Editorial [Hot Topic:Continuous Glucose Monitoring Systems: Toys or Tools (Guest Editor: Christophe De Block)]

Diabetic complications can be reduced by achieving good metabolic control, which for insulin-requiring diabetic subjects, requires frequent self-monitoring of blood glucose (SMBG). However, SMBG represents only a snapshot of the glucose concentration and it does not provide trend information, nor does it reflect glycaemic fluctuations. In contrast, accurate and reliable devices sensing glucose on a (near)-continuous basis provide information about the direction, magnitude, duration, frequency of glycaemic fluctuations, and may facilitate specific therapeutic adjustments that need to be made to avoid hypoand hyperglycaemic excursions, thereby improving metabolic control. Particularly patients with brittle diabetes, hypoglycaemia unawareness, gastroparesis, pregnant women, or pump users, who are motivated to participate in their diabetes care and are technologically adept, may benefit from continuous glucose monitoring (CGM). In this special issue of Current Diabetes Reviews, leading researchers review the current evidence for CGM. This issue begins with a review of the indications, advantages, technical and clinical aspects of minimally invasive (needle-type glucose electrodes, and microdialysis-based systems), and non-invasive CGM sensors [1]. Next, an overview is given of fully implantable glucose monitoring systems (IGMS) (e.g. I.V. sensors for hospital use, e.g. ICU) [2]. Long-term use reducing impact of invasiveness due to implantation, less frequent calibration needs because of a more stable tissue environment around the sensor and potential easier inclusion in a closed-loop insulin delivery system are the expected benefits of IGMS. Although CGM systems may represent a breakthrough for glucose monitoring, the patient and the treating physician must be aware of the limitations of current CGM systems, that originate from physiological and technical aspects. Most of the systems monitor invasively glucose in the subcutaneous tissue. It is important to realize that there are discrepancies between blood and interstitial glucose concentration, which may affect the quality of the system calibration and thereby the accuracy of the data, as reviewed in the next article [3]. A clinically important task in diabetes management is the prevention of hypo/hyperglycemic events, as discussed in the next manuscript. By complex mathematical trend analysis, real-time CGM sensors can serve as a tool to predict impending glucose excursions for 20-30 minutes ahead, thereby providing alarm signals of hypo- and hyperglycaemic values warning the patient to take preventative actions [4]. The next two papers review criteria for evaluation of CGM accuracy and clinical performance. Clarke et al. describe the Continuous Glucose-Error Grid Analysis (CG-EGA), which reports point- and rate-accuracy for each of the relevant glycemic ranges; hypo-, eu-, and hyperglycemia [5]. Wentholt et al. discuss pros and cons of other methods to analyse accuracy, including regression analysis and correlation coefficient, relative difference measures, Bland Altman plot, ISO criteria, combined curve fitting, and epidemiological analyses. In this paper, recommendations for much needed head-to-head studies are given [6]. Next, three papers critically review the current clinical evidence of CGM sensors in type 1 diabetes and in diabetic pregnancy [7-9]. As written, only a few short-term randomized controlled trials using real-time CGM have provided us with evidence in favour of improved metabolic control, reductions in HbA1c, reductions in hypo- and hyperglycaemic episodes, and improved quality-of-life [7,8]. In a mini-review, the likely advantages of using CGM in pregnancy, aiming to improve a patient's overall glucose profile, thereby decreasing the risks of poor fetal outcomes, are described [9]. The next three papers review the importance of strict blood glucose control in critically ill patients [10-12]. Implementation of a strict glycemic control protocol, which is primordial to obtain normoglycemia, in the intensive care unit is feasible and cost-effective, but asks for careful consideration of some practical aspects, such as prevention of hypoglycaemia, training of nurses and selection of accurate blood glucose measurement tools. CGM devices and closed-loop systems are being developed and might be of great benefit to overcome these issues. Novel insights into post-stroke hyperglycaemia derived from CGM are also discussed in greater detail [12]. In a next article, Braithwaite et al. describe algorithms for intravenous insulin infusion [13]. Specific distinguishing algorithm design features and choice of parameters may be important to establish freedom from hypoglycemia, eliminate the need for administration of concentrated dextrose during euglycemia, control variability within the treatment course of individual patients, achieve adaptability to differing blood glucose targets, and minimize variability of glycemic control between treatment courses of different patients or patient populations.