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CGM Accuracy vs. Fingerstick Glucometers

Learn about the key differences between continuous glucose monitors (CGMs) and fingerstick glucometers for testing blood sugar levels, and how accurate they are.

Side by side of fingerstick glucometer and a cgm device
Table of Contents

How Does a CGM Measure Glucose? 

A continuous glucose monitor (CGM) sensor is painlessly implanted into your arm or upper stomach and can be worn continuously for 10-14 days. A small filament on the backside of the CGM sensor sits just below the skin. The filament tracks and records the glucose levels in the interstitial fluid (fluid that fills the spaces between your cells). 

The sensor takes a glucose reading every 5-15 minutes, even while you sleep. The data is sent via Bluetooth to your smartphone where you can review trends or any noteworthy changes in blood sugar levels. 

How Does a Fingerstick Glucometer Measure Glucose? 

To measure your blood sugar with a fingerstick glucometer you will need several pieces of equipment: 

  • Lancet (a small needle)
  • Fresh glucose test strips 
  • Calibrated glucometer 

The blood glucose data can be stored within your glucometer or sent to your smartphone, which you can interpret by yourself or under the guidance of your healthcare provider. 

<p class="pro-tip"><strong>Learn more about </strong><a href="/blog/what-is-glucose">glucose levels and your health</a></p>

Do CGMs and Glucometers Show Differences in Reported Values?

CGMs and glucometers both detect enzymes to measure the blood glucose level. The core difference between these two devices is that a CGM takes a reading from interstitial fluid, while a fingerstick test uses a blood sample. 

Although both are FDA-approved methods to test blood sugar levels, you can anticipate different readings from a CGM sensor and fingerstick glucometer device. Interstitial glucose levels lag behind blood glucose levels with a median lag time of 9 minutes14. Keep in mind differences between CGM and fingerstick readings will be most noticeable when glucose levels are changing (as opposed to stable).

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Average Margin of Error for CGMs vs. Glucometers

Every blood glucose testing device will carry a different margin of error (referred to as MARD: the mean absolute relative difference). 

CGM

One comprehensive article published in European Endocrinology suggests that the MARD range across CGM devices on the market in 2018 is 9-15%11. For a reading to be considered 100% accurate, the MARD would need to fall between 3.3-5.3%11

CGM technology continues to improve year by year, and future devices will offer even greater accuracy. 

Fingerstick testing

Per FDA guidelines, a fingerstick blood testing device is expected to deliver readings with 95% accuracy2. This 2017 study published in the Journal of Diabetes Science and Technology reviewed several glucometers and observed a range of MARD from 5.6-20%3

Why is there such a large range? Several factors can detract from the accuracy of fingerstick testing9

  • Using outdated or expired test strips
  • Test strips can be damaged if exposed to light or air for an extended period 
  • Glucose meters need to be recalibrated before using new strips 
  • Missing a step in the manufacturer's instructions 
  • Certain substances on the strips or on your hands may interfere with enzyme functioning2 

Similar to CGM devices, there is great variability between individual fingerstick glucometer devices1. To truly get accurate results, the gold standard is to use a CGM device in conjunction with a blood glucometer

Why Use CGM 

No finger poking! This provides major relief for people who have a fear of needles and would prefer not to draw their own blood. 

The continuous data collection can provide greater insight into your blood sugar levels around the clock. You may discover surprising trends or changes in your blood glucose during periods when you otherwise could not check, such as overnight. 

CGM devices offer additional support for people with diabetes, who rely on frequent blood glucose testing to make sure their numbers are within range. 

Non-diabetics may also benefit from continuous glucose monitoring data to gain a deeper understanding of how their blood sugar levels may be affecting weight loss efforts, sleep patterns, and athletic performance. More research in this area is required. 

Woman smiling and wearing a CGM on her right arm

Why Use Fingerstick Testing 

The fingerstick method is appropriate for people who are trying to better understand their blood glucose range at a specific point in the day. If you only want to target one meal of the day or only focus on your glucose level in the mornings, a fingerstick test is all you need. 

