Diabetes Basics Part 3: How is Glucose Measured?

Diabetes is diagnosed when the level in the blood is persistently higher than the thresholds described in the last blog. So how is the glucose level measured?

First, the glucose level at a point in time can be measured in a standard blood sample, in which the amount of glucose is detected by another chemical (the enzyme glucose oxidase) that binds to it. Second, this enzyme can be used to measure the level in a drop of blood from a finger prick. 

Third, the glucose oxidase enzyme can be bound to a tiny metal or plastic needle that is inserted just under the skin, and that detects the level of glucose repeatedly throughout the day (instead of just at one point in time). This is what happens with continuous glucose monitoring (CGM) technologies. Every time the enzyme detects glucose it activates an electrical signal that indicates the level – the higher the level, the stronger the signal. Note that CGM detects glucose in the fluid under the skin that comes from the blood, but not in the blood. Therefore, the glucose needs to leave the blood vessel before it can be detected, and this can take time. That is why people who wear a CGM notice that levels take about 15 to 30 minutes to rise after eating or drinking. 

Finally, one other way is routinely used. The test is called the hemoglobin A1c level. It does not directly measure glucose. Instead, it measures a type of hemoglobin (which is a protein in red blood cells that transports oxygen from the lungs to the body) called hemoglobin A1c. This protein – also called the A1c – binds to glucose levels and tightly sticks to it. So people with higher average glucose levels in their blood have higher A1c levels. Because A1c levels fluctuate a lot slower than the glucose levels, it is a good test of the average glucose levels over a 2–3-month window. Importantly, the A1c level is not the same thing as the glucose level and should be thought of as an indicator or index that is linked to it, in the same way that body weight is an indicator of the amount of muscle or fat. 

In summary, glucose can be measured in a blood tube, a drop of blood, or under the skin with CGM. The A1c can also provide an indicator of the average amount of glucose in the prior 2-3 months. These tests are used to guide and assess glucose management in people with diabetes. 

About the author

Hertzel Gerstein

Hertzel is an endocrinologist and professor at McMaster University who is in high demand as a speaker, advocate, and educator on diabetes-related topics. His research focuses on using large, international randomized trials to identify and test new ways of preventing type 2 diabetes, reducing serious health outcomes like strokes and death, and achieving type 2 diabetes remissions.