# Toward Long-Term Implantable Glucose Biosensors for Clinical Use
## 2 Materials and Methods
### Accuracy
#### 2.2.1 Evaluation  
Accuracy of systems for blood glucose monitoring has been widely evaluated based on the [PERSON]'s error grid analysis. The [PERSON]'s error grid analysis analyzes clinical implication of blood glucose monitoring system by comparing between system-generated glucose values and the reference blood glucose values. The grid is divided into five zones that explains accuracy of glucose sensors. Zone A indicates that sensor reading deviates from the reference blood glucose by less than 20% or sensor reading shows hypoglycemia (<70 mg/dL) when the reference is also in the hypoglycemia. If the estimated values of blood glucose fall into zone A, the value means clinically correct. Zone B explains that sensor reading deviate from the reference with benign error or no treatment. Zone C means that the determined values lead to overcorrect acceptable blood glucose concentrations. Such values would result the actual blood glucose concentration into hypoglycemia or hyperglycemia. Zone D represents \"dangerous failure to detect and treat\" errors. The determined values fall into the target range whereas the actual blood glucose is not in the target range. Zone E is an \"erroneous treatment\" zone. In zone E, the determined values would lead opposite results to the reference values. In this conventional analysis, the determined blood glucose values in zones A and B are clinically acceptable (Figure 2) [22].  
A new evaluation method for CGMS is needed as research and industrial efforts on the development of CGMS increase. Since CGMS provides the time sequence of blood glucose data, error grid analysis has to reflect the temporal characteristics of blood glucose. [PERSON] et al. introduced a new concept of \"rate-error grid analysis (R-EGA)\" and a new \"point-error grid analysis (P-EGA)\" that is modified from the traditional the [PERSON]'s error grid analysis. In R-EGA, estimated rate (rate of change of sensor blood glucose (SBG)) is plotted against reference rate (rate of change of reference blood glucose (RBG)). R-EGA is divided into five zones: AR, BR, CR, DR, and ER. Clinical meaning of each error zone is similar to the conventional [PERSON]'s error grid analysis. Zone AR means that SBG rate perfectly fit to RBG rate or has errors within \(\pm\)1 mg\(\cdot\)dL\({}^{-1}\cdot\)min\({}^{-1}\). Zone AR is expanded to error within \(\pm\)2 mg\(\cdot\)dL\({}^{-1}\cdot\)min\({}^{-1}\) at extreme RBG rates of \(\pm\)4 mg\(\cdot\)dL\({}^{-1}\cdot\)min\({}^{-1}\). Zone BR indicates benign errors; they do not lead clinically-wrong decision or negative outcome. Zone CR causes overcorrection. Zone DR fails to detect significant change of RBG. In zone ER, SBG rate is opposite to RBG rate. In P-EGA, RBG is plotted against SBG. P-EGA is also divided into five zones: AP, BP, CP, DP, and EP. In this analysis, rate of blood glucose is also considered and the zones are expanded according to the rates of blood glucose. When RBG rate is within \(\pm\)1 mg\(\cdot\)dL\({}^{-1}\) min\({}^{-1}\), P-EGA zones are the same with the zones of the traditional EGA. In the case of RBG rate of \(-\)2 to \(-\)1 mg\(\cdot\)dL\({}^{-1}\) min\({}^{-1}\), the upper limits of upper AP, BP, and DP zones are added by 10 mg\(\cdot\)dL\({}^{-1}\) compared to the traditional EGA. If RBG rate is faster than \(-\)2 mg\(\cdot\)dL\({}^{-1}\cdot\)min\({}^{-1}\), the upper limits of upper AP, BP, and DP zones are expanded by 20 mg\(\cdot\)dL\({}^{-1}\) compared to the traditional EGA. With RBG rate if 1-2 mg\(\cdot\)dL\({}^{-1}\cdot\)min\({}^{-1}\), the lower limits of lower AP,  
Figure 2: [PERSON] error grid. Reproduced with permission [22]. Copyright 1988, Elsevier.  
BP, and DP zones are added by 10 mg-dL\({}^{-1}\). If RBG rate is faster than 2 mg-dL\({}^{-1}\)-min\({}^{-1}\), the lower limits of lower AP, BP, and DP zones are added by 20 mg-dL\({}^{-1}\). By combining R-EGA and P-EGA, sensor readings of CGMS can be estimated according to the error grid matrix [23].