You have diabetes if your blood glucose after fasting (and before a meal) is 126 milligrams per deciliter (mg/dL) or higher; if two hours after a 75 gram oral glucose tolerance test (OGTT), it is 200 mg/dL or higher; or if your A1C is 6.5 or higher.
Hemoglobin A1C Test
The A1C test can be used to diagnose prediabetes and diabetes. “Because it is a measure of the patient’s average blood sugar over the previous three months, it gives much more information than simply getting a few blood sugar readings on the spot and is useful as an initial baseline as the patient gets treated,” says Rettinger. A 6.5 or higher result on two separate days is in the range of diabetes, while a result of 5.7 to 6.4 is in the prediabetes range.
How often you are tested once you receive a diagnosis depends on a number of factors. Typically, if you have prediabetes, you will be tested once a year. If you have type 2 diabetes, you will often be tested twice a year — though that might go up to four times a year if you use insulin to manage your blood sugar or if you otherwise have trouble stabilizing your blood sugar.
Keep in mind that the A1C test may not be accurate in people who have anemia or other disorders that affect red blood cell turnover. Additionally, people of Mediterranean, Southeast Asian, or African descent may have results that are falsely high or low because they have less common forms (variants) of hemoglobin in their blood.
For instance, people of African descent are more likely than those in other groups to have a hemoglobin variant that carries the sickle cell trait. That variant also gives falsely low A1C readings, according to a retrospective cohort study published in 2017.
Other tests besides A1C are used to diagnose diabetes. After taking a medical history, which would determine any family incidence of the disease, your doctor may order lab tests to measure your blood glucose levels. A healthcare professional can make a diagnosis on the basis of two abnormal test results from the same blood sample or two separate results from different dates.
Fasting Plasma Glucose Test
This test is conducted in a doctor’s office on blood drawn after an eight-hour fast with no food or drink except water. The blood is normally drawn in the morning before you have eaten breakfast. You will be diagnosed according to the following parameters:
- Normal blood sugar is less than 100 mg/dL.
- Prediabetes (impaired fasting glucose) is from 100 to 125 mg/dL.
- Diabetes is 126 mg/dL or higher.
Oral Glucose Tolerance Test
The OGTT is another diabetes test performed after you haven’t eaten for about eight hours. It allows your doctor to see how quickly glucose is cleared from your blood.
The technician will take a sample of your blood and then give you a prepared sugary drink. You will then have your blood drawn every hour, so that you will end up with blood glucose level results after:
- Fasting
- One hour after ingesting the sugary drink
- Two hours after ingesting the sugary drink
- Sometimes three hours after ingesting the sugary drink
You will be diagnosed according to the following parameters:
- Normal blood sugar at two hours after testing is less than 140 mg/dL.
- Prediabetes (impaired glucose tolerance) is 140 to 199 mg/dL.
- Diabetes is 200 mg/dL or above.
The OGTT is also used in screening pregnant women for gestational diabetes and is usually performed between 24 and 28 weeks of pregnancy.
The OGTT is one of the best detectors of diabetes and prediabetes, but it is given less often in nonpregnant adults because it is more expensive to perform and not as easy to administer as some other tests.
Random Plasma Glucose Test
This test may be done at any time to measure blood sugar levels, usually when you have severe diabetes symptoms. It allows for the identification of high blood levels immediately, but it should not be used as a tool to diagnose diabetes.
Still, it can indicate that you need additional testing or treatment if your blood sugar is at least 200 mg/dL. The other classic symptoms of high blood sugar are:
- You’re urinating more than usual.
- You’re drinking more fluid than you normally do.
- You’ve lost weight when you didn’t mean to.
Any diagnosis is generally confirmed with two results from a fasting plasma glucose test, an oral glucose tolerance test, or an A1C test.
