Commentary: The technology of continuous glucose monitoring

The continuous glucose monitor used by Murphy and colleagues can be thought of as diabetology’s answer to cardiology’s 24 hour electrocardiography monitor. It is used as an investigative tool, is attached to the patient (usually for 72 hours), and requires the user to return to hospital for its removal. The data are downloaded and examined for abnormal glucose levels and trends.

The sensor is inserted through the abdominal wall, so that its glucose oxidase tip lies within subcutaneous tissue, where it measures the glucose level of interstitial fluid, not of blood. The sensor is wired to a palm sized monitor, which can be clipped to clothing. The sensor cannot be disconnected, even temporarily—for example, when showering. The monitor captures and stores the average glucose measurement every five minutes, yielding 288 readings in 24 hours.1

The patient is taught to key in the times of food, exercise, insulin, and symptoms of hypoglycaemia. The device also requires at least four self monitored blood glucose readings to be entered each day for calibration, obtained by finger prick.

The device used by Murphy and colleagues does not give “real time” readings; both patient and professional are unaware of the measurements until the end of the 3-7 day period of recording, determined by the life of the sensor. The data are then downloaded. Software ensures adequate correlation between the sensor’s interstitial fluid readings and the more reliable2 self monitored glucose readings.

Continuous glucose monitoring “fleshes out” the information obtained from conventional self monitored glucose readings, helping inform changes to management and providing a graphic educational tool. Such monitoring is often used when patients and professionals cannot reconcile self monitored readings with those of measured HbA_1c levels. Its usefulness in this situation seems limited.3 More specific indications are suspected asymptomatic nocturnal hypoglycaemia especially with “rebound” hyperglycaemia; suspected “dawn phenomenon”—when endogenous corticosteroids drive blood glucose levels up to a misleadingly normal or high level on waking; unawareness of hypoglycaemia; pregnancy, when postprandial hyperglycaemia needs tight control; and proved or suspected gastroparesis, when postprandial hyperglycaemia is unpredictable.

Continuous glucose monitors cost around £2000 (€2509; $3566) and each sensor around £50.4 The accuracy of the monitor is limited2 especially at lower glucose levels, and readings still require validation with those of the self monitored blood glucose. A lag exists between changes in blood glucose levels and those of interstitial fluid, and readings from interstitial fluid are slightly lower than those from blood. This is of particular concern at times of rapidly changing blood glucose levels and in hypoglycaemia. The National Institute for Health and Clinical Excellence has yet to appraise the technology.

Real time continuous glucose monitoring displays the most recent glucose reading and alerts the user when a preset glucose range is breached. The advantages of rapid response, particularly to hypoglycaemia, are evident but in practice there are problems with false alarms; the temptation to give extra insulin so often that it accumulates, causing hypoglycaemia; the temptation not to validate with self monitored blood glucose readings; and the psychological morbidity of being attached to a device for a defined period.