers, well-known organizations, research institutions, and medical associations.

  1. Abbott Freestyle Libre Safety Information
  2. National Library of Medicine Freestyle Libre Glucose Monitoring System
  3. Access Science Osmosis

About the Author

Ely Fornoville

Hi, I'm Ely Fornoville, and I am the founder of Diabetic Me. Being a type 1 diabetic since 1996, I developed a passion to help people learn more about diabetes. I write about diabetes and share stories from other diabetics around the world. I currently use a Medtronic Guardian 4 CGM and a MiniMed 780G insulin pump with Humalog insulin.

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