Should I get one of the new continuous glucose monitors?

There are now three types of continuous glucose monitors available. They all transmit glucose results wirelessly from a small sensor placed just beneath the skin via a transmitter whose signal is received by the monitor placed anywhere from 5 to 10 feet away. Results are sent from every minute to every 5 minutes and the trend of the readings can be shown on graphs. One, the Medtronic Realtime System®, can transmit the results into the same unit that is used as the insulin pump. However, even though the same unit acts as both monitor and pump, it is still necessary for the wearer to program and set the amount of insulin to be delivered. Studies have shown that the additional information provided by the frequently delivered values and the graphed trends reduces high and low glucose events in the wearer by about half. It is important to note that all the current continuous glucose monitors are approved only for use with and alongside conventional glucose meters. This means that before acting on the information the continuous monitor provides, you should verify it by obtaining a reading with your regular monitor. Also, the two technologies provide similar but slightly different information. The conventional monitor measures blood glucose from the blood droplet resulting from the finger prick. The continuous meter does not use blood. Instead, it measures the glucose level in the fluid bathing the tissue under the skin. This is in fact derived from the blood plasma itself, but it takes several minutes to adjust to reflect the blood level. The available continuous meters need to be calibrated twice daily (Medtronic Guardian®, Dexcom 7®) with a conventional fingerstick reading, although the newer Abbott Freestyle Navigator® needs only four calibration readings in a 5-day period. Once introduced, the sensor/transmitter can be worn for 3 (Guardian), 5 (Navigator), or 7 (Dexcom-7) days before the sensor must be changed. Features of an available continuous monitor are shown in Figure 7 and Table 12.

The decision as to whether to get one of these monitors depends on the value to the wearer of knowing his or her readings on a minute-by-minute basis. For people early in the course of diabetes, on oral medications, and in good control, they are probably not necessary. For people on insulin, with a history of low and high readings, especially if they are hard to predict or explain, the information provided by continuous glucose sensing may be very valuable. However, approval for insurance coverage is often limited to specific circumstances, such as the frequent occurrence of very high or low blood sugars that cannot otherwise be prevented.

Figure 7 A continuous glucose monitor.

Source: Dexcom.

Table 12 Features of Two Available Continuous Glucose Monitors


Dexcom STS

Medtronic Guardian

Sample site

Subcutaneous ISF

Subcutaneous ISF

Sensor life (days)



User age (years)



Set-up cost (!)



Clark Error Grid A&B



Probe gauge





Disposable AAA

Initialization (minutes)



Calibration (hours)

2, 8, then q12

0.5 × 2, then q12


High and low

High and low



Can transmit to Paradigm insulin pump

Transmitter range (feet)



Transmitter waterproof



Graphs trends (hours)

1, 3, and 9

3 and 24

How accurate are glucose monitors?

Glucose monitors are quite accurate in that any given reading on the same sample of blood should only vary by a few percentage points (i.e., 5-10). In order to obtain approval from the Food and Drug Administration, such medical devices have to show that they are both precise and accurate. This means that they will obtain a similar result under similar conditions and that the result will be close to the real true value, as best as we are able to ascertain it, i.e., according to the accepted gold standard, which is glucose measured in the laboratory. Furthermore, when such a device is compared to the gold standard measurement in clinical trials, a specified number of readings have to fall very close to the readings obtained by the gold standard device. This value is more than 95%. The rules for approval of such devices have been published by the FDA and can be viewed at Although users are often surprised by the variation in results that they will see upon obtaining a reading within a very short space of time from a prior reading, it has to be acknowledged that the variation is seldom such that a different course of action would be taken. For example, insulin would be given or not given or a low blood sugar level would be treated or not treated. Actual numerical accuracy tends to be less when the glucose level is very low. However, since such a low value should always be treated, the accuracy of the actual reported number is not critical.

When a surprising glucose value that does not fit with the expected circumstances is obtained, it is always prudent to recheck that reading, on either the same or a different monitor. Also, it is advisable to check that the monitor is functioning well, by checking the low battery indicator, assuring that the correct coding has been entered and that the test strips are within date and inserted properly Sometimes, variation in technique by the user accounts, at least partially, for variation in the result obtained. This can include liquid on the puncture site, contamination with glucose, excessive squeezing of the finger, and sweating.

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