You’ll need to get your blood sugar tested to find out for sure if you have prediabetes or type 1, type 2, or gestational diabetes. Testing is simple, and results are usually available quickly.
Tests for Type 1 Diabetes, Type 2 Diabetes, and Prediabetes
Your doctor will have you take one or more of the following blood tests to confirm the diagnosis:
A1C Test
The A1C test measures your average blood sugar level over the past 2 or 3 months. An A1C below 5.7% is normal, between 5.7 and 6.4% indicates you have prediabetes, and 6.5% or higher indicates you have diabetes.
Fasting Blood Sugar Test
This measures your blood sugar after an overnight fast (not eating). A fasting blood sugar level of 99 mg/dL or lower is normal, 100 to 125 mg/dL indicates you have prediabetes, and 126 mg/dL or higher indicates you have diabetes.
Glucose Tolerance Test
This measures your blood sugar before and after you drink a liquid that contains glucose. You’ll fast (not eat) overnight before the test and have your blood drawn to determine your fasting blood sugar level. Then you’ll drink the liquid and have your blood sugar level checked 1 hour, 2 hours, and possibly 3 hours afterward. At 2 hours, a blood sugar level of 140 mg/dL or lower is considered normal, 140 to 199 mg/dL indicates you have prediabetes, and 200 mg/dL or higher indicates you have diabetes.
Random Blood Sugar Test
This measures your blood sugar at the time you’re tested. You can take this test at any time and don’t need to fast (not eat) first. A blood sugar level of 200 mg/dL or higher indicates you have diabetes.
Random Blood Sugar Test| 6.5% or above | 126 mg/dL or above | 200 mg/dL or above | 200 mg/dL or above |
| 5.7 – 6.4% | 100 – 125 mg/dL | 140 – 199 mg/dL | N/A |
| Below 5.7% | 99 mg/dL or below | 140 mg/dL or below | N/A |
*Results for gestational diabetes can differ. Ask your health care provider what your results mean if you’re being tested for gestational diabetes.
Source: American Diabetes Association
If your doctor thinks you have type 1 diabetes, your blood may also tested for autoantibodies (substances that indicate your body is attacking itself) that are often present in type 1 diabetes but not in type 2 diabetes. You may have your urine tested for ketones (produced when your body burns fat for energy), which also indicate type 1 diabetes instead of type 2 diabetes.
Tests for Gestational Diabetes
Gestational diabetes is diagnosed using blood tests. You’ll probably be tested between 24 and 28 weeks of pregnancy. If your risk is higher for getting gestational diabetes (due to having more risk factors), your doctor may test you earlier. Blood sugar that’s higher than normal early in your pregnancy may indicate you have type 1 or type 2 diabetes rather than gestational diabetes.
Glucose Screening Test
This measures your blood sugar at the time you’re tested. You’ll drink a liquid that contains glucose, and then 1 hour later your blood will be drawn to check your blood sugar level. A normal result is 140 mg/dL or lower. If your level is higher than 140 mg/dL, you’ll need to take a glucose tolerance test.
Glucose Tolerance Test
This measures your blood sugar before and after you drink a liquid that contains glucose. You’ll fast (not eat) overnight before the test and have your blood drawn to determine your fasting blood sugar level. Then you’ll drink the liquid and have your blood sugar level checked 1 hour, 2 hours, and possibly 3 hours afterward. Results can differ depending on the size of the glucose drink and how often your blood sugar is tested. Ask your doctor what your test results mean.
Prevent Type 2 Diabetes
If your test results show you have prediabetes, ask your doctor or nurse if there is a lifestyle change program offered through the CDC-led National Diabetes Prevention Program in your community. You can also search for an online or in-person program. Having prediabetes puts you at greater risk for developing type 2 diabetes, but participating in the program can lower your risk by as much as 58% (71% if you’re over age 60).
Diabetes Treatment Plan
If your test results show you have type 1, type 2, or gestational diabetes, talk with your doctor or nurse about a detailed treatment plan—including diabetes self-management education and support services—and specific steps you can take to be your healthiest.
