The two primary tests and their results
In diagnosing diabetes, physicians primarily depend upon the results of specific glucose tests. However, test results are just part of the information that goes into the diagnosis of diabetes. Doctors also take into account your physical exam, presence or absence of symptoms, and medical history. Some people who are significantly ill will have transient problems with elevated blood sugars which will then return to normal after the illness has resolved. Also, some medications may alter your blood glucose levels (most commonly steroids and certain diuretics (water pills)). The two main tests used to measure the presence of blood sugar problems are  the direct measurement of glucose levels in the blood during an overnight fast, and  measurement of the body’s ability to appropriately handle the excess sugar presented after drinking a high glucose drink.
 Fasting Blood Glucose (Blood Sugar) Level:
The “gold standard” for diagnosing diabetes is an elevated blood sugar level after an overnight fast (not eating anything after midnight). A value above 140 mg/dl on at least two occasions typically means a person has diabetes. Normal people have fasting sugar levels that generally run between 70-110 mg/dl.
 The Oral Glucose Tolerance Test
An oral glucose tolerance test is one that can be performed in a doctor’s office or a lab. The person being tested starts the test in a fasting state (having no food or drink except water for at least 10 hours but not greater than 16 hours). An initial blood sugar is drawn and then the person is given a “glucola” bottle with a high amount of sugar in it (75 grams of glucose), (or 100 grams for pregnant women). The person then has their blood tested again 30 minutes, 1 hour, 2 hours and 3 hours after drinking the high glucose drink.
For the test to give reliable results, you must be in good health (not have any other illnesses, not even a cold). Also, you should be normally active (for example, not lying down or confined to a bed like a patient in a hospital) and taking no medicines that could affect your blood glucose. The morning of the test, you should not smoke or drink coffee. During the test, you need to lie or sit quietly.
The oral glucose tolerance test is conducted by measuring blood glucose levels five times over a period of 3 hours. In a person without diabetes, the glucose levels in the blood rise following drinking the glucose drink, but then then fall quickly back to normal (because insulin is produced in response to the glucose, and the insulin has a normal effect of lowing blood glucose.) In a diabetic, glucose levels rise higher than normal after drinking the glucose drink and come down to normal levels much slower (insulin is either not produced, or it is produced but the cells of the body do not respond to it) (see details on type 1 and type 2 diabetes for more information on this topic).
As with fasting or random blood glucose tests, a markedly abnormal oral glucose tolerance test is diagnostic of diabetes. However, blood glucose measurements during the oral glucose tolerance test can vary somewhat. For this reason, if the test shows that you have mildly elevated blood glucose levels, the doctor may run the test again to make sure the diagnosis is correct.
Glucose tolerance tests may lead to one of the following diagnoses:
A person is said to have a normal response when the 2-hour glucose level is less than or equal to 110 mg/dl.
Impaired Fasting Glucose
When a person has a fasting glucose equal to or greater than 110 and less than 126 mg/dl, they are said to have impaired fasting glucose. This is considered a risk factor for future diabetes, and will likely trigger another test in the future, but by itself, does not make the diagnosis of diabetes.
Impaired Glucose Tolerance
A person is said to have impaired glucose tolerance when the 2-hour glucose results from the oral glucose tolerance test are greater than or equal to 140 but less than 200 mg/dl. This is also considered a risk factor for future diabetes. There has recently been discussion about lowering the upper value to 180 mg/dl to diagnose more mild diabetes to allow earlier intervention and hopefully prevention of diabetic complications.
A person has diabetes when oral glucose tolerance tests show that the blood glucose level at 2 hours is equal to or more than 200 mg/dl. This must be confirmed by a second test (any of the three) on another day. There has recently been discussion about lowering the upper value to 180 mg/dl to diagnose more people with mild diabetes to allow earlier intervention and hopefully prevention of diabetic complications.
A woman has gestational diabetes when she is pregnant and has any two of the following: a fasting plasma glucose of more than 105 mg/dl, a 1-hour glucose level of more than 190 mg/dl, a 2-hour glucose level of more than 165 mg/dl, or a 3-hour glucose level of more than 145 mg/dl.