Diagnosing Diabetes 

In order to accurately diagnose 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). 

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 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 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 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: 

Normal Response 

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. 

Diabetes  

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 (either one) 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. 

Gestational Diabetes 

A woman has gestational  diabetes when she  is pregnant  and has any two of the following:  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.