Evaluation of the accuracy of a microdialysis-based glucose sensor during insulin-induced hypoglycemia, its recovery, and post-hypoglycemic hyperglycemia in humans

Blood, sweat, and tears: developing clinically relevant protein biosensors for integrated body fluid analysis

Biosensors are being developed to provide rapid, quantitative, diagnostic information to clinicians in order to help guide patient treatment, without the need for centralised laboratory assays. The success of glucose monitoring is a key example of where technology innovation has met a clinical need at multiple levels – from the pathology laboratory all the way to the patient's home. However, few other biosensor devices are currently in routine use. Here we review the challenges and opportunities regarding the integration of biosensor techniques into body fluid sampling approaches, with emphasis on the point-of-care setting.

Glucose Measurement by Affinity Sensor and Pulsed Measurements of Fluidic Resistances: Proof of Principle

Affinity sensors for glucose are based on a different measuring principle than the commercially available amperometric needle type sensors: reversible affinity interaction of glucose with specific receptors is the primary recognition mechanism instead of an enzymatic glucose oxidation. A novel pulsed-flow micro-fluidic system was used to characterize first the viscosity of a sensitive liquid containing the glucose receptor Concanavalin A and dextran and in a second approach to characterize the geometry of a fluidic resistance. In the viscometric sensor, glucose of the sensitive liquid is equilibrated, while passing through a dialysis chamber, with the surrounding medium. With the membrane flow sensor, the viscosity of the liquid remains constant but the pores of the flow-resisting membrane contain a swellable hydrogel affecting the width of the pores. Two types of hydrogel were tested with the membrane flow sensor; one is highly sensitive to pH and salt concentration, the other contains receptors of phenyl boronic acids to obtain sensitivity to glucose. The viscometric affinity sensor (first approach) showed a linear response over 0 to 30 mmol/L glucose concentration range. The disturbing effect of air bubbles could be compensated for. The sensing proof of principle of the second approach could be demonstrated by its linear response to different saline concentrations; however, the glucose-sensitive membrane developed showed only a small response to glucose. Glucose monitoring based on this pulsed flow measuring principle offers interesting alternatives for the development of CGM systems with different options for the glucose sensing part.

Microdialysis-coupled enzymatic microreactor for in vivo glucose monitoring in rats

Continuous glucose monitoring (CGM) is an important aid for diabetic patients to optimize glycemic control and to prevent long-term complications. However, current CGM devices need further miniaturization and improved functional performance. We have coupled a previously described microfluidic chip with enzymatic microreactor (EMR) to a microdialysis probe and evaluated the performance of this system for monitoring subcutaneous glucose concentration in rats. Nanoliter volumes of microdialysis sample are efficiently reacted with continuously supplied glucose oxidase (GOx) solution in the EMR. The hydrogen peroxide produced is amperometrically detected at a (polypyrrole (PPy)-protected) thin-film Pt electrode. Subcutaneous glucose concentration was continuously monitored in anesthetized rats in response to intravenous injections of 20% glucose (w/v), 5 U/kg insulin, or saline as a control. In vitro evaluation showed a linear range of 2.1-20.6 mM and a sensitivity of 7.8 ± 1.0 nA/mM (n = 6). The physical lag time between microdialysis and the analytical signal was approximately 18 min. The baseline concentration of blood glucose was 10.2 ± 2.3 mM. After administering glucose to the rats, glucose levels increased by about 2 mM to 12.1 ± 2.3 mM in blood and 11.9 ± 1.5 mM in subcutaneous interstitial fluid (ISF). After insulin administration, glucose levels decreased by about 8 mM relative to baseline to 2.1 ± 0.6 mM in blood and 2.1 ± 0.9 mM in ISF. A microfluidic device with integrated chaotic mixer and EMR has been successfully combined with subcutaneous microdialysis to continuously monitor glucose in rats. This proof-of-principle demonstrates the feasibility of improved miniaturization in CGM based on microfluidics.

