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 type 1 or type 2 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, such as water pills).
Fasting Blood Glucose (Blood Sugar) Level
A value above 126 mg/dL on at least 2 occasions typically means a person has diabetes.
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 you should not be taking any 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 5 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 they 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).
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: