Written by Kara Collier
Traditional glucose laboratory values such as fasting glucose, hemoglobin a1c, and oral glucose tolerance tests only provide a snapshot of your health and often miss important trends.
A CGM provides value beyond traditional measures by looking at the larger picture, including daily trends and fluctuations.
Laboratory value ranges are based on averages from the general population. A 'normal' result as defined by a laboratory does not mean that the value is optimal or even preferred for healthy individuals.
Optimal post-meal glucose is less than 140 mg/dL 2-3 hours after a meal.
Despite good intentions, there are two fundamental flaws with traditional lab testing. First, most tests only show a static snapshot of your health and may miss the bigger picture. Second, the classification of a lab result as “normal” is based on a general reference range, not necessarily with health optimization in mind. A reference range is defined by sampling a large population of “normal” individuals, sometimes leading to controversial results. More importantly, the same reference ranges are used, independent of your health goals.
Traditional glucose measurements:
To assess glucose and metabolic status in a traditional setting, you will likely receive a fasting blood glucose test and a test of hemoglobin A1c. Occasionally you may also receive an oral glucose tolerance test (OGTT) or random glucose draw.
Fasting Blood Glucose Level:
This will be part of a larger blood panel taken when you show up for your annual physical. This test is exactly what it sounds like - it measures your baseline glucose level when you are in the fasted state (have not eaten for at least 8 hours).
According to the American Diabetes Association (ADA), normal is less than 100 mg/dL, prediabetes is between 100-125 mg/dL and anything above 126 mg/dL is considered Type 2 diabetes. (Remember, these are blood glucose levels measured in a lab setting. Your CGM is not accurate enough to make a pre-diabetes diagnosis, so please don't worry if your fasting levels are slightly higher than expected.)
Hemoglobin A1c (or simply referred to as A1c)
Your A1c measures the average amount of sugar that is bound to an individual red blood cell at any given time. Because the average lifespan of a red blood cell is 3 months, an a1c tells you on average how much glucose has been in your bloodstream for the past 3 months.
According to ADA, normal is less than 5.7%, prediabetes is between 5.8-6.4%, and anything above 6.5% is diabetic
Oral Glucose Tolerance Test (OGTT)
During this test, your glucose levels are monitored for 2 hours. You begin the test in a fasted state and then consume 75 grams of pure glucose (similar to a 22 oz can of coca-cola). Then you sit and wait to see what happens to your glucose levels.
At the two hour mark, if your glucose is below 140 mg/dL this is considered normal, between 140-199 mg/dL is pre-diabetes and anything above 200 mg/dL is considered diabetes.
This may occasionally be measured at random, and not when you have been fasting.
According to ADA, anything above 200 mg/dL is a diabetes diagnosis.
Now, let’s discuss some flaws with the existing tests. A fasting blood glucose level is going to be the primary test performed during the first half of your life unless you specifically ask for additional labs or you have a family history of diabetes. Unfortunately, knowing your fasting glucose level is not enough, since we spend the majority of our day in a fed or semi-fed state. Seeing a one-time snapshot during a fasted state fails to capture the dynamic nature of your metabolism and how it reacts to the variety of foods you eat day-to-day.
An A1c gives you more information than a fasted glucose draw, but it is still imperfect. This test measures your average glucose levels over time but fails to capture any highs or lows. A person who fluctuates all day between 100-140 mg/dL may have the same average as someone who is going as low as 60 mg/dL and as high as 180 mg/dL. Additionally, there is a genetic component that may affect the results of an A1c test. It assumes that your red blood cells are recycled every 120 days on average. For many people, this isn’t the case and may throw off the results. Anemia is one example of a very common condition that affects your red blood cells, causing inaccurate a1c results.
Next up is the OGTT, which is a step above the other two tests. It is helpful because it is capturing glucose levels after ingestion of simple sugars and helps determine how your body deals with the load. However, the “meal” you are eating is essentially a large soda, which not representative of real-life for most people. Additionally, it is only a single day measurement and misses the day-to-day variability in glucose response. Factors like exercise, meals from the day before, stress, and sleep will impact your results.
So what is the optimal way to measure glucose response? Performing real-time, all day, data collection and analysis is the gold standard, and a continuous glucose monitor comes as close to this ideal as currently possible. By looking at your glucose curve under normal, day to day circumstances, over a long period of time, we can characterize your physiological response to food much better than any of the other tests.
While fasting levels are important to monitor and keep within range, postprandial (after eating) glucose levels are better correlated with overall health. Recent studies have shown that when postprandial glucose levels consistently stay above 140 mg/dL, we start to see an increased risk for many types of disease. This measurement is performed approximately 2-3 hours after a meal, and the ideal range is between 70-100 mg/dL. As a general rule, aim to have postprandial values lower than 140 mg/dL, and as close to your pre-meal levels as possible.
Understanding the difference between traditional ranges and optimal ranges will provide meaningful insight into your overall health. In a future lesson, we will take a deeper dive into interpreting your CGM trends, highs, lows, and variability.
A Note on Accuracy (Lab Values vs CGM vs Finger Sticks)
It's possible that the CGM values you see do not perfectly line up with blood glucose values from the lab or a finger stick device that you already own. The reason for this is two-fold.
First and foremost, consumer continuous glucose monitors and finger stick glucometers are not required by the FDA to be perfectly accurate. Their accuracy must fall within +/-15% of true lab values. This means that a reading of 100mg/dL could actually be equivalent to a lab reading of 85mg/dL or 115mg/dL. This is why the CGM is NOT a diagnostic device and should not be used to self-diagnose diabetes. Comparing two inaccurate devices can make this discrepancy even worse. Let's say you compare a finger stick that reads 10mg/dL lower than the true lab value to a CGM that reads 15mg/dL higher. The total difference you see will be 25mg/dL. This might be alarming if you assumed that the finger stick is always a source of truth.
Second, the CGM does not actually measure blood glucose, instead, it measures glucose levels in your interstitial fluid. This means that the CGM reading will not react as quickly as a finger stick to a change in blood glucose (it can be delayed by 10-20 minutes on average). This makes the two difficult to compare at any point when your glucose levels are fluctuating (the best time to compare CGM readings to finger stick readings is first thing in the morning before you have eaten or done any sort of exercise).
That being said, these small differences in baseline levels are not very relevant to what we are trying to achieve when personalizing our diet. Post-meal glucose variations are generally much larger and well outside of this room for error. This is why, throughout the program, we will primarily focus on how your body responds to the foods you eat, instead of obsessing about small differences in baseline numbers.