Finger Stick vs. CGM Accuracy
Which is more accurate? Your finger stick meter (BGM) or your continuous glucose monitor (CGM)? This is a very difficult question to answer which, unfortunately, has no clear answer.
Simply put, methods for accuracy and performance are inconsistent between the two systems. BGM is assessed according to the FDA and ISO 15197:2013 accuracy standards (see below), whereas CGM accuracy is mainly assessed by MARD (mean absolute relative difference). Comparing the two systems is quite difficult because ISO standards do not directly translate into MARD values. The datasets created by each system cannot be easily reconciled for a meaningful comparison.
Finger Stick Meter (BGM) Accuracy Standards
What Does This Mean?
Let’s say you went to get labs drawn and your blood glucose value (measured by the lab test) was 100 mg/dL. According to ISO standards, your home blood glucose meter could show as low as 85 mg/dL or as high as 115 mg/dL, and the FDA would consider any value between 85 mg/dL –115 mg/dL as accurate!
Continuous Glucose Monitoring Accuracy
CGM accuracy is mainly assessed by MARD (mean absolute relative difference).
As a reference point, the typical MARD for commercially available CGMs ranges from 10%–12%, whereas commercially available finger stick meters can achieve MARD results below 5%. CGM accuracy is not yet at the same level that is required for SMBG systems. In order to meet ISO standards, MARD would need to meet 3.3%–5.3%. Currently, there are no CGM systems on the market that reach this level of analytical performance.
Despite the differences, CGM systems provide significantly more context to overall glucose control than BGM systems.
Most CGM systems approved for use in the U.S. meet FDA standards for guiding diabetes treatment decisions.
New and Newsworthy
The US Food and Drug Administration (FDA) has announced a new route for some CGM systems, designated as “integrated CGM” (iCGM). iCGM systems will have to meet additional accuracy requirements. iCGM systems are required to transmit glucose data to digitally connected devices (i.e., insulin pumps). These new recommendations are intended to improve awareness and transparency surrounding CGM accuracy and help the scientific community compare and contrast BGM and CGM data.
iCGM standards (right hand column)
While exact comparisons are difficult, it is important to remember that all care and decision making in diabetes management are based upon trends. Rarely are decisions made from a single result.
Finger stick meters and CGM systems are both accurate.* The degree to which each meets accuracy standards is the challenging part to assess.
To be clear, accuracy does not equate to precision. This is an important concept to understand when evaluating meters. Some meters have higher accuracy whereas some have higher precision.
*Accuracy = degree to which a result of a measurement conforms to the correct value or standard
Precision = a measure of test result consistency. Of note, this is not mandated or monitored by the FDA.
Finger stick meters are regulated on accuracy.
The FDA looks at company-reported trials when it reviews new BGM meters. The FDA does not continue to test meter accuracy after this. In a 2018 study of meter accuracy, it was found that “Cleared BGMs do not always meet the level of analytical accuracy currently required for regulatory clearance.”
“Patients assume their glucose monitor is accurate because it is FDA cleared, but often that is not the case. There is substantial variation in the accuracy of widely used blood glucose monitoring systems. In a recent analysis, the program found that only 6 of the top 18 glucose meters met the accuracy standard.” Standards of Medical Care in Diabetes—2019 American Diabetes Association
Limiting Factors for Accuracy
- Counterfeit Strips
- Over exposure to oxygen
- Interfering Substances
- Size of adipocytes
- Metabolic rate of adjacent cells
- Local temperature
- Oxygen tension in the interstitial space