By Lindsay Hodgson, Dietetic Intern at Hailey Crean Nutrition LLC
What To Know About Blood Glucose Monitoring Accuracy
For many people with diabetes, self-monitoring blood glucose is an ongoing part of their care. The technology allows them to make important treatment and lifestyle decisions regarding medication doses, diet, and exercise. With such a reliance on self-monitoring, the technology should be working effectively for them. Here are some key things to know about the accuracy of blood glucose meters (BGM) and continuous glucose monitoring (CGM) systems.
What are the standards for blood sugar meter accuracy?
BGMs work by measuring the glucose concentration in a drop of blood from the body’s capillaries. They display the blood glucose level at that moment. Typically individuals use a fingerstick to draw a drop of blood, and then a testing strip in the BGM collects the sample to read the blood glucose level. People with diabetes perform these blood glucose checks as indicated in their care plan.
There are standards for BGM accuracy developed by the International Organization for Standardization (ISO), as well as Food and Drug Administration (FDA) standards (1,2). Both sets of standards outline the accuracy and validity requirements for BGM systems where the BGM values are compared to lab-measured blood glucose values (1,2). The ISO standards were developed in 2013 for lab-controlled settings, and state that 95% of the BGM lot results need to fall within 15 mg/dL of the comparison when blood glucose is less than 100 mg/dL, and within 15% when blood glucose exceeds 100 mg/dL (1,2). The 2016 FDA standards, set specifically for BGMs sold over the counter, are that 95% of all meter values must be within 15% of the reference value, and 99% of meter values must be within 20% of the reference (2). Adherence to these accuracy requirements is important for BGM manufacturers to ensure the quality of their products.
How do you find your meter’s accuracy?
There are a few ways to test the accuracy of a BGM (3). One can use the control solution that comes in the BGM monitoring kit or examine the product specifications that usually outline the accuracy expectations of the device (3).
Another way to verify BGM accuracy is to do a simple experiment.3 A person with diabetes can bring their BGM testing kit to their doctor’s office when they know they will have blood work drawn. They can perform a self-check using their BGM immediately before or after the technician draws the labs, and note the level. When the labs are returned, they can check the blood glucose generated by the doctor’s lab versus the one from their BGM to assess accuracy. An accurate BGM should have a difference of no more than 15%.
How is CGM accuracy determined?
CGMs, as the name implies, differ from BGMs because they work continuously. A sensor device attaches to a very small tube that sits below the skin. This apparatus collects glucose data from the fluid in between the person’s cells called interstitial fluid. While a CGM doesn’t read blood glucose directly, the interstitial fluid glucose concentration mirrors the concentration in the blood.
Currently, the most common way to determine CGM accuracy is by a math equation called the Mean Absolute Relative Difference (MARD) that compares the blood glucose data on the CGM to a reference value (1,4,5). The lower the MARD, the higher the CGM accuracy because it indicates less variability between monitoring devices (1). For instance, the Dexcom G6 CGM system has a MARD of 9% when the CGM detected glucose values have been studied versus lab reference values (6).
Whatever the preferred method of self-monitoring, it is important that the equipment is working properly. In the meantime, using best practices for blood glucose testing will also help generate more accurate results.
Freckmann G, Pleus S, Grady M, Setford S, Levy B. Measures of Accuracy for Continuous Glucose Monitoring and Blood Glucose Monitoring Devices. J Diabetes Sci Technol. 2019 May;13(3):575-583. doi: 10.1177/1932296818812062.
Freckmann G, Baumstark A, Pleus S. Do the New FDA Guidance Documents Help Improving Performance of Blood Glucose Monitoring Systems Compared With ISO 15197?. J Diabetes Sci Technol. 2017;11(6):1240-1246. doi:10.1177/1932296817713220
R Castro. Blood Glucose Monitors: What Factors Affect Accuracy?. Mayo Clinic. Accessed September 29, 2021. https://www.mayoclinic.org/diseases-conditions/diabetes/expert-answers/blood-glucose-monitors/faq-20057902
Ajjan RA, Cummings MH, Jennings P, Leelarathna L, Rayman G, Wilmot EG. Accuracy of flash glucose monitoring and continuous glucose monitoring technologies: Implications for clinical practice. Diab Vasc Dis Res. 2018 May;15(3):175-184. doi: 10.1177/1479164118756240. Epub 2018 Feb 15. PMID: 29446646.
Pardo S, Simmons DA. The Quantitative Relationship Between ISO 15197 Accuracy Criteria and Mean Absolute Relative Difference (MARD) in the Evaluation of Analytical Performance of Self-Monitoring of Blood Glucose (SMBG) Systems. J Diabetes Sci Technol. 2016 Aug 22;10(5):1182-7. doi: 10.1177/1932296816644468.
Shah VN, Laffel LM, Wadwa RP, Garg SK. Performance of a Factory-Calibrated Real-Time Continuous Glucose Monitoring System Utilizing an Automated Sensor Applicator. Diabetes Technol Ther. 2018 Jun;20(6):428-433. doi: 10.1089/dia.2018.0143.