How is Diabetes diagnosed?
Common tests to diagnose Diabetes:
Hemoglobin A1C
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 diagnose Diabetes and Prediabetes. 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 Prediabetes 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:
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.
Random plasma glucose test
A plasma glucose test is a measure of how much sugar/glucose you have circulating in your blood. “Random” 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 glucose test, or to complete a different type of test to support the diagnosis (i.e. Hemoglobin A1C, 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. Impaired fasting glucose (Prediabetes) is defined by a glucose level of 100-125 mg/dl. Diabetes is not confirmed until 2 separate fasting plasma glucose tests each measure 126 mg/dl 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). This is usually completed in pregnancy.
Your fasting plasma glucose normally is less than 100 mg/dl. Values from 100 mg/dl to 125 mg/dl are diagnostic of Prediabetes. 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 140 mg/dl to 199 mg/dl indicate that you have Prediabetes. You have Diabetes if the plasma glucose levels are equal to or greater than 200 mg/dl.
ADA Diagnostic Criteria to diagnose Diabetes
| Indicator | 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 | ||
| A1c | <5.7% | 5.7-6.4% | 6.5% or greater |
Antibody Testing for Type 1 Diabetes
If Type 1 Diabetes is suspected, your medical provider may order antibody testing to assess for autoimmune markers that may help diffentiate from Type 2 Diabetes. These antibody tests include: glutamic acid decarboxylase (GADAs), islet cell antigen 512 (ICA512s or ICA IA-2s), insulin auto-antibodies (IAAs), islet cell antibodies (ICAs), zinc transporter 8 (ZnT8).