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Your health care professional can diagnose diabetes, prediabetes, and gestational diabetes through blood tests. The blood tests show if your blood glucose, also called blood sugar, is too high.
Do not try to diagnose yourself if you think you might have diabetes. Testing equipment that you can buy over the counter, such as a blood glucose meter, cannot diagnose diabetes.
Anyone who has symptoms of diabetes should be tested for the disease. Some people will not have any symptoms but may have risk factors for diabetes and need to be tested. Testing allows health care professionals to find diabetes sooner and work with their patients to manage diabetes and prevent complications.
Testing also allows health care professionals to find prediabetes. Making lifestyle changes to lose a modest amount of weight if you are overweight may help you delay or prevent type 2 diabetes.
Blood tests help health care professionals diagnose diabetes and prediabetes.
Health care professionals most often use the fasting plasma glucose (FPG) test or the A1C test to diagnose diabetes. In some cases, they may use a random plasma glucose (RPG) test.
The FPG blood test measures your blood glucose level at a single point in time. For the most reliable results, it is best to have this test in the morning, after you fast for at least 8 hours. Fasting means having nothing to eat or drink except sips of water.
The A1C test is a blood test that provides your average levels of blood glucose over the past 3 months. Other names for the A1C test are hemoglobin A1C, HbA1C, glycated hemoglobin, and glycosylated hemoglobin test. You can eat and drink before this test. When it comes to using the A1C to diagnose diabetes, your doctor will consider factors such as your age and whether you have anemia or another problem with your blood.1 The A1C test is not accurate in people with anemia.
If you’re of African, Mediterranean, or Southeast Asian descent, your A1C test results may be falsely high or low. Your health care professional may need to order a different type of A1C test.
Your health care professional will report your A1C test result as a percentage, such as an A1C of 7 percent. The higher the percentage, the higher your average blood glucose levels.
people with diabetes also use information from the A1C test to help manage their diabetes.Sometimes health care professionals use the RPG test to diagnose diabetes when diabetes symptoms are present and they do not want to wait until you have fasted. You do not need to fast overnight for the RPG test. You may have this blood test at any time.
Pregnant women may have the glucose challenge test, the oral glucose tolerance test, or both. These tests show how well your body handles glucose.
If you are pregnant and a health care professional is checking you for gestational diabetes, you may first receive the glucose challenge test. Another name for this test is the glucose screening test. In this test, a health care professional will draw your blood 1 hour after you drink a sweet liquid containing glucose. You do not need to fast for this test. If your blood glucose is too high—135 to 140 or more—you may need to return for an oral glucose tolerance test while fasting.
The OGTT measures blood glucose after you fast for at least 8 hours. First, a health care professional will draw your blood. Then you will drink the liquid containing glucose. For diagnosing gestational diabetes, you will need your blood drawn every hour for 2 to 3 hours.
High blood glucose levels at any two or more blood test times during the OGTT—fasting, 1 hour, 2 hours, or 3 hours—mean you have gestational diabetes. Your health care team will explain what your OGTT results mean.
Health care professionals also can use the OGTT to diagnose type 2 diabetes and prediabetes in people who are not pregnant. The OGTT helps health care professionals detect type 2 diabetes and prediabetes better than the FPG test. However, the OGTT is a more expensive test and is not as easy to give. To diagnose type 2 diabetes and prediabetes, a health care professional will need to draw your blood 1 hour after you drink the liquid containing glucose and again after 2 hours.
Each test to detect diabetes and prediabetes uses a different measurement. Usually, the same test method needs to be repeated on a second day to diagnose diabetes. Your doctor may also use a second test method to confirm that you have diabetes.
The following table helps you understand what your test numbers mean if you are not pregnant.
| Diagnosis | A1C (percent) | Fasting plasma glucose (FPG)a | Oral glucose tolerance test (OGTT)ab | Random plasma glucose test (RPG)a |
|---|---|---|---|---|
| Normal | below 5.7 | 99 or below | 139 or below | |
| Prediabetes | 5.7 to 6.4 | 100 to 125 | 140 to 199 | |
| Diabetes | 6.5 or above | 126 or above | 200 or above | 200 or above |
| Glucose values are in milligrams per deciliter, or mg/dL. | ||||
At 2 hours after drinking 75 grams of glucose. To diagnose gestational diabetes, health care professionals give more glucose to drink and use different numbers as cutoffs.
Even though the tests described here can confirm that you have diabetes, they can’t identify what type you have. Sometimes health care professionals are unsure if diabetes is type 1 or type 2. A rare type of diabetes that can occur in babies, called monogenic diabetes, can also be mistaken for type 1 diabetes. Treatment depends on the type of diabetes, so knowing which type you have is important.
To find out if your diabetes is type 1, your health care professional may look for certain autoantibodies. Autoantibodies are antibodies that mistakenly attack your healthy tissues and cells. The presence of one or more of several types of autoantibodies specific to diabetes is common in type 1 diabetes, but not in type 2 or monogenic diabetes. A health care professional will have to draw your blood for this test.
If you had diabetes while you were pregnant, you should get tested no later than 12 weeks after your baby is born to see if you have type 2 diabetes.