The American Diabetes Association (ADA) recommends that blood glucose screening for adults begin at age 45, or sooner if you are overweight and have additional risk factors for prediabetes or type 2 diabetes.
There are several blood tests for prediabetes.
Glycated hemoglobin (A1C) test
This test indicates your average blood sugar level for the past two to three months. Specifically, the test measures the percentage of blood sugar attached to the oxygen-carrying protein in red blood cells (hemoglobin). The higher your blood sugar levels, the more hemoglobin you'll have with sugar attached.
- An A1C level below 5.7 percent is considered normal
- An A1C level between 5.7 and 6.4 percent is considered prediabetes
- An A1C level of 6.5 percent or higher on two separate tests indicates type 2 diabetes
Certain conditions can make the A1C test inaccurate — such as if you are pregnant or have an uncommon form of hemoglobin (hemoglobin variant).
Fasting blood sugar test
A blood sample is taken after you fast for at least eight hours or overnight.
- A fasting blood sugar level below 100 milligrams per deciliter (mg/dL) — 5.6 millimoles per liter (mmol/L) — is considered normal.
- A fasting blood sugar level from 100 to 125 mg/dL (5.6 to 7.0 mmol/L) is considered prediabetes. This result is sometimes called impaired fasting glucose.
- A fasting blood sugar level of 126 mg/dL (7.0 mmol/L) or higher indicates type 2 diabetes.
Oral glucose tolerance test
This test is usually used to diagnose diabetes only during pregnancy. A blood sample is taken after you fast for at least eight hours or overnight. Then you'll drink a sugary solution, and your blood sugar level will be measured again after two hours.
- A blood sugar level less than 140 mg/dL (7.8 mmol/L) is considered normal.
- A blood sugar level from 140 to 199 mg/dL (7.8 to 11.0 mmol/L) is considered prediabetes. This is sometimes referred to as impaired glucose tolerance.
- A blood sugar level of 200 mg/dL (11.1 mmol/L) or higher indicates type 2 diabetes.
If you have prediabetes, further testing may be needed. At least once a year, your doctor will likely check your:
- Fasting blood sugar
- Hemoglobin A1C
- Total cholesterol, HDL, low-density lipoprotein (LDL), and triglycerides
Testing might occur more frequently if you have additional risk factors for diabetes.
Children and prediabetes testing
Type 2 diabetes is becoming more common in children and adolescents, likely due to the rise in childhood obesity. The ADA recommends prediabetes testing for children who are overweight or obese and who have at least two other risk factors for type 2 diabetes.
These other risk factors include:
- Family history of type 2 diabetes.
- Race. Children who are African-American, Hispanic or Native American are at higher risk.
- Sex and age. Type 2 diabetes is more common among girls than boys. A diagnosis of childhood type 2 diabetes often occurs during puberty — as early as age 10.
- Low birth weight.
- Being born to a mother who had gestational diabetes.
The ranges of blood sugar level considered normal, prediabetic and diabetic are the same for children and adults.
Children who have prediabetes should be tested annually for type 2 diabetes — or more often if the child experiences a change in weight or develops signs or symptoms of diabetes, such as increased thirst, increased urination, fatigue or blurred vision.