FreeStyle Navigator Continuous Glucose Monitoring System with TRUstart algorithm, a 1-hour warm-up time

Abstract

Background: The first-generation FreeStyle Navigator® Continuous Glucose Monitoring System (FreeStyle Navigator CGM) requires a 10 h warm-up period to avoid inaccurate glucose readings caused by sensor insertion trauma and wound-healing processes. The performance of a second-generation FreeStyle Navigator CGM that begins reporting glucose 1 h after sensor insertion is described.

Methods: Second-generation FreeStyle Navigator CGM performance was evaluated in an in-clinic study using the YSI Model 2300 STATPlus Glucose Analyzer as reference with 47 subjects with type 1 diabetes. The reference readings were taken at 15 min intervals, and the study was designed to emphasize the first 10 h of use.

Results: The second-generation FreeStyle Navigator CGM exhibited continuous glucose error grid analysis ratings of 93.7% "clinically accurate," 3.6% "benign errors," and 2.8% "clinical errors" and a mean and median absolute relative difference of 14.5% and 10.7%, respectively. The second-generation algorithm detected signal instability in the first 10 h of use and suspended the reporting of 14.1% of first day continuous glucose readings. The clinical accuracy of the second-generation FreeStyle Navigator CGM was similar for the first 10 h versus subsequent hours, with 92.6% and 94.2% "clinically accurate" readings, respectively.

Conclusion: The warm-up period for the second-generation FreeStyle Navigator CGM was reduced from 10 to 1 h, with minimal interruption of glucose reporting and without sacrificing clinical performance.

Figure 1

Normalized sensitivity as a function of time (n = 9125). Values of normalized sensitivity include BG tests that do not meet the screening limits for calibration. The line is a statistically significant linear regression for days 2–5 (p = .020) and represents a total increase of 1.8% from day 2 to the end of sensor life.

Figure 2

Normalized sensitivity for BG tests that meet screening limits for calibration as a function of rate (n = 8584). The line is a nonstatistically significant linear regression (p = .060) and represents a calibration bias of ±0.8% for rates of ±3.5 mg/dl/min. The sensitivity check makes it very unlikely that the outlier values in this plot would be used for calibration.

Figure 3

Normalized sensitivity for BG tests that meet screening limits for calibration as a function of glucose (n = 8584). The variability in calibration decreases with increasing glucose. The sensitivity check makes it very unlikely that the outlier values in this plot would be used for calibration.

Figure 4

Point EGA of the FreeStyle Navigator CGM clinical data. Zone A represents clinically accurate (80.3%); zone B represents benign or no treatment errors (18.0%); zone C represents overcorrection errors (0.2%); zone D represents a potentially dangerous failure to treat (1.5%); and zone E represents erroneous treatment (0.0%). A slight modification of the Point-EGA was made for the purposes of this figure. Normally the ranges of the grid are adjusted for rate. To fit all points onto one grid, the adjustments to the grid were applied to the data points instead. For example, when the rate is 1–2 mg/dl/min, appropriate zones of the grid are adjusted by -10 mg/dl; for the purposes of this figure, the CGM glucose was adjusted by +10 mg/dl instead. This modification was only used for this figure; the CG-EGA data presented were performed as the analysis was described in the original publication.

Figure 5

Point EGA of the FreeStyle CGM data during the period when readings were suppressed. The modification of the Point EGA described in Figure 4 was also applied to this figure.