Despite labeling for nonadjunctive use, recommendations for minimum CGM accuracy performance are still at the formative stage. It might also be argued that, at least for those CGM systems that are intended to replace BGM systems, that it is reasonable to provide capillary-like glucose values. The inverse, of course, results in a negative bias (indicates under-forecast). Expression. The Clarke EG may seem outdated in light of the more modern consensus and surveillance EGs. Calculate the accuracy of the ruler. value of the estimate is from the true value of the parameter being As with MARD, application of accuracy limits like those of ISO 15197:2013 does not allow differentiation between imprecision and bias (see ISO 15197:2013 percentage values in Figure 1, plots A, B, and D). Bias analysis, on the other hand, can provide high-level estimators of both the location and dispersion of data points. Assessing sensor accuracy for non-adjunct use of continuous glucose monitoring, Improved accuracy of continuous glucose monitoring systems in pediatric patients with diabetes mellitus: results from two studies. Accuracy determines whether the measured value is close to the true value. Accuracy refers to the closeness of a measured value to a standard or known value. Bias is a distinct concept from consistency: consistent estimators converge in probability to the . Currently, additional measurement functionality is available in some episodic (BGM) systems, for example the ability to measure analytes such as ketones, by simply using a different reagent strip in the same episodic meter. Keeping Things Positive So, in average, we are really measuring what we say we are measuring. estimated. MARD results are sometimes reported as aggregated MARD, that is, the average of all individual pairs between CGM/BGM system measurement results and corresponding comparison method results within a given study. Modeled data (n = 600) for a glucose monitoring system showing constant bias and glucose-concentration-dependent variability. It may therefore be concluded that reducing measurement accuracy to a single parameter (like MARD or accuracy limits of ISO 15197:2013) is inappropriate, especially when looking to compare results between different studies. Evaluating the accuracy of continuous glucose-monitoring sensors: continuous glucose-error grid analysis illustrated by TheraSense FreeStyle Navigator data, 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, Impact of blood glucose self-monitoring errors on glucose variability, risk for hypoglycemia, and average glucose control in type 1 diabetes: an in silico study. Measurement Accuracy Criteria of ISO 15197:2013 and FDA Requirements for Integrated CGM (iCGM) Systems. Email: accuracy, blood glucose monitoring, continuous glucose monitoring, ISO 15197, MARD, performance, Standards of medical care in diabetes2018, PMA P120005/S041: FDA summary of safety and effectiveness data, Premarket approval (PMA): FreeStyle Libre flash glucose monitoring system (P160030), Self-monitoring of blood glucose (SMBG) in insulin- and non-insulin-using adults with diabetes: consensus recommendations for improving SMBG accuracy, utilization, and research. Bias often referred as 'Accuracy . Place the pipette, tips and distilled water in the test room 2 hours before starting measurements. In statistics, the bias of an estimator (or bias function) is the difference between this estimator's expected value and the true value of the parameter being estimated. ( % Result / 100). This concept is important as bad equipment, poor data processing or human error can lead to inaccurate results that are not very close to the truth. Determining Inaccuracy. Here is how I usually set up a simply formula in excel: First I calculate the absolute variance between sales and forecast. A laboratory analyzer such as a YSI (Yellow Springs Instrument) glucose analyzer is often used as the comparison method for BGM systems while for CGMs a laboratory analyzer, or in many cases a BGM (particularly in studies performed in a nonclinical setting), is used as the comparison method to substantiate accuracy. Typically, location and dispersion estimators are provided for either differences (expressed in the same unit as the measurement results) or relative differences (expressed in percent), and it is not standard practice to combine the two in bias analysis. Despite measuring glucose using similar enzyme-based reagents in the episodic strip or CGM sensor, the two types of systems measure from different compartments (blood vs ISF) and each is exposed to differing glucose concentrations that require specific algorithmic compensations and/or real-time calibrations to improve accuracy. Determine the bias = Xbar - reference value (bias = 0.00667) Determine the standard deviation (s = 0.21202) Determine the degrees of freedom (df = n-1 = 14) Determine the alpha level (confidence coefficient) you want (alpha = 0.05) Find the t value for the t distribution for df and alpha (t = 2.144787) Calculate the upper confidence limit: The quantity in which the trueness is measured is called bias (b), which is the systematic difference between the test result and the accepted reference value. BGM, being an episodic measurement process initiated by the end-user, is recommended to be performed at least 3 or 4 times per day by intensive insulin-using people with type 2 diabetes4 and actually performed on average 5 to 6 times per day by people with type 1 diabetes.5 Guidance recommends people with type 1 diabetes perform self-monitoring of blood glucose (SMBG) between 4 and 106 or even 6 and 101 times per day. . Pairs of glucose measurement results are categorized by risk scores (eg, surveillance error grid [EG]22) or risk zones (eg, consensus EG9 and continuous glucose EG23). Mean Average Deviation (MAD) MAD shows how much, on average, your forecasts have deviated from actual demand. While laboratory analyzers may provide higher analytical quality than most BGM systems under optimal conditions, they might be affected by preanalytical errors that can more easily