In principle, it is important for all people with diabetes to measure their glucose levels. There are, however, significant differences between spot (intermittent) glucose measurement and continuous glucose monitoring. The former refers to self-monitoring of blood glucose (SMBG), which patients have practiced for about 40 years, while the latter involves the use of a continuous glucose monitoring (CGM) system. In CGM, a glucose sensor placed in the subcutaneous fat tissue continuously measures the glucose concentration in the interstitial fluid.

With the exception of the Eversense glucose sensor (Senseonics), which is implanted long-term (currently up to one year) in the subcutaneous tissue, other CGM systems are minimally invasive. This means that the user inserts the sensor into the tissue themselves, where it can measure for between 7 and 15 days before it needs to be replaced.
Therefore, spot blood glucose measurement and CGM measure glucose levels in different compartments. Physiologically, glucose values in both compartments are identical when glucose concentrations are stable. However, during rising or falling glucose levels, there is a time lag until the same value appears in both the blood and the interstitial fluid. This time lag is physiological in nature and not a measurement error.

Glucose Self-Monitoring Is Important for People with Diabetes

Traditionally, self-monitoring of blood glucose was primarily recommended for people with diabetes who were being treated with insulin. The classical method for self-monitoring was capillary blood glucose measurement (SMBG). Some modern blood glucose monitoring systems now achieve a level of accuracy comparable to that of laboratory measurements.

The importance and frequency of SMBG largely depend on the type of diabetes treatment. For individuals who manage their own insulin therapy, SMBG is indispensable. Knowing the current glucose level is essential for properly adjusting insulin doses. Furthermore, it helps prevent acute complications, especially hypoglycemia, which can occur due to incorrect insulin dosing. Both hypoglycemia and very high glucose levels leading to diabetic ketoacidosis are dangerous and must be avoided in certain situations—for example, hypoglycemia must not occur while driving.

Of course, food intake affects glucose levels. However, physical activity, inflammation, fever, stress, and various medications can also cause glucose fluctuations. Even patients who are not on insulin therapy, and therefore cannot immediately lower elevated glucose levels, gain a sense of security and awareness through SMBG in many life situations. This sense of safety and control is further enhanced by the use of CGM.