Continuous Glucose Monitoring by Means of the Microdialysis Technique: Underlying Fundamental Aspects

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Abstract
The microdialysis technique allows extraction of substances (e.g., glucose) from fluids in the human body for quantitative measurements ex vivo. The microdialysis catheter can be inserted in many different tissues; for continuous glucose monitoring it is most often implanted in the subcutaneous fat tissue in the abdominal region. Perfusion of the thin catheter with an isotonic solution without glucose leads to a diffusion of glucose available in the interstitial fluid along the concentration gradient across the semipermeable membrane into the catheter. The glucose levels in the dialysate are measured quantitatively outside the body by means of specific sensors. A number of factors have a profound impact on the amount of glucose extracted (i.e., the glucose levels in the dialysate can be considerably lower than that in the interstitial fluid). However, as long as this proportion remains constant (independent of the prevailing glucose level), the sensor signal, which is related to the glucose level in the interstitial fluid, can be calibrated to the blood glucose level by means of a conventional blood glucose measurement. The microdialysis systems that are commercially available or in clinical development allow (after a run-in phase of some hours) continuous glucose monitoring with a good reliability over several days. Insertion of the microdialysis catheters cannot be performed by the patients themselves but requires professional help. From a technological point of view the microdialysis technique is demanding; consequently the costs of continuous glucose monitoring using this approach are considerable. However, further developments probably will allow development of cheaper patient self-care systems that can be used for longer periods of time.