be avoided with BGM systems, for example, inadequate sample handling leading to hemolysis or glycolysis. Accuracy and Precision - Overview 2001 BMV guidance states that at least four of every six QCs should be 15% of nominal (chromatographic) or 20% of Any residual bias should be small compared with the stated accuracy requirement. Laboratories must determine if bias exists in their methods, and estimate the severity of that bias. Test if the pipette is leaking. While BGM is the standard and more established approach for informing diabetes therapy, CGM has seen a rapid increase in users in recent years. CGM systems, however, are also influenced by the time delay between glucose changes in the interstitial fluid compartment and the compartment (ie, blood) in which comparative measurements are obtained. When a forecast, for instance, is generated by considering the last 24 observations, a forecast history totally void of bias will return a value of zero. According to ISO 15197:2013, and as described in Table 1, at least 95% of results for each of three different reagent system lots shall fall within 15 mg/dl of the comparison method result at glucose concentrations <100 mg/dl (ie, based on the difference between the paired values) and within 15% at glucose concentrations 100 mg/dl (ie, based on the relative difference between the paired values).9 In addition, at least 99% of pooled results shall fall within zones A and B of the consensus error grid.9, In some publications, additional accuracy criteria are reported with regard to these difference and/or relative difference values between the test system and comparison method results. Positive and negative fluctuations of the observed value from the accepted one can yield important information. Modeled data (n = 600) for a glucose monitoring system with (A) MARD 4.8%, 99.3% of results within accuracy limits of ISO 15197, and no bias, medium precision; (B) MARD 10.0%, 99.3% of results within accuracy limits of ISO 15197, and medium bias, high precision; (C) 10.0% MARD, 81.5% of results within accuracy limits of ISO 15197, and positive bias, low precision; (D) MARD 10.0%, 99.3% of results within accuracy limits of ISO 15197, and negative bias, high precision. National Institute for Health and Care Excellence. Thennadil SN, Rennert JL, Wenzel BJ, Hazen KH, Ruchti TL, Block MB. Most CGM systems currently in use can be categorized as either real-time CGM (rtCGM) or as intermittently scanned CGM (iscCGM; also referred to as flash glucose monitoring). Add all the absolute errors across all items, call this A; Add all the actual (or forecast) quantities across all items, call this B EG analysis differs from the other assessments described, since it focuses on the clinical relevance of differences between CGM and BGM results and the comparison method. Early postprandial hyperglycemia is common in people with diabetes, because currently available subcutaneously injected insulin analogues do not act sufficiently rapidly to effectively blunt increases in glucose concentration.46 If patients were delivering corrective insulin doses too early after meals, that is, based on these expected hyperglycemic results, insulin-induced hypoglycemia would likely follow. In addition, real-world implications of accuracy and its relevance are discussed. Calculating forecast accuracy, in relation to the supply chain, is typically measured using the Mean Absolute Percent Error (MAPE). If you're only interested in the accuracy of the new thermometer and don't care whether the temperature is lower or higher than the accepted value, you would use an absolute value in the numerator to calculate the percent accuracy: (78-81)/78 X 100 = (81-78)/78 X 100 = 3/78 X 100 = 0.0385 X 100 = 3.85 percent. Currently, patients with diabetes may choose between two major types of system for glucose measurement: blood glucose monitoring (BGM) systems measuring glucose within capillary blood and continuous glucose monitoring (CGM) systems measuring glucose within interstitial fluid. Therefore, the datasets that are obtained for each system cannot be easily reconciled for a meaningful comparison to be made. However, the International Organization for Standardization (ISO) uses "trueness" for the above definition while keeping the word "accuracy" to refer to the combination of trueness and precision. It helps to begin by sorting your data in numerical order, from lowest to highest. Calculate the accuracy and the precision and compare them with the . When the bias is a positive number, this means the prediction was over-forecasting, while a negative number suggests under forecasting. It seems inconsistent of regulatory authorities and expert groups to require minimum BGM system accuracy and, at the same time, not require minimum CGM system accuracy, especially regarding nonadjunctive use of CGM systems, independent of whether the system is an integrated CGM system or not. Freckmann G, Pleus S, Link M, Zschornack E, Klotzer HM, Haug C. Performance evaluation of three continuous glucose monitoring systems: comparison of six sensors per subject in parallel. In an experiment observing a parameter with an accepted value of VA and an observed value VO, there are two basic formulas for percent accuracy: If the observed value is smaller than the accepted one, the second expression produces a negative number. The term accuracy refers to the closeness of a measurement or estimate to the TRUE value. 