Diabetes is checked through blood tests that measure how much sugar is in your bloodstream. The most common options are a fasting blood sugar test, an A1C test, and an oral glucose tolerance test. Each one measures blood sugar in a slightly different way, and your doctor may use one or a combination to confirm a diagnosis. Here’s what each test involves and what the results mean.
The A1C Test
The A1C test is one of the most convenient ways to check for diabetes because it doesn’t require fasting or drinking anything special. It measures your average blood sugar over the past two to three months by looking at how much sugar has attached to your red blood cells. The results come back as a percentage:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
Because it reflects a longer window of time rather than a single moment, the A1C gives a more stable picture of your blood sugar patterns. A stressful morning or a skipped meal won’t throw off the number. That said, certain conditions like anemia or recent blood loss can affect the accuracy of the test, so your doctor may pair it with another method to be sure.
Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten for at least 8 to 12 hours. You’ll typically schedule it for the morning so most of the fasting happens while you sleep. During the fasting window, you can drink plain water but nothing else: no coffee, juice, soda, flavored water, or gum. Smoking and exercise should also be avoided, since both can temporarily shift your blood sugar levels.
A fasting blood sugar below 100 mg/dL is normal. Between 100 and 125 mg/dL falls into the prediabetes range. A result of 126 mg/dL or higher on two separate tests points to diabetes. That second test is important. One elevated reading can sometimes reflect illness, stress, or a lab error, so doctors typically confirm with a repeat test before making a diagnosis.
Oral Glucose Tolerance Test
The oral glucose tolerance test, often called the OGTT, is a two-part process. You start by fasting overnight, then your blood sugar is measured. After that baseline reading, you drink a very sweet liquid containing 75 grams of glucose (roughly the equivalent of drinking a large soda’s worth of pure sugar). Two hours later, your blood is drawn again to see how well your body handled the sugar load.
A two-hour result below 140 mg/dL is normal. Between 140 and 199 mg/dL indicates prediabetes. A reading of 200 mg/dL or higher means diabetes. This test takes more time than the others, so it’s less commonly used for routine screening. It’s more often reserved for situations where other results are borderline or during pregnancy screening for gestational diabetes.
Random Blood Sugar Test
Unlike the other tests, a random blood sugar test doesn’t require any fasting or preparation. It simply measures your blood sugar at whatever time the test is taken. A result of 200 mg/dL or above, combined with classic diabetes symptoms like excessive thirst, frequent urination, or unexplained weight loss, is enough to diagnose diabetes. This test is typically used when symptoms are severe enough that waiting for a fasting test doesn’t make sense.
Screening for Gestational Diabetes
Pregnant women are usually screened for gestational diabetes between 24 and 28 weeks of pregnancy. The most common approach is a two-step process. First, you drink a smaller sugar solution (50 grams) and have your blood drawn one hour later. If the result is at or above 140 mg/dL, you move on to a longer three-hour test with a larger sugar load to confirm the diagnosis. Some providers use a one-step approach that works more like the standard OGTT. Your OB will let you know which method they use.
Who Should Get Tested
Current guidelines from both the American Diabetes Association and the U.S. Preventive Services Task Force recommend that all adults start screening for prediabetes and type 2 diabetes at age 35, then repeat the test every three years. If you have overweight or obesity plus at least one additional risk factor, screening may start earlier. Those risk factors include a family history of diabetes, a history of gestational diabetes, polycystic ovary syndrome, or belonging to a racial or ethnic group with higher diabetes rates (including Black, Hispanic, Native American, Asian American, and Pacific Islander populations).
Children and younger adults aren’t routinely screened unless they show symptoms or have significant risk factors like obesity combined with a strong family history.
Symptoms That Warrant Testing
Many people with type 2 diabetes have no obvious symptoms in the early stages, which is why routine screening matters. When symptoms do appear, they tend to develop gradually. The most common ones include frequent urination, increased thirst and hunger, unexplained weight loss, fatigue, blurry vision, and recurring urinary tract infections or yeast infections.
Type 2 diabetes can also cause slower-healing cuts or sores, numbness or tingling in the hands and feet, and dark patches of skin around the neck, armpits, or groin. Those dark patches, called acanthosis nigricans, signal insulin resistance and often show up before blood sugar levels reach the diabetic range.
Type 1 diabetes tends to come on faster, sometimes over a matter of weeks, and can include nausea, vomiting, and stomach pain. In children, unexplained bed-wetting or increased accidents can be an early sign. Type 1 can develop at any age, though it most often appears in childhood or adolescence.
How to Prepare for Your Test
If you’re getting a fasting blood sugar test or an OGTT, plan to fast for 8 to 12 hours before your appointment. Schedule the blood draw for the morning so you’re mostly sleeping through the fast. Drink plain water freely, but skip everything else, including coffee, tea, and flavored water, since even small amounts of sugar or artificial sweetener can affect results. Avoid chewing gum, smoking, and exercising during the fasting period.
For an A1C or random blood sugar test, no preparation is needed. You can eat and drink normally beforehand. If you’re unsure which test your doctor ordered, call the office ahead of time so you know whether fasting is required. Showing up for a fasting test after eating means you’ll likely need to reschedule.
What Happens After an Abnormal Result
A single abnormal blood sugar reading usually isn’t enough for a diagnosis on its own (the exception being a random test of 200 mg/dL or higher with clear symptoms). Doctors typically confirm results with a second test, either repeating the same one or using a different method. For example, if your A1C comes back at 6.6%, your doctor might order a fasting blood sugar test to verify.
If your results fall in the prediabetes range, that’s not a diabetes diagnosis, but it is a signal that your blood sugar is trending upward. Prediabetes is the stage where lifestyle changes have the most impact. Losing 5% to 7% of your body weight and getting about 150 minutes of moderate physical activity per week has been shown to cut the risk of progressing to type 2 diabetes by more than half.
If your results confirm diabetes, your doctor will work with you on a management plan that depends on whether it’s type 1 or type 2, how high your blood sugar is, and what other health factors are in play. Early detection gives you the widest range of options, which is one of the strongest arguments for getting tested on schedule rather than waiting for symptoms to show up.