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tissue by the patient (Fig. 2A). This classic electrochemical sensor uses a small amount of glucose oxidase enzyme to covert ISF glucose to hydrogen peroxide. Electrons generated by the chemical reaction are measured as an electric current. The sensor's electrodes are connected to a battery and transmitter that relays the averaged output signal to a patient-worn display every 1-5 min (Figs. 2B and 2C). The sensor output signal (milliamp current) is correlated to the BG concentration using two or more finger-stick SMBG and a calibrated glucose meter.
The electrodes and glucose oxidase enzyme are protected beneath a biocompatible porous membrane (Fig. 3) that is permeable only to small molecules such as oxygen and glucose. Following insertion, the pores of the membrane become fouled with plasma proteins and white blood cells, causing a dynamic change in sensitivity (decreased sensor output in response to a change in glucose concentration). Initial calibration is typically performed after a run-in period of 2-10 h following sc tissue insertion.
A second calibration is typically performed 2-6 h later, and then once every 6-12 h, in an attempt to compensate for sensor instability and drift. Recalibration should be performed when the ISF glucose concentration and BG concentration are changing slowly. A close
FIGURE 3 Classic needle-type electrochemical glucose sensor where the distal tip of the flexible wire is inserted for 3 to 7 days within the loose connective tissue under the skin. Glucose and oxygen molecules within the ISF diffuse through the outer membrane to interact with the enzyme glucose oxidase. The hydrogen peroxide that is produced travels through the inner membrane and electrolyte solution to interact with the working electrode. An increase or decrease in the number of glucose molecules reaching the enzyme correlates directly with an increase or decrease in the electric current output signal of the glucose sensor. Source: Courtesy of Brian Hipszer.
correlation between blood and ISF glucose typically occurs before meals and several hours after an insulin bolus. When properly inserted and calibrated, the needle-type CGM sensors exhibit satisfactory sensitivity, specificity, accuracy and precision over the physiological range (40-400 mg/dL). Long-term sensor function is limited owing to foiling of the enzyme and electrodes (38,39). The risk of infection necessitates the placement of a new sensor at an alternate location every three to seven days (http://www.fda.gov; 17-19,21,22,24,26).
The needle-type CGM systems developed by Medtronic Diabetes, DexCom Corporation and Abbott Diabetes have received FDA approval for glucose monitoring by adult ambulatory patient with type 1 and type 2 diabetes (http://www.medtronicdiabetes.com, http://www. dexcom.com, http://www.abbottdiabetescare.com). CGM monitors display the current ISF glucose level, the direction of glucose change (stable, rising or falling) and the rate of glucose change over time.
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