8600 Rockville Pike Stefan Pleus, MSc, Institut fr Diabetes-Technologie, Forschungs- und Entwicklungsgesellschaft mbH an der Universitt Ulm, Lise-Meitner-Strae 8/2, D-89081 Ulm, Germany. Schmelzeisen-Redeker G, Schoemaker M, Kirchsteiger H, Freckmann G, Heinemann L, Del Re L. Time delay of CGM sensors: relevance, causes, and countermeasures, Journal of Diabetes Science and Technology, https://www.accessdata.fda.gov/cdrh_docs/pdf12/P120005S041b.pdf, https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfpma/pma.cfm?id=P160030, http://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/UCM380325.pdf, http://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/UCM380327.pdf, 99% of within consensus error grid zones A and B, 1% of glucose rates of change >1 mg/(dl*min) if true rate of change. multiple estimates are to each other, not how close they are to the true value What 2 formulas are used for the Accuracy and Precision Calculator? forecast bias and systematic errors occur. So as to know how accurate a value is, we find the percentage error. To solve this, it is common to divide MAE by the average demand to get a %: MAPE/MAE Confusion It seems that many practitioners use the MAE formula and call it MAPE. sharing sensitive information, make sure youre on a federal High-quality BGM systems might therefore provide comparable accuracy depending on the specific conditions. Review of Method Validation Plan. This harmonization had no impact on the requirements and procedures in ISO 15197:2013; changes were made to the foreword and an informative annex added. Precision is usually expressed in terms of the deviation of a set of results from the arithmetic mean of the set (mean and standard deviation to be discussed later in this section). Zijlstra E, Heise T, Nosek L, Heinemann L, Heckermann S. Continuous glucose monitoring: quality of hypoglycaemia detection, Clinical implications of accuracy measurements of continuous glucose sensors. and transmitted securely. The average of these multiple samples is called the expected If BGM or CGM systems are calibrated with capillary blood samples, using samples other than capillary blood for comparison measurements may introduce additional bias.31-33 Because laboratory analyzers typically require larger blood volumes than BGM systems, obtaining capillary blood samples with sufficient volume may be an issue. One of senior analysts told me I could report the recovery as following. Finally, you need to calculate the % of the error, again at the item level. If you are playing football and you always hit the right goal post instead of scoring, then you are not accurate, but you are precise! If a manually calibrated CGM system is used, additional influencing factors should be considered, especially compartmental differences and possible measurement bias between the instruments used for calibration and comparison measurements. In the literature, BGM system accuracy is assessed mainly according to ISO 15197:2013 accuracy requirements, whereas CGM accuracy has hitherto mainly been assessed by MARD, although often results from additional analyses such as bias analysis or error grid analysis are provided. Please consider supporting us by disabling your ad blocker. Save my name, email, and website in this browser for the next time I comment. When we measure the effectiveness of this process, the forecast may have both bias and inaccuracy (measured as MAPE, e.g.) This is a simple but Intuitive Method to calculate MAPE. Federal government websites often end in .gov or .mil. Calculation of Bias & variance (For Classifiers): For classifier, we are going to use the same library the only difference is the loss function. In terms of providing evidence that end users can have confidence in the product when transitioning from one lot to another in the field, ISO 15197:2013, for example, explicitly states that three different strip lots must be used in system accuracy. Whereas rtCGM systems typically provide new glucose values every 5 minutes, provided that the device used for display is in range of the wireless transmission, iscCGM systems require the user to scan the sensor to obtain current glucose values, either on a specific reader or app-enabled smartphone. 1-5) (Eq. Venous, arterialized-venous, or capillary glucose reference measurements for the accuracy assessment of a continuous glucose monitoring system, Capillary and venous blood glucose accuracy in blood glucose meters versus reference standards: the impact of study design on accuracy evaluations. The true value is that value which, under operational conditions, characterizes perfectly the variable to be measured/observed over the representative time, area and/or volume interval required, taking into account siting and exposure. Bethesda, MD 20894, Web Policies The formula in my bias binding calculator will help you figure out how much fabric you will get from yardage from fabric square and how much bias you get from the fabric you own. For CGM systems, even those intended for nonadjunctive use (eg, insulin dosing decisions), minimum accuracy requirements have, until very recently, not been defined, and recommendations on CGM accuracy assessments, although in the process of being updated, are around 10 years old.10 The US Food and Drug Administration (FDA) has recently outlined a new 510K (premarket approval) route for some CGM systems, designated as integrated CGM (iCGM) with additional special controls governing accuracy (see Table 1). Charleer S, Mathieu C, Nobels F, Gillard P. Accuracy and precision of flash glucose monitoring sensors inserted into the abdomen and upper thigh compared with the upper arm. Schmelzeisen-Redeker G, Staib A, Strasser M, Muller U, Schoemaker M. Overview of a novel sensor for continuous glucose monitoring. The physiologic time delay between the interstitial and capillary compartments differs between individuals, but has been estimated to be around 6 minutes, with a conservative estimation of approximately 10 minutes.47 In addition to this physiologic time delay, a physical (or technical) time delay is introduced via the sensor, for example by the time taken for glucose to diffuse through a biocompatible membrane and the signal de-noising algorithms.48,49 It is expected that shorter time delays reported for CGM systems are most likely achieved through the use of algorithms aimed at compensating for this time delay.50 Regarding nonadjunctive use, compensation of this time delay may be helpful: if BGM is intended to be replaced by CGM, diabetes therapy parameters may otherwise have to be adjusted to compensate for any differences between the capillary and interstitial compartments.
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