Blood glucose monitoring has gotten easier, more accurate, and less painful over the years, but it still provides only snapshots of a person’s blood glucose level. Even a person who monitors several times a day really only knows what his blood glucose level is several times a day; the rest of the time, he simply can’t be sure. However, an increasingly popular continuous glucose monitoring device called the CGMS System Gold can help fill in some of those gaps.
The CGMS System Gold is the successor of the CGMS, a Medtronic MiniMed product that was approved for marketing by the Food and Drug Administration (FDA) in 1999. The system measures the glucose level of the interstitial fluid (the fluid between the cells of the body) just under the skin every 10 seconds, then averages the readings over five-minute intervals and stores those averages in memory. (Readings and averages are not displayed in real time.) That adds up to 288 recorded glucose levels during the course of a 24-hour period. Although there may be a slight time lag between changes in blood glucose levels and changes in interstitial glucose levels, the difference is negligible. The CGMS System Gold can be used for up to three days at a time.
The main advantage of a continuous glucose monitoring system such as the CGMS System Gold is its ability to uncover patterns that are missed by conventional blood glucose monitoring. Many people find, for example, that their blood glucose level after meals is much higher than they realized because they were not monitoring when their blood glucose was high. It is also not uncommon for continuous monitoring to reveal previously undetected hypoglycemia during the night—again, a time when most people do not monitor regularly.
Sometimes, the CGMS System Gold is simply worn for three days to see what blood glucose patterns emerge. In many cases, however, a health-care provider will recommend using it to gather information on a specific aspect of diabetes care. For instance, a pump user might use continuous monitoring to assess his basal insulin rates. This is often done by skipping a meal each day to see whether blood glucose levels stay in target range without the influence of food or bolus doses. The CGMS System Gold would give a complete look at what’s happening between the two meals that are eaten. Continuous monitoring can also be used to assess a person’s insulin-to-carbohydrate ratios (the number of grams of carbohydrate one unit of insulin covers at meals). In this case, the person would eat preplanned meals with known amounts of carbohydrate and would inject or infuse preplanned amounts of insulin before meals to see whether the doses of insulin were sufficient to cover the food.
While the CGMS System Gold itself is not approved by the FDA for making insulin dosing decisions, the data it provides can help determine when conventional blood glucose monitoring should detect out-of-range blood glucose levels. Changes in insulin dosing or timing of insulin injections or boluses might then be made on the basis of such monitoring, and the CGMS System Gold could then be used again to evaluate the effects of those changes.
It should be noted that the CGMS System Gold has no direct effect on diabetes control while it is being used; its data can only be viewed and analyzed after the fact. The user must carry out his usual diabetes self-care regimen, including blood glucose monitoring, for blood glucose control while using the system. In fact, to get the most out of continuous monitoring, the user may want to keep more complete records than usual, noting timing and content of meals, timing and doses of medicines or insulin, duration and exertion level of physical activity, and any feelings of stress or unusual occurrences. With detailed records like these, the user can see not just what his glucose level was doing but also what influenced any rises or falls.
Using the system
The CGMS System Gold consists of three components, all of which are extremely compact. They include a small, needlelike sensor that is inserted just beneath the skin, usually on the abdomen; a monitor about the size of a pager that collects and temporarily stores the data from the sensor; and a cable that connects the sensor to the monitor. Insertion of the sensor, which should feel much like the insertion of a needle, is accomplished via a spring-loaded mechanism similar to an insulin pump infusion set inserter. The sensor is held in place by transparent tape.