So, okay, diabetes (mellitus) is a disease (or maybe a group of diseases) where there is too much sugar. Fine.
Now the question is, how much sugar is too much? This is a question that is relevant for many reasons, and this question actually contains many questions. Here are some of the questions it contains:
1. What is the lowest blood sugar that is dangerous?
2. Is high blood sugar dangerous if it's just for a little while?
3. Is the dangerous part an average high blood sugar or time spent above a certain value?
4. Or is the danger actually in the thing that causes high blood sugar and not in high blood sugar itself?
5. Or maybe it's not high blood sugar, but unstable blood sugar that's dangerous?
6. Is blood sugar that's within the mathematically normal range necessarily good for you?
7. At what threshhold of high blood sugar (or urine sugar) is treatment a good thing?
8. Are different blood sugar levels normal or safe for some people compared to others?
Pretty much none of these questions have been fully answered (although some we are well on our way to answering) although they are important. These questions matter when we try to decide:
1. If we are diabetic, what blood sugar level do we correct highs at?
2. If we are doctors, what blood sugars do we call "controlled"?
3. How do we define diabetes?
4. At what point do we start or change treatment for diabetes?
5. Can you use anything other than blood sugar to diagnose diabetes?
6. If a person's blood sugar is measured as being high just once, is the person diabetic?
7. If a person has no symptoms, is the person diabetic?
Therefore, the definition of diabetes (and pre-diabetes!) has been and is different in different times and places.
Currently, the most common diagnostic criteria say that diabetes is diagnosed if:
1. One or both of these things happen at least twice:
a)A person has a fasting blood sugar (a blood sugar after nothing has been eaten in six hours) taken with a venous blood draw, that is 127 mg/dl or higher.
b)A person has a blood sugar at any time that is over 200 mg/dl- especially if it is in an oral glucose tolerance test.
Side note: The oral glucose tolerance test was developed and introduced as a way to diagnose diabetes in 1917. In the oral glucose tolerance test most commonly used, a person comes in fasting, has blood sugar checked, drinks a glucose solution (like flat soda) and then has blood sugar measured every half hour or hour for two to four hours. Diabetes or impaired glucose tolerance is diagnosed when any readings are high, or in a four hour test when the readings don't go back down. An intravenous glucose tolerance test has also been proposed (more than once), wherein a person is injected with glucose (or dextrose) into the vein, and time until the body gets a normal blood sugar back is measured. Source
2. Or the person's A1c is 6.5% or greater. This is a bit controversial because there are things that artificially elevate the Hemoglobin A1c reading, and when I was diagnosed, a scant five years ago, this was not considered good enough to diagnose diabetes- but now it is. The change was made in July 2009. Source
3.If a person is in ketoacidosis or a hyperosmolar coma, only one blood sugar reading above 200 is needed to diagnose diabetes.
Note: The cut-off value of 200 is arbitrary and has been set and reset over time. In some places a cut-off of 180 or 220 is used instead.
The American Diabetes Association also says that a person can be in remission from diabetes. If the person does not take any diabetes medication at all, and hasn't taken any in over a year, and has an A1c below 6%. Previously it has a definition of remission that said a person could have either normal fasting blood sugar or an A1c below 6% to be termed in remission.
Many places currently diagnose type 2 diabetes either by criteria 1 or 2, rather than both, and seem to see the 2009 change as having the A1c replace the blood sugar criteria instead of being in addition to it- a definition I see as problematic, given that a person can have very high blood sugar without a high A1c, in cases of quick onset diabetes. My mother (an MD) says she has seen patients in DKA whose A1cs were below 6%- although everybody does consider DKA diagnostic of diabetes (DKA is defined as acidosis- meaning venous pH < 7.3 with blood sugar above 200).
Although all of the common definitions of diabetes call it chronic, a person does not have to have a history of high blood sugar to be called diabetic- newborns can be diagnosed with diabetes.
Much of the controversy over borderline diabetes has been settled in recent years by calling people who have blood sugar and A1c levels that may or may not be problematic "pre-diabetic". Despite the term, people with pre-diabetes sometimes do develop ill effects of chronic hyperglycemia.
I think the most interesting fact in here is that the oral glucose tolerance test was developed in 1917.