There is a known lag in the blood sugar reading of a CGM device14 which does not exist in fingerstick testing. This is because a CGM sensor is measuring glucose levels in the interstitial fluid rather than blood. This lag may concern people with diabetes, or people taking insulin, who need to know their actual blood glucose readings in the moment. 

Fingerstick glucometers continue to be the more affordable option compared to CGM devices because they are covered by most US insurance plans4. For many people, this will be the deciding factor in which type of blood glucose testing device they use. As CGMs become more popular, insurance plans may change.  

Man taking blood glucose reading with a fingerstick glucometer

Tips for interpreting and making the most out of your CGM data 

Blood glucose trends before and after meals can be some of the most valuable data collected through your CGM device. Observing blood sugar patterns after eating will provide you with greater insight into how your body is metabolizing carbohydrates, and how any changes to your diet may affect your scores. 

Regular physical activity also influences blood glucose. The more active you are, the more efficiently your cells turn glucose into energy12. A workout can even lower your blood sugar levels for up to 24 hours after13.

Analyzing your data, including any spikes in glucose, may prompt you to change up your exercise timing and even your daily routine. Visible evidence of how movement can improve your blood glucose is a powerful tool and motivator to help you make lifestyle changes.  

William Dixon, MD, shared his greatest “aha” moments on The Body Signals Podcast after using a CGM. It brought greater awareness to the negative impact shift work was having on his sleeping habits, nutrition choices, and sugar metabolism. He thought he was already living a very healthy life, but access to insightful data motivated him to make beneficial changes where he could. 

Takeaway 

Tracking your glucose levels can help you make different choices to promote more stable blood sugar levels. Staying away from foods that cause glucose spikes is recommended, but without the data collected by a CGM and analyzed with an AI-driven app, it is very difficult to understand how your diet and lifestyle are directly affecting your glucose levels. Investing in personalized health data enables you to better invest your time and energy into meaningful changes that can impact your future wellbeing.

<p class="pro-tip"><strong>Learn more about </strong><a href="https://portal.signos.com/plan">getting started with Signos</a></p>

FAQs

What Is CGM? 

CGM is an abbreviation for continuous glucose monitor(ing). The first CGM device was approved for use in the United States in 1999 by the FDA6. The first continuous glucose monitor recorded three full days of data.

Technology used for continuous blood glucose monitoring has continued to evolve. Current options on the market are the most compact and sophisticated models yet, and can track blood glucose for almost five times as long as the original device.  

<p class="pro-tip"><strong>Learn more about </strong><a href="/blog/what-is-a-continuous-glucose-monitor">how CGMs work</a></p>

Who Uses a CGM? 

People diagnosed with both type 1 and type 2 diabetes are the number one users of CGM sensors. The abundance of blood glucose data helps them build a lifestyle plan and adjust medications as needed to meet target blood sugar levels.

People without diabetes may also benefit from wearing a CGM device, especially those with a high risk of developing diabetes in the future. Continuous glucose monitors are also used by non-diabetics to help control blood sugar levels in an effort to avoid weight gain associated with glucose spikes. Currently, there are no peer-reviewed studies to show that a CGM device directly contributes to weight loss. 

Signos is currently studying the effectiveness of wearing a CGM to help with weight loss. Signos has designed a clinical trial with 20,000 participants which is estimated to finish in 2026. 

