Continuous glucose monitors (CGMs) have transformed diabetes care. Devices such as the Abbott FreeStyle Libre 3 allow people with diabetes to check their glucose levels continuously without the frequent finger sticks that were once routine. The small sensor, worn on the arm, sends readings to a handheld reader or smartphone every few minutes. For many patients, the technology has made diabetes management easier and more convenient.

But convenience should not be confused with perfection.

Recent news reports and patient experiences have raised concerns about inaccurate or misleading readings from some sensors. In rare but serious situations, incorrect readings could lead patients to take the wrong action, such as administering insulin when it is not needed, or failing to treat dangerously high blood sugar. These errors can potentially lead to serious complications and have led to deaths.

Many people with Type 2 diabetes, including this writer, rely heavily on these numbers to guide daily decisions: what to eat, when to exercise, and whether medication or insulin is required. Because of that reliance, it is important for users to understand exactly what these devices are measuring.

A CGM such as the Libre 3 does not measure blood glucose directly. Instead, it measures glucose levels in the interstitial fluid, the fluid that surrounds the body’s cells. While this value usually tracks closely with blood glucose, it can sometimes lag behind actual blood levels, particularly when glucose levels are changing rapidly, such as after meals, exercise, or insulin administration.

This difference can occasionally produce readings that are significantly off from the true blood glucose value.

For example, there have been cases where a sensor reading appeared normal—around 96 mg/dL, while a traditional finger-stick test showed the blood glucose level was actually 170 mg/dL. A difference that large should not occur under normal circumstances and could easily mislead a patient making treatment decisions.

Fortunately, most experienced diabetics know that numbers are not the only signal the body provides. People living with diabetes often learn to recognize the physical sensations associated with high or low blood sugar, such as dizziness, sweating, fatigue, confusion or unusual thirst. When the body’s signals do not match what the sensor is reporting, that is a warning sign.

In those situations, the safest course of action is simple: do a finger-stick test.

Traditional glucose meters that use a drop of blood remain the gold standard for accuracy. Even patients who use CGMs regularly should keep a reliable finger-stick meter available as a backup. It is also wise to periodically compare readings between the sensor and a finger-stick test to make sure the system is functioning properly.

Some patients even keep a second meter from another manufacturer so that results can be cross-checked when something seems wrong.

If the sensor continually gives you readings that you know are incorrect, change the sensor. Abbott will replace it at no charge. If your finger stick readings do not correspond to, or are close to the readings on your back-up meter, Abbott will replace the reader free of charge. 

Continuous glucose monitoring technology is valuable and has helped millions manage diabetes more effectively. But despite marketing claims that finger sticks are “a thing of the past,” ask any diabetic user: they are not.

For now, the best advice for CGM users is straightforward: use the technology—but trust your body more. If the reading doesn’t make sense, or if you simply do not feel right, confirm it the old-fashioned way.

Sometimes, a simple finger stick can make all the difference.