Diagnosing Diabetes

closeup of stethoscope

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How is diabetes diagnosed?

Four common tests are used to diagnose diabetes:

Random or casual plasma glucose test

A plasma glucose test is a measure of how much sugar/glucose you have circulating in your blood. “Random” or “Casual” simply means that you have blood drawn at a laboratory at any time. Whether you have fasted or recently eaten will not affect the test. A plasma glucose test measurement equal to or greater than 200 milligrams per deciliter (mg/dl) indicates that you may have diabetes. To be sure, you will need to have the test results confirmed on another day through another random test, or by taking a fasting plasma glucose test or an oral glucose tolerance test.

Fasting plasma glucose test

This simple blood test is taken after you have abstained from food and drink (except water) for at least 8 hours. A normal plasma glucose level after fasting is between 60 and 99 mg/dl. Diabetes is not confirmed until 2 separate fasting plasma glucose tests each measure 126 or greater.

Oral glucose tolerance test

It’s possible for people with diabetes – even those with symptoms – to have a normal fasting plasma glucose test. If you fall into this category, you will again be asked to abstain from food and drink (except water) for 8 hours and then drink a liquid containing a known amount of glucose, usually 75 grams.Your blood is drawn before drinking the glucose mixture and  2 hours later. You will be asked to refrain from eating until the test is completed.  This test is called an Oral Glucose Tolerance Test (OGTT).

Your fasting plasma glucose normally is less than 100 mg/dl. Values from 100mg/dl to 126 mg/dl are diagnostic of  pre-diabetes. Fasting plasma glucose levels equal or above 126 mg/dl are diagnostic of diabetes.

Two hours after the glucose drink  your plasma glucose normally is less than 140 mg/dl. Values from 140mg/dl to 199 mg/dl indicate that you have pre-diabetes. You have diabetes if the plasma glucose levels are equal to or greater than 200 mg/dl.  


An A1c or glycosylated hemoglobin test is a measure of your average blood glucose control over the previous three months. The American Diabetes Association recommends using the A1c test to diagnosis diabetes and pre-diabetes. According to the American Diabetes Association (ADA), A1c levels over 6.5 % are considered diagnostic of diabetes; values between 5.7-6.4% are diagnostic of pre-diabetes and test results less than 5.6% are normal . Glycosolated Hemoglobin or Hemoglobin A1c tests are similar to the A1c and also evaluate long term blood glucose control.

A note of caution:

  1. The A1c measurement is an indirect measurement of the average blood glucose and may inaccurately represent the average glucose level. For example, if someone has a certain type of hemoglobin mutation (variation in the hemoglobin structure), is severely anemic (low red blood cell count), or is being treated blood transfusions or medications to increase the production of new red blood cells, the A1c test may not be accurate.
  2. The use of the A1c test to diagnosis diabetes is controversial. Currently, only the American Diabetes Association endorses use of the A1c for diabetes diagnosis.   

This table lists the ADA Diagnostic Criteria for diagnosing diabetes:

Fasting plasma glucose< 100 mg/dl100-125 mg/dl126 mg/dl or greater
OGTT – 2 hr. post glucose-rich beverage< 140 mg/dl140-199 mg/dl200 mg/dl or greater
Casual or random plasma glucose and symptoms200 mg/dl or greater
A1c<5.7%5.7-6.4%6.5% or greater

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