Italian contributions to the development of continuous glucose monitoring sensors for diabetes management

Monitoring glucose concentration in the blood is essential in the therapy of diabetes, a pathology which affects about 350 million people around the World (three million in Italy), causes more than four million deaths per year and consumes a significant portion of the budget of national health systems (10% in Italy). In the last 15 years, several sensors with different degree of invasiveness have been proposed to monitor glycemia in a quasi-continuous way (up to 1 sample/min rate) for relatively long intervals (up to 7 consecutive days). These continuous glucose monitoring (CGM) sensors have opened new scenarios to assess, off-line, the effectiveness of individual patient therapeutic plans from the retrospective analysis of glucose time-series, but have also stimulated the development of innovative on-line applications, such as hypo/hyper-glycemia alert systems and artificial pancreas closed-loop control algorithms. In this review, we illustrate some significant Italian contributions, both from industry and academia, to the growth of the CGM sensors research area. In particular, technological, algorithmic and clinical developments performed in Italy will be discussed and put in relation with the advances obtained in the field in the wider international research community.

A multiple local models approach to accuracy improvement in continuous glucose monitoring

BACKGROUND

Continuous glucose monitoring (CGM) devices estimate plasma glucose (PG) from measurements in compartments alternative to blood. The accuracy of currently available CGM is yet unsatisfactory and may depend on the implemented calibration algorithms, which do not compensate adequately for the differences of glucose dynamics between the compartments. Here we propose and validate an innovative calibration algorithm for the improvement of CGM performance.

METHODS

CGM data from GlucoDay(®) (A. Menarini, Florence, Italy) and paired reference PG have been obtained from eight subjects without diabetes during eu-, hypo-, and hyperglycemic hyperinsulinemic clamps. A calibration algorithm based on a dynamic global model (GM) of the relationship between PG and CGM in the interstitial space has been obtained. The GM is composed by independent local models (LMs) weighted and added. LMs are defined by a combination of inputs from the CGM and by a validity function, so that each LM represents to a variable extent a different metabolic condition and/or sensor-subject interaction. The inputs best suited for glucose estimation were the sensor current I and glucose estimations Ĝ, at different time instants [I(k), I(k)(-1), Ĝ(k)(-1)] (IIG). In addition to IIG, other inputs have been used to obtain the GM, achieving different configurations of the calibration algorithm.

RESULTS

Even in its simplest configuration considering only IIG, the new calibration algorithm improved the accuracy of the estimations compared with the manufacturer's estimate: mean absolute relative difference (MARD)=10.8±1.5% versus 14.7±5.4%, respectively (P=0.012, by analysis of variance). When additional exogenous signals were considered, the MARD improved further (7.8±2.6%, P<0.05).

CONCLUSIONS

The LM technique allows for the identification of intercompartmental glucose dynamics. Inclusion of these dynamics into the calibration algorithm improves the accuracy of PG estimations.

Technologies for continuous glucose monitoring: current problems and future promises

Devices for continuous glucose monitoring (CGM) are currently a major focus of research in the area of diabetes management. It is envisioned that such devices will have the ability to alert a diabetes patient (or the parent or medical care giver of a diabetes patient) of impending hypoglycemic/hyperglycemic events and thereby enable the patient to avoid extreme hypoglycemic/hyperglycemic excursions as well as minimize deviations outside the normal glucose range, thus preventing both life-threatening events and the debilitating complications associated with diabetes. It is anticipated that CGM devices will utilize constant feedback of analytical information from a glucose sensor to activate an insulin delivery pump, thereby ultimately realizing the concept of an artificial pancreas. Depending on whether the CGM device penetrates/breaks the skin and/or the sample is measured extracorporeally, these devices can be categorized as totally invasive, minimally invasive, and noninvasive. In addition, CGM devices are further classified according to the transduction mechanisms used for glucose sensing (i.e., electrochemical, optical, and piezoelectric). However, at present, most of these technologies are plagued by a variety of issues that affect their accuracy and long-term performance. This article presents a critical comparison of existing CGM technologies, highlighting critical issues of device accuracy, foreign body response, calibration, and miniaturization. An outlook on future developments with an emphasis on long-term reliability and performance is also presented.

