The content below is provided by Kristie Lambert, MSN, APRN

Kristie Lambert is an Adult Nurse Practitioner who specializes in Endocrinology. She completed her Bachelor of Science in Nursing at the University of Toledo and then earned her Master’s of Science in Nursing at the University of Akron. She has a passion for diabetes and has been working in this field since 2011.

In her spare time she enjoys reading, boating and spending time with family and her dog CJ.

CGM vs. Finger Prick: Which One is Right?

Have you ever checked your continuous glucose monitor (CGM) and then compared it to your finger prick reading and found them to be different? If both of these devices are supposed to be fairly accurate, how do you know which reading to use? 

This can be very frustrating, but this is not uncommon and is actually to be expected. CGMs and finger prick glucometer readings are measuring sugar concentrations from different sources within the body. Due to this, you can check your CGM and finger prick reading at the same time and won’t get the same result. Why is this? 

Sugar first enters the bloodstream and then makes its way to the interstitial fluid (ISF). The CGM is measuring sugar concentration in the ISF and the finger prick measures sugar concentration in the blood. You will see the sugar concentration change first in the blood (finger prick) before it changes in the ISF (CGM). It takes the sensor a little bit longer to catch up to the sugar changes that have already happened in the blood. This is commonly referred to as lag time. It is the lag time that generally leads to the differences between finger prick reading and the CGMs reading. 

One factor that affects lag time is how quickly the sugar is changing. When the sugar is stable, there isn’t a lot of lag time and the readings will be similar. When there is a rapid change in sugar, there is an increase in lag time. When the sugar is increasing quickly, the CGM reading will be falsely low until it catches up. The opposite is also true, when the sugar is quickly falling, the CGM will be falsely high until it is able to catch up. 

One way to help factor in lag time is to pay attention to the rate of change displayed on the CGM. The rate of change is typically arrows that help indicate how rapidly the sugar is rising or falling. Knowing what the arrows are indicating can help with management decisions. There are several different CGM options so it is important to know how yours operates. 

While diabetic technology is advancing, making management a little less “hands on” for the individual, it is important to remember that technology is not always 100% accurate and can fail. Knowing the limitations of these devices is important. It is also important to know your body, if you feel low, but your CGM isn’t telling you your sugar is low, trust your instinct, you don’t want to delay treatment of a blood sugar issue.