Adjunctive and non-adjunctive CGM sensors

An adjunctive sensor is a continuous glucose monitoring (CGM) sensor that works as an additional aid, not as a replacement for the glucometer. Its reading shows you the estimated glucose value and the direction in which it is moving, but before a treatment decision — such as the insulin dose or treating a hypoglycemia — you must rely on the value measured with the glucometer [1]. The very word “adjunctive” means “helping”: the sensor complements the glucometer, without taking its place.

This requirement comes from the way older sensors were built, whose real accuracy was lower, especially at very low or very high values, exactly where decisions matter most [2]. That is why the manufacturer required confirmation by a finger prick for any important action, and the sensor remained useful mainly for tracking the glucose trend between measurements.

A non-adjunctive sensor, also called a “therapeutic” sensor, is approved to be used directly for treatment decisions, without needing to confirm each time with the glucometer. You can rely on the value displayed by the sensor to adjust your insulin dose or to correct a hyperglycemia or a hypoglycemia, and this approval is stated explicitly in the package leaflet [1].

A sensor receives this approval because its accuracy has been properly evaluated and considered sufficient by the regulatory agencies. Any sensor may seem very precise in the tests carried out by the manufacturer, but things change when standardized, academic evidence is required, which is why only some sensors obtain approval as a non-adjunctive device [3] [4].

It means that a regulatory agency has examined the sensor and confirmed that its values are precise enough for you to base your insulin dosing and other decisions on them, without confirmation by a finger prick. In practice, this approval is the same thing as the terms “non-adjunctive” or “therapeutic”: your treatment decision can rely directly on what the sensor displays [1].

To receive this approval, the sensor must demonstrate high accuracy across the entire glucose range, especially at low and high values, where the most important decisions are made [4]. There is also a special category, the integrated sensor (iCGM), which meets even stricter accuracy standards [2].

Yes, you can rely on the value of a non-adjunctive sensor to calculate your mealtime insulin dose, to make a correction for a high glucose, or to treat a hypoglycemia. This is precisely the purpose for which this type of sensor was created: to spare you the repeated finger pricks and to give you information of a medically certified quality [5].

Even so, the replacement is not complete. Even when you use a non-adjunctive sensor it is good to always have a glucometer at hand, because a few situations remain in which confirmation is necessary. The sensor greatly reduces the number of finger pricks, but does not bring it all the way to zero [1].

Even if you use a non-adjunctive sensor, there are situations in which it is good to do a glucose test with the glucometer. The most common is when your symptoms do not match the value displayed by the sensor — for example you feel a hypoglycemia, but the sensor shows a normal value. Confirm with the glucometer also after you have treated a hypoglycemia, until you return to normal, and check before critical activities, such as driving [1].

There are also a few technical situations: during the warm-up period of a new sensor, when the transmission is interrupted, or when an error message appears. Sometimes you also use the glucometer for calibration, as well as on the first day of using the sensor, when accuracy may be lower [1].

The difference is not that an adjunctive sensor would be dangerous, but that its technology — whether older or inadequately replicated — has a lower real accuracy (not the one declared by the manufacturer). That is exactly why in the package leaflet you will not find approval for making a therapeutic decision. Used correctly, that is, together with measuring glucose with the glucometer at every insulin dose, an adjunctive sensor remains a useful and reliable tool for tracking the evolution of glucose [2].

By comparison, a non-adjunctive sensor gives you more convenience and fewer finger pricks, because its accuracy has been properly validated medically as being sufficient for treatment decisions [4]. Risks arise if you rely on the value of an adjunctive sensor for treatment decisions without also doing a glucometer test each time.

The safest source is the package leaflet, that is, the sensor's instructions for use, where it is clearly stated whether it can be used for treatment decisions without confirmation with the glucometer. You can also ask your medical team or the pharmacist, and in today's practice a great many modern personal systems are non-adjunctive, so the chances that you have a therapeutic model are high [1].

There are, however, some countries where local sensor production is largely on the adjunctive system. If the sensor can be connected to an automated insulin delivery system, it is almost certainly non-adjunctive and possibly integrated; when you are not sure, confirm with your medical team before giving up the finger pricks [1].