Whether you have diabetes or prediabetes, you’re probably paying a lot more attention to your blood sugar (glucose) level now. A blood sugar level that remains high over time can lead to a number of long-term health complications, including kidney damage, eye damage, nerve problems and heart disease.
That's why testing your blood sugar level is important not only for diagnosing diabetes but also when treating the condition. So if your doctor suspects you have diabetes, or if you already know you have the condition, you can expect to have your blood sugar level checked with one or more tests.
Blood sugar testing to diagnose diabetes
For people with early symptoms of diabetes, or prediabetes, having your blood sugar tested in a doctor’s office is critical for diagnosing diabetes. You may have several tests, and you may have some of these tests done on two separate days to confirm the diagnosis.
Glycated hemoglobin, otherwise known as A1C, is the primary test you'll have if your doctor suspects you have diabetes. The A1C test measures the amount of sugar attached to hemoglobin, which is the protein in red blood cells that transports oxygen.
The A1C test has a few advantages over other diagnostic blood sugar tests. For one thing, you don't have to fast or do any other special preparations beforehand. It also estimates your average blood sugar level over the past two to three months, so it provides a more comprehensive picture of your blood sugar levels over time.
Here are the ranges of the A1C test and what each means to your health:
- Below 5.7 percent is considered a normal A1C level.
- Between 5.7 and 6.4 percent indicates prediabetes.
- Above 6.5 percent indicates diabetes.
Fasting plasma glucose test
If the results of the A1C test are inconclusive, your doctor may recommend a fasting plasma glucose test (FPG) to confirm a diabetes diagnosis.
This exam tests your blood sugar level after you haven't had anything to eat or drink for at least 8 hours. These results show a clear picture of your blood glucose without the impact of food or drink, which can skew results. For convenience, this test is often done first thing in the morning.
Here are the ranges of the FPG test and what each means to your health:
- Less than 100 milligrams per deciliter (mg/dl) is considered a normal FPG level.
- Between 100 mg/dl and 125 mg/dl indicates prediabetes.
- Above 126 mg/dl indicates diabetes.
Oral glucose tolerance test
The oral glucose tolerance test (OGTT) is a variation of the fasting plasma glucose test. Your doctor might recommend this diagnostic test to determine the proper course of treatment.
According to the Mayo Clinic, you begin this test by fasting overnight and taking a blood sugar test, just as with the FPG test. But then you drink a special sugary liquid and have your blood sugar tested occasionally over the next two hours. This shows your doctor how your blood sugar reacts to sugar in the diet.
Here are the ranges for the OGTT, after the two-hour testing period is over:
- Less than 140 milligrams per deciliter (mg/dl) is considered a normal OGTT level.
- Between 140 mg/dl and 199 mg/dl is considered prediabetes.
- Above 200 mg/dl is considered diabetes.
Random plasma glucose test
If you have enough significant diabetes symptoms, more elaborate forms of diabetes testing may not be necessary, according to the American Diabetes Association. In these instances, having the random plasma glucose test (sometimes called a casual plasma glucose test) is often enough to diagnose the condition.
You can have this blood test at any time of day, regardless of whether you had anything to eat or drink beforehand. If your blood sugar level in the random plasma glucose test is 200 mg/dl or above, it means you have diabetes.
Blood sugar testing to monitor diabetes
If you have diabetes, you'll need to monitor your blood sugar regularly at home, with equipment prescribed by your doctor, as well as having it checked at your appointments.
Keeping your blood sugar in a healthy range is the key to having good control of your diabetes, and doing self-checks at various times throughout the day is the way to keep track of it. You may need to check your blood sugar even more frequently if you take insulin.
Depending on your doctor’s recommendation, you can monitor your blood sugar with a blood glucose meter or by using a continuous glucose monitor.
Here are the general guidelines for blood sugar monitoring, according to the American Diabetes Association, though this may vary based on your specific circumstances:
- Before a meal, the normal range for blood sugar is 80 to 130 milligrams per deciliter (mg/dL).
- In the two-hour period after a meal, the target is less than 180 mg/dL.
- A1C, which measures your average blood sugar over the last two to three months, should be less than 7 percent.
Besides having an A1C test to diagnose diabetes, you may also have this test during regular doctor visits to monitor your progress in controlling your blood sugar when you have diabetes. The A1C test is an excellent tool for this purpose because it provides an average of your blood sugar level over the past few months.
- Diabetes, Mayo Clinic, 2019, https://www.mayoclinic.org/diseases-conditions/diabetes/diagnosis-treatment/drc-20371451
- The Big Picture: Checking Your Blood Glucose, American Diabetes Association, 2019, https://www.diabetes.org/diabetes/medication-management/blood-glucose-testing-and-control/checking-your-blood-glucose
- Diagnosis, American Diabetes Association, 2019, https://www.diabetes.org/a1c/diagnosis