<p class="pro-tip"><strong>Learn more about </strong><a href="/science">the science of using CGMs for weight loss</a></p>

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References

  1. Diabetes Technology Society. (2018). Diabetes Technology Society. Retrieved May 2022, from https://www.diabetestechnology.org/surveillance.shtml
  2. Diabetes Technology: Standards of Medical Care in Diabetes—2019. (2018). Diabetes Care, 42(Supplement_1), S71–S80. https://doi.org/10.2337/dc19-s007 
  3. Ekhlaspour, L., Mondesir, D., Lautsch, N., Balliro, C., Hillard, M., Magyar, K., Radocchia, L. G., Esmaeili, A., Sinha, M., & Russell, S. J. (2016). Comparative Accuracy of 17 Point-of-Care Glucose Meters. Journal of Diabetes Science and Technology, 11(3), 558–566. https://doi.org/10.1177/1932296816672237 
  4. Heinemann, L., & Stuhr, A. (2018). Self-measurement of Blood Glucose and Continuous Glucose Monitoring – Is There Only One Future? European Endocrinology, 14(2), 24. https://doi.org/10.17925/ee.2018.14.2.24 
  5. Howard, R., Guo, J., & Hall, K. D. (2020). Imprecision nutrition? Different simultaneous continuous glucose monitors provide discordant meal rankings for incremental postprandial glucose in subjects without diabetes. The American Journal of Clinical Nutrition, 112(4), 1114–1119. https://doi.org/10.1093/ajcn/nqaa198 
  6. Irl B. Hirsch, MD. (2012). History of Glucose Monitoring. University of Washington. https://professional.diabetes.org/sites/professional.diabetes.org/files/media/db201811.pdf 
  7. Medtronic Diabetes. (2014a, May 14). Why Sensor Glucose does not equal Blood Glucose. Retrieved May 2022, from https://www.medtronicdiabetes.com/customer-support/sensors-and-transmitters-support/why-sensor-glucose-does-not-equal-blood-glucose 
  8. Medtronic Diabetes. (2014b, May 14). Why Sensor Glucose does not equal Blood Glucose. Retrieved May 2022, from https://www.medtronicdiabetes.com/customer-support/sensors-and-transmitters-support/why-sensor-glucose-does-not-equal-blood-glucose 
  9. Monitoring Your Blood. (n.d.). Diabetes Education Online. Retrieved May 2022, from https://dtc.ucsf.edu/types-of-diabetes/type1/treatment-of-type-1-diabetes/monitoring-diabetes/monitoring-your-blood/ 
  10. Staal, O., Hansen, H., Christiansen, S., Fougner, A., Carlsen, S., & Stavdahl, Y. (2018). Differences Between Flash Glucose Monitor and Fingerprick Measurements. Biosensors, 8(4), 93. https://doi.org/10.3390/bios8040093 
  11. Heinemann, L., Stuhr, A., Brown, A., Freckmann, G., Breton, M. D., Russell, S., & Heinemann, L. (2018). Self-measurement of Blood Glucose and Continuous Glucose Monitoring - Is There Only One Future?. European endocrinology, 14(2), 24–29. https://doi.org/10.17925/EE.2018.14.2.24
  12. Cell Press. (2017, March 7). How exercise -- interval training in particular -- helps your mitochondria stave off old age. ScienceDaily. Retrieved May 20, 2022 from www.sciencedaily.com/releases/2017/03/170307155214.htm
  13. American Diabetes Association. (n.d.) Blood Sugar and Exercise. Retrieved May 20, 2022 from https://www.diabetes.org/healthy-living/fitness/getting-started-safely/blood-glucose-and-exercise
  14. Schmelzeisen-Redeker, G., Schoemaker, M., Kirchsteiger, H., Freckmann, G., Heinemann, L., & Del Re, L. (2015). Time Delay of CGM Sensors: Relevance, Causes, and Countermeasures. Journal of diabetes science and technology, 9(5), 1006–1015. https://doi.org/10.1177/1932296815590154

About the Author

Julia Zakrzewski is a Registered Dietitian and nutrition writer. She has a background in primary care, clinical nutrition, and nutrition education. She has been practicing dietetics for four years.
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Please note: The Signos team is committed to sharing insightful and actionable health articles that are backed by scientific research, supported by expert reviews, and vetted by experienced health editors. The Signos blog is not intended to diagnose, treat, cure or prevent any disease. If you have or suspect you have a medical problem, promptly contact your professional healthcare provider. Read more about our editorial process and content philosophy here.

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