Diagnosing Diabetes
How is diabetes diagnosed?
Three 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 by taking the test after fasting, through another random test, or by taking a fasting plasma glucose 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 2 hours later. You will be asked to refrain from eating until the test is completed.
After 2 hours, your blood glucose would normally be less than 140 mg/dl. Values equal to or greater than 200 mg/dl indicate that you have diabetes.
Note on the A1c or glycosylated hemoglobin test
If you have already been diagnosed with diabetes, the A1c or glycosylated hemoglobin test is very useful in keeping track of your average blood glucose for the past three months. This is not an appropriate test for diabetes diagnosis.
This table lists the Diagnostic Criteria for diagnosing diabetes:
| Normal | Pre-diabetes | Diabetes | |
|---|---|---|---|
Fasting plasma glucose |
< 100 mg/dl |
100-125 mg/dl |
126 mg/dl or greater |
OGTT – 2 hr. post glucose-rich beverage |
< 140 mg/dl |
140-199 mg/dl |
200 mg/dl or greater |
Casual or random plasma glucose and symptoms |
200 mg/dl or greater |