Your questions are thoughtful and thought-provoking.
There certainly is a wide range of normal data out there. Remember that “normal” ranges are based on population statistics. We extrapolate those statistics to individuals, with the understanding that we are not necessarily comparing apples-to-apples. (If the statistics were done on middle-aged women, we cannot necessarily extrapolate to teenage boys.) However, we do have much data on “normal” (or usual) glucose readings.
Whole blood glucose levels are lower than serum (or plasma) levels by about 15-20%. This is due, in part, because the red blood cells, still present in whole blood (but absent in serum) will continue to utilize the glucose for energy, thus lowering the value. There may indeed be some differences, probably minor, in values depending on where the blood was obtained from e.g. fingerstick capillary vs forearm lancet vs a true right-out-of-the-vein venipuncture. So it is true — you need to know what your meter is measuring. (My pet peeve with my staff is that they download a meter on the computer not recognizing that it is a plasma glucose reading meter, but present the information is “whole blood glucose.” In these cases, I am not seeing the same data that my patient sees, and this causes miscommunication!)
Confounding the picture more is that not all devices that measure the capillary glucose value have 100% specificity and 100% sensitivity. Even the machine in the laboratory from your hospital is not 100% accurate. We generally accept an error rate of less than10%. Indeed, many hospital labs shoot for these statistical variances to be less than 5%. So that if the reading is “100 mg/dl [5.6mmol/L]”, the actual heaven-only-knows-true-value may really be 95-105 mg/dl [5.3-5.8 mmol/L], but both are normal, so who cares. If the reading is 200 mg/dl [mmol/L], then the actual value may range from 190-210 mg/dl [10.6-11.7 mmol/L], but both are abnormal so who really cares?
With that in mind, in general a “normal” fasting whole blood glucose value is between 60-120 mg/dl [3.3-6.7 mmol/L]. A fasting level greater than 125 mg/dl [6.7 mmol/L] has been the new (statistical) definition of diabetes mellitus since about 1997. Between 120 and 125 mg/dl [6.7 and 6.9 mmol/L]? Questionable results which might indicate impaired glucose tolerance — a sort of “pre-diabetes,” if you will. (I actually do not like the term “pre-diabetes” as it implies an unalterable path.)
After a meal, depending on the food intake, the two-hour level should be less than 140 mg/dl [7.8 mmol/L], but here is the rub — the glycemic index may play a huge role.
The current definitions of diabetes must be understood carefully:
A fasting serum (or plasma) level greater than 125 25 mg/dl [6.7 mmol/L] or;
A random serum level greater or equal to 200 mg/dl [11.1 mmol/L] with symptoms of diabetes or;
A serum value greater or equal to 200 mg/dl [11.1 mmol/L], two hours after a standard oral glucose tolerance test.which is very specific (at least for the non-pregnant individual): 1.75 grams of glucose to a maximum of 75 grams is the glucose load. However, the patient should have prepared themselves properly by consuming a diet over the preceding days comprising at least 60% of carbohydrates.
[Editor’s comment: Concerning what’s “normal”: I think if you lock a dozen endocrinologists in a room, and ask them to decide on what a normal blood sugar is before letting them out, they’ll never come out.