Blood Glucose Test
Test Overview
A blood glucose test measures the amount of a type of sugar, called glucose, in your blood. Glucose comes from carbohydrate foods. It is the main source of energy used by the body. Insulin is a hormone that helps your body's cells use the glucose. Insulin is produced in the pancreas and released into the blood when the amount of glucose in the blood rises.
Normally, your blood glucose levels increase slightly after you eat. This increase causes your pancreas to release insulin so that your blood glucose levels do not get too high. Blood glucose levels that remain high over time can damage your eyes, kidneys, nerves, and blood vessels.
There are several different types of blood glucose tests.
- Fasting blood sugar (FBS) measures blood glucose after you have not eaten for at least 8 hours. It is often the first test done to check for prediabetes and diabetes.
- Random blood sugar (RBS) measures blood glucose regardless of when you last ate. Several random measurements may be taken throughout the day. Random testing is useful because glucose levels in healthy people do not vary widely throughout the day. Blood glucose levels that vary widely may mean a problem. This test is also called a casual blood glucose test.
- A 2-hour postprandial blood sugar test measures blood sugar exactly 2 hours after you start eating a meal. This test is most often done at home when you have diabetes. It can see if you are taking the right amount of insulin with meals.
- The hemoglobin A1c test and the oral glucose tolerance test (OGTT) are other tests used to look at blood sugar levels. The A1c test can be used to estimate your average blood sugar level over the past 2 to 3 months. The OGTT is commonly used to diagnose diabetes that occurs during pregnancy (gestational diabetes).
Why It Is Done
Blood glucose tests are done to:
- Check for prediabetes and diabetes.
- Monitor treatment of diabetes.
- Check for diabetes that occurs during pregnancy (gestational diabetes).
- Determine if an abnormally low blood sugar level (hypoglycemia) is present. A test to measure blood levels of insulin and a protein called C-peptide may be done along with a blood glucose test to determine the cause of hypoglycemia.
How To Prepare
Fasting blood sugar (FBS)
For a fasting blood sugar test, do not eat or drink anything other than water for at least 8 hours before the blood sample is taken.
If you have diabetes, you may be asked to wait until you have had your blood tested before you take your morning dose of insulin or diabetes medicine.
Random blood sugar (RBS)
No special preparation is needed before having a random blood sugar test.
How It Is Done
A health professional uses a needle to take a blood sample, usually from an arm.
Watch
How It Feels
You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
Risks
There is very little chance of having a problem from this test. A small bruise may form at the site.
Results
Normal
Results are often ready in 1 to 2 hours. Glucose levels in a blood sample taken from your vein (called a blood plasma value) may differ a little from glucose levels checked with a finger stick.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
High values
You may have diabetes. To make a diagnosis of type 2 diabetes, your doctor will use the American Diabetes Association's criteria.
Other conditions that can cause high blood glucose levels include:
- Severe stress.
- Heart attack.
- Stroke.
- Cushing's syndrome.
- Medicines such as corticosteroids.
- Excess production of growth hormone (acromegaly).
Low values
A fasting glucose level below 40 mg/dL (2.2 mmol/L) in women or below 50 mg/dL (2.8 mmol/L) in men that is accompanied by symptoms of hypoglycemia may mean you have an insulinoma, a tumor that produces abnormally high amounts of insulin.
Low glucose levels also may be caused by:
- Addison's disease.
- Decreased thyroid hormone levels (hypothyroidism).
- A tumor in the pituitary gland.
- Liver disease, such as cirrhosis.
- Kidney failure.
- Malnutrition or an eating disorder, such as anorexia.
- Medicines used to treat diabetes.
Credits
Current as of: April 13, 2022
Author: Healthwise Staff
Medical Review:
E. Gregory Thompson MD - Internal Medicine
Kathleen Romito MD - Family Medicine
Adam Husney MD - Family Medicine
Matthew I. Kim MD - Endocrinology
David C.W. Lau MD, PhD, FRCPC - Endocrinology