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Diabetes Management Blood Glucose Monitoring Table 2: Uses of Continuous Glucose Monitoring

Replacement for self-blood glucose monitoring Prevention of hypoglycaemia Reduction of glycated haemoglobin Reduction of glycaemic variability Clinical research studies Development of artificial pancreas

Over the past few years, studies have reported the importance of maintaining BG levels within narrow limits (80–110 mg/dl) in order to reduced morbidity in critically ill adults.14,15

levels, rather they measure interstitial glucose levels that usually lag behind BG levels, and that lag time can vary depending on the rate of BG change. Analytical accuracy as well as clinical accuracy of these new systems are not similar to SBGM accuracy, especially when the BG level is low (<70 mg/dl).22

Unfortunately, the criteria for marketing Initial studies

demonstrating this reduction relied on arterial blood gas/glucose analysers to measure BG levels. Subsequent studies using SBGM devices failed to show similar positive results,16

and it has been

suggested that SBGM results are not sufficiently accurate to use in such critical situations.9,17

Few of these studies have been performed

in critically ill paediatric populations, even though ‘critical illness hyperglycaemia’ is prevalent and associated with poor outcomes.18–20 Since hypoglycaemia has been frequently reported in both children and adults being treated with ‘tight glucose control’ insulin-dosing algorithms, SBGM systems may not be the method of choice for measuring BG levels in these hospitalised patients.9

Continuous Glucose Monitoring

CGM was developed and introduced into diabetes care during the past decade. The concept of continuous monitoring is attractive to both healthcare professionals and to patients and their families because CGM ‘fills in’ critical pieces of information that are missing from SBGM data. In addition to providing glucose readings every one to five minutes, these systems also calculate and display the rate and direction of glucose change. Thus, patients no longer have to estimate whether their BG level is rapidly or slowly rising or falling, based on recall of previous BG readings, most recent insulin, food and exercise, and current subjective symptoms associated with their previous high or low BG levels. Such critical information is displayed on the liquid crystal display (LCD) screen of a hand-held receiver. A reading of 100 mg/dl no longer sits alone as an isolated value. It is now accompanied by the information needed to make an accurate treatment decision. The analogy between a digital snapshot and a video recording is often used to explain the importance of this additional information.21

The clinical applications of CGM that have been explored to date include replacement for SBGM testing,

prevention of hypoglycaemia, reduction in HbA1c values, reduction of glycaemia variability, evaluation of the effects of new drugs or other treatments on glycaemia, and the development of a closed-loop artificial pancreas for insulin delivery (see Table 2). Much of the data recorded with CGM have been collected by the Diabetes Research in Children Network (DirectNet) study group, a National Institutes of Health (NIH)-funded consortium of diabetes centres with strong clinical research programmes.

, DexCom, Inc., San Diego, CA) have been approved by the FDA for outpatient use as adjuncts to, but not replacements for, SBGM. It is recommended that treatment decisions not be based on CGM results alone, but that BG be measured with a SBGM device before any treatment decision is taken. CGM systems do not measure BG

Accuracy of Continuous Glucose Monitoring Three CGM devices (Guardian® Freestyle Navigator® STS-7®

24 RT, Medtronic, Northridge, CA; , Abbott Diabetes Care, Alameda, CA; DexCom

The ability to record glucose continuously permits the evaluation of effects of new treatments that may not have been observable from

routine SBGM and HbA1c data alone. For instance, in a study of the drug pramlintide in adults with type 1 diabetes, there was no

difference in mean BG levels or in HbA1c levels between the controls and the treatment group. But a statistically significant reduction in


approval for these devices is identical to those for SBGM systems and do not take into account the additional clinical information presented with each glucose reading. Our research group has modified the EGA to take into account the lag time between BG and interstitial glucose based on the rate of BG change, and has added a rate error grid to quantify the clinical importance of accuracy of rate and direction of change of CGM results.21

It is important that clinical

accuracy is calculated and reported separately for each of the critical BG ranges (hypoglycaemia <70 mg/dl, euglycaemia 70–180 mg/dl, hyperglycaemia >180 mg/dl). When used with currently recommended insulin dosing algorithms, recent evidence suggests that CGM results within the euglycaemic or hyperglycaemic ranges can be associated with clinically accurate treatment decisions.23

The potential for current CGM systems to serve as hypoglycaemia alarms is limited to some extent by their level of analytical and clinical accuracy.22,24,25

When alarm thresholds are set higher than the low BG target, the sensitivity of the systems increase. In other words, setting the alarm at 80 mg/dl to detect BG levels of 70 mg/dl detects more BG <70 mg/dl than setting the alarm at 70 mg/dl.24,25

However, that

increase in sensitivity is associated with a significant increase in false-positive alarms, which is a nuisance to patients and parents and could result in terminating the use of the system. Despite this shortcoming, CGM has provided important information concerning the previously unreported significant amount of time children spend with low BG overnight.26–28

Since CGM systems calculate rate and

direction of BG change, they are able to use algorithms to predict impending hypoglycaemia.29,30

Such information has been used

successfully in ‘partial’ closed-loop systems to signal the termination of basal insulin infusion for up to 90 minutes or until glucose levels rise to euglycaemia. Partial closed-loop systems used to prevent nocturnal hypoglycaemic are currently being marketed in Europe.

Although early studies of CGM use demonstrated modest reductions in

HbA1c levels among patients unblinded to their CGM results, the most complete information in terms of this effect comes from the Juvenile Diabetes Research Foundation (JDRF), which sponsored a six-month prospective study of CGM use and its effects in children and adults.31

Adults (>25 years of age) significantly reduced their HbA1c levels while children (8–14 years of age) and adolescents (14–25 years of age) did not. Adults used CGM six or more days a week more frequently than adolescents (83 versus 30 %) and recorded more time (minutes/day) with glucose levels within the 70–180 mg/dl target range. This suggests that CGM use can be associated with a reduction in glycaemic

variability. Other investigators have shown reductions in HbA1c as well as increases in time spent within the 71–180 mg/dl target range in children as young as three years of age.32,33

These improvements in glycaemic control were also related to frequency of CGM use.

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