Clinical need for continuous glucose monitoring in the hospital

Automation and standardization of the glucose measurement process have the potential to greatly improve glycemic control, clinical outcome, and safety while reducing cost. The resources required to monitor glycemia in hospitalized patients have thus far limited the implementation of intensive glucose management to patients in critical care units. Numerous available and up-and-coming technologies are targeted for the hospital patient population. Advantages and limitations of these devices are discussed herewith in.

Glucose monitoring by microdialysis: performance in a multicentre study

AIMS

The aim of this study was to assess the performance of the Continuous Research Tool (CRT) in a multicentre clinical-experimental study.

METHODS

Three patient groups totalling 28 subjects with diabetes [group A 10 Type 1 (Ulm), group B 10 Type 1 (Neuss), group C eight Type 2 (Aarhus)] participated in this trial. Two CRT microdialysis probes were inserted in parallel in the abdominal subcutaneous tissue for 120 h in each subject. In subjects in group A, glucose excursions were induced on one study day and those in group B underwent a glucose clamp (eu-, hypo- or hyperglycaemic) on one study day. CRT data were calibrated once with a retrospective calibration model based on a run-in time of 24 h and three blood glucose measurements per day.

RESULTS

All analysable experiments, covering a broad range of blood glucose values, yielded highly accurate data for the complete experimental time with a mean relative absolute difference of 12.8 +/- 6.0% and a predictive residual error sum of squares of 15.6 +/- 6.3 (mean +/- SD). Of all measurement results, 98.2% were in zones A and B of the error grid analysis. The average absolute differences were 1.14 mmol/l for Type 1 and 0.88 mmol/l for Type 2 diabetic patients. Relative absolute differences were 16.0% for Type 1 and 12.6% for Type 2 diabetic patients.

CONCLUSIONS

These results demonstrate that this microdialysis system allows reliable continuous glucose monitoring in patients with diabetes of either type.

Multicentre, randomised, controlled study of the impact of continuous sub-cutaneous glucose monitoring (GlucoDay) on glycaemic control in type 1 and type 2 diabetes patients

AIM

This randomised study was designed to investigate the impact of continuous glucose monitoring (CGM) for 48h on glycaemic control with a 3-month follow-up in patients with type 1 (T1D) or type 2 (T2D) diabetes.

METHODS

A total of 48 patients with poor glycaemic control (HbA(1c): 8-10.5%) underwent CGM for 48h using the GlucoDay((R)) system (A. Menarini Diagnostics), after which they were randomly assigned to treatment adjustments based on either their CGM profile (CGM group) or their usual self-monitoring of blood glucose (SMBG group). HbA(1c) measurement and 48-h CGM were repeated 3 months later.

RESULTS

Altogether, 34 patients with either T1D (n=9) or T2D (n=25) completed the study; seven patients chose to leave the study, and seven patients in the CGM group were excluded because their baseline CGM graphs were not interpretable. HbA(1c) levels decreased significantly in the CGM group (n=14, -0.63+/-0.27%; P=0.023), but not in the controls (n=20, -0.28+/-0.21%; P=0.30). In T2D patients, the improvement associated with CGM vs SMBG was due to HbA(1c) decreases (mean: -0.63+/-0.34%; P=0.05 vs -0.31+/-0.29%; P=0.18, respectively). However, HbA(1c) did not change significantly with CGM in T1D patients. Comparisons of CGM data at baseline and after 3 months showed no significant changes in glucose control, glucose variability or hypoglycaemia. No major adverse events related to the GlucoDay system were reported.

CONCLUSION

This is the first randomised study showing that CGM improves glycaemic control in patients with T2D.

