Diabetes is checked through blood tests that measure how much sugar is in your blood, either at a single point in time or averaged over several months. The most common tests are the A1C, fasting blood sugar, and oral glucose tolerance test. Any of these can confirm a diagnosis, and most require little preparation beyond skipping breakfast.
The A1C Test
The A1C test is one of the most convenient ways to check for diabetes because it doesn’t require fasting. 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 higher
Because it captures a longer window, the A1C isn’t thrown off by what you ate yesterday or how stressed you were that morning. That makes it a reliable first test for most adults. Your doctor may order a second A1C or a different blood test to confirm the result before making an official diagnosis.
Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten overnight. You’ll need to skip food and drinks other than water for at least eight hours before the blood draw, which is why it’s usually scheduled first thing in the morning. The thresholds are straightforward:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
A result in the prediabetes range is sometimes called impaired fasting glucose. It means your body is starting to struggle with blood sugar regulation but hasn’t crossed into diabetes territory yet. If your number falls in the diabetes range, a repeat test on a different day is typically needed to confirm the diagnosis.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) is the most involved option. You fast overnight, then drink a sugary liquid containing 75 grams of glucose. Your blood is drawn before you drink it and again two hours later. The two-hour reading tells your doctor how efficiently your body clears sugar from the bloodstream.
A two-hour result under 140 mg/dL is normal. Between 140 and 199 mg/dL indicates prediabetes. A reading of 200 mg/dL or higher points to diabetes. This test is especially useful for catching prediabetes because some people have normal fasting levels but struggle to process a sugar load.
Screening During Pregnancy
Gestational diabetes has its own testing timeline. Most pregnant women are screened between 24 and 28 weeks of pregnancy using one of two approaches.
The more common two-step method starts with a one-hour screening. You drink a glucose solution (no fasting required) and have your blood drawn an hour later. If that result comes back high, you return for a longer three-hour test. For the second visit, you fast for 8 to 14 hours, drink a liquid with 100 grams of glucose, and have blood drawn four times: before drinking and then once every hour for three hours.
Some providers use a one-step approach instead. You fast beforehand, drink 75 grams of glucose, and have blood drawn three times over two hours. Your provider will let you know which method they use.
Can a Home Glucose Monitor Diagnose Diabetes?
Home glucose meters are designed for people who already have diabetes to track their blood sugar day to day. They’re useful for adjusting diet, exercise, and medication, but they aren’t accurate enough to serve as a formal diagnostic tool. A reading that looks high on a home meter doesn’t equal a diagnosis, and a normal-looking reading doesn’t rule diabetes out.
If you’re concerned about your numbers, bring your meter to your next appointment. Your doctor can compare your meter’s reading against a lab blood test taken at the same time to check whether your device is accurate. But the official diagnosis will come from a lab-grade blood test, not a home device.
Urine Tests Are Mostly Outdated
Urine glucose testing used to be a common screening method, but it has largely been replaced by blood tests. The problem is accuracy: you can have high blood sugar and still show little or no glucose in your urine, so a normal urine result doesn’t mean you’re in the clear. Urine tests are now reserved for situations where blood draws are difficult, such as when veins are scarred from repeated testing. An abnormal urine glucose result will always be followed up with a blood test to confirm what’s going on.
Telling Type 1 Apart From Type 2
The blood tests above detect diabetes but don’t always reveal which type you have. For most adults diagnosed later in life, Type 2 is assumed based on age, weight, and other risk factors. But when the picture is unclear, especially in younger adults or people who don’t fit the typical Type 2 profile, doctors can order autoantibody tests.
These tests look for immune system proteins that attack the cells in your pancreas responsible for making insulin. There are five main autoantibodies doctors check for. Finding two or more of them gives a greater than 90% likelihood that the diabetes is Type 1, which is an autoimmune condition requiring insulin from the start. This distinction matters because the treatment paths are quite different.
When and How Often to Get Tested
Current guidelines from the American Diabetes Association recommend that all adults begin routine screening at age 35, even without symptoms. If your results come back normal, retesting every three years is reasonable. If you gain weight or develop new risk factors like high blood pressure or a family history of diabetes, earlier retesting makes sense.
People with certain risk factors may need screening before 35. These include being overweight, having a parent or sibling with diabetes, a history of gestational diabetes, or belonging to an ethnic group with higher diabetes rates (including Black, Hispanic, Native American, Asian American, and Pacific Islander populations). If any of these apply, ask about testing at your next routine visit regardless of your age.
Prediabetes results deserve attention, not panic. A fasting glucose between 100 and 125 mg/dL or an A1C between 5.7% and 6.4% means your blood sugar is elevated but still in a range where lifestyle changes, like losing 5 to 7% of your body weight and getting regular physical activity, can meaningfully lower your risk of progressing to full diabetes.