TheClinical Research Tool: a high-performance microdialysis-based system for reliably measuring interstitial fluid glucose concentration

BACKGROUND

A novel microdialysis-based continuous glucose monitoring system, the so-called Clinical Research Tool (CRT), is presented. The CRT was designed exclusively for investigational use to offer high analytical accuracy and reliability. The CRT was built to avoid signal artifacts due to catheter clogging, flow obstruction by air bubbles, and flow variation caused by inconstant pumping. For differentiation between physiological events and system artifacts, the sensor current, counter electrode and polarization voltage, battery voltage, sensor temperature, and flow rate are recorded at a rate of 1 Hz.

METHOD

In vitro characterization with buffered glucose solutions (c(glucose) = 0 - 26 x 10(-3) mol liter(-1)) over 120 h yielded a mean absolute relative error (MARE) of 2.9 +/- 0.9% and a recorded mean flow rate of 330 +/- 48 nl/min with periodic flow rate variation amounting to 24 +/- 7%. The first 120 h in vivo testing was conducted with five type 1 diabetes subjects wearing two systems each. A mean flow rate of 350 +/- 59 nl/min and a periodic variation of 22 +/- 6% were recorded.

RESULTS

Utilizing 3 blood glucose measurements per day and a physical lag time of 1980 s, retrospective calibration of the 10 in vivo experiments yielded a MARE value of 12.4 +/- 5.7. Clarke error grid analysis resulted in 81.0%, 16.6%, 0.8%, 1.6%, and 0% in regions A, B, C, D, and E, respectively.

CONCLUSION

The CRT demonstrates exceptional reliability of system operation and very good measurement performance. The ability to differentiate between artifacts and physiological effects suggests the use of the CRT as a reference tool in clinical investigations.

Glucose sensors: a review of current and emerging technology

Glucose monitoring technology has been used in the management of diabetes for three decades. Traditional devices use enzymatic methods to measure glucose concentration and provide point sample information. More recently continuous glucose monitoring devices have become available providing more detailed data on glucose excursions. In future applications the continuous glucose sensor may become a critical component of the closed loop insulin delivery system and, as such, must be selective, rapid, predictable and acceptable for continuous patient use. Many potential sensing modalities are being pursued including optical and transdermal techniques. This review aims to summarize existing technology, the methods for assessing glucose sensing devices and provide an overview of emergent sensing modalities.

Continuous glucose monitoring and diabetes health outcomes: a critical appraisal

This paper presents a critical review of continuous glucose monitoring studies that address accuracy, utility in assessing glucose variability, detection of hypoglycemia for improved metabolic control or reduced glucose variability, impact on quality of life, reduced fear of hypoglycemia, and applicability in helping patients to overcome their reluctance to intensify insulin therapy. This analysis can be used to delineate the evidence that is still missing when considering continuous glucose monitoring as an established tool in clinical practice and for reimbursement of the system and services.

Recent progress in analytical instrumentation for glycemic control in diabetic and critically ill patients

Implementing strict glycemic control can reduce the risk of serious complications in both diabetic and critically ill patients. For this reason, many different analytical, mainly electrochemical and optical sensor approaches for glucose measurements have been developed. Self-monitoring of blood glucose (SMBG) has been recognised as being an indispensable tool for intensive diabetes therapy. Recent progress in analytical instrumentation, allowing submicroliter samples of blood, alternative site testing, reduced test time, autocalibration, and improved precision, is comprehensively described in this review. Continuous blood glucose monitoring techniques and insulin infusion strategies, developmental steps towards the realization of the dream of an artificial pancreas under closed loop control, are presented. Progress in glucose sensing and glycemic control for both patient groups is discussed by assessing recent published literature (up to 2006). The state-of-the-art and trends in analytical techniques (either episodic, intermittent or continuous, minimal-invasive, or noninvasive) detailed in this review will provide researchers, health professionals and the diabetic community with a comprehensive overview of the potential of next-generation instrumentation suited to either short- and long-term implantation or ex vivo measurement in combination with appropriate body interfaces such as microdialysis catheters.