Diabetes is diagnosed through blood tests that measure how your body handles sugar. There’s no single at-home check that can confirm it. You’ll need a lab test ordered by a doctor, and in most cases, an abnormal result must be confirmed with a second test on a different day before you receive an official diagnosis. The three main tests used are the A1C, fasting blood glucose, and oral glucose tolerance test, each with specific cutoff numbers that separate normal from prediabetes from diabetes.
The A1C Test
The A1C test measures your average blood sugar over the past two to three months. It works by looking at how much sugar has attached to your red blood cells. Because it reflects a longer window, a single high-sugar meal or stressful day won’t throw off the result.
The ranges are straightforward:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
No fasting is required, so you can eat and drink normally before the test. This makes it the most convenient option for many people. However, certain conditions like sickle cell trait, recent blood transfusions, or heavy bleeding can affect the accuracy of A1C results. Your doctor may choose a different test if any of those apply.
Fasting Blood Glucose Test
This test measures your blood sugar after you haven’t eaten for a period of time. You’ll need to fast for 8 to 12 hours beforehand. Plain water is fine during the fast, but skip flavored water, gum, cigarettes, and exercise, as all of these can affect results. Ask your doctor whether to take your usual medications that morning, and mention any vitamins or supplements you use.
The cutoffs:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
Most clinics schedule this test first thing in the morning so you can sleep through most of the fasting window.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) checks how efficiently your body clears sugar from the bloodstream. You fast overnight, then drink a solution containing 75 grams of glucose (roughly the sugar equivalent of two cans of soda consumed at once). Your blood is drawn two hours later.
Two-hour results:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
This test is more time-consuming than the others, so it’s used less often for routine screening. It’s most commonly ordered during pregnancy or when other test results are borderline and the doctor wants a clearer picture.
Random Blood Sugar Test
If you’re already showing symptoms of diabetes, like excessive thirst, frequent urination, unexplained weight loss, or blurred vision, your doctor can order a random blood sugar test at any time of day, regardless of when you last ate. A result of 200 mg/dL or higher, combined with symptoms, is enough to diagnose diabetes without a second confirmatory test.
Why Home Glucose Monitors Aren’t Used for Diagnosis
If you already own a blood glucose meter, you might wonder whether you can test yourself at home. These devices are useful for people who already have diabetes and need to track their levels day to day, but they aren’t accurate enough to make a diagnosis. Studies show home glucometer readings can vary by 10 to 15% from laboratory values. Regulatory standards allow 95% of readings to fall within 15 mg/dL of the true value, which is acceptable for daily monitoring but problematic at the exact cutoff points where a diagnosis is made. A reading of 128 mg/dL on a home meter could actually be 113 in the lab, which would be prediabetes rather than diabetes. For this reason, diagnosis always requires a lab-grade blood draw.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends screening for prediabetes and type 2 diabetes in adults aged 35 to 70 who are overweight or obese (a BMI of 25 or higher). For Asian Americans, screening is recommended at a lower BMI of 23 or higher, because type 2 diabetes tends to develop at lower body weights in this population.
Outside of those guidelines, testing makes sense earlier or more frequently if you have risk factors: a family history of diabetes, a history of gestational diabetes, polycystic ovary syndrome, or if you belong to a racial or ethnic group with higher diabetes rates (including Black, Hispanic, Native American, and Pacific Islander communities). If you fall into the prediabetes range on any test, your doctor will typically recheck within one to three years.
Gestational Diabetes Screening
Pregnant women are typically screened for gestational diabetes between 24 and 28 weeks of pregnancy. The most common approach in the U.S. is a two-step process. First, you drink a 50-gram glucose solution and have your blood drawn one hour later. If that result is elevated, you return for a longer test using a 100-gram glucose load, with blood drawn at fasting, one hour, two hours, and three hours. A diagnosis is made when two or more of those values exceed the thresholds (for example, fasting above 95, one-hour above 180, two-hour above 155, or three-hour above 140 mg/dL).
Some providers use a one-step approach instead, which involves a single 75-gram glucose test with blood drawn at fasting, one hour, and two hours. Only one elevated value is needed for diagnosis with this method. Your OB provider will tell you which protocol they use.
Testing for Type 1 Diabetes
The blood sugar tests described above can diagnose diabetes but don’t tell you which type you have. Type 1 diabetes is an autoimmune condition where the immune system attacks the cells in the pancreas that produce insulin. To confirm type 1, doctors test for specific antibodies in the blood. The most commonly measured are antibodies against GAD65, IA-2, insulin itself, and a protein called ZnT8. The presence of two or more of these antibodies strongly suggests type 1 or autoimmune diabetes.
This distinction matters even in adults. About 7% of people initially diagnosed with typical adult-onset diabetes actually have antibodies against GAD65, meaning their diabetes is autoimmune in nature. This form, sometimes called latent autoimmune diabetes in adults (LADA), progresses more slowly than childhood-onset type 1 but will eventually require insulin. If you’re diagnosed with diabetes at a relatively young age, are not overweight, or don’t respond well to standard type 2 medications, antibody testing can clarify what’s going on.
What to Expect on Test Day
For an A1C or random glucose test, there’s nothing to prepare. You show up, get a blood draw, and results are typically back within a day or two. For fasting glucose, plan to have your blood drawn in the morning after an overnight fast of at least 8 hours. Bring a snack for afterward.
The oral glucose tolerance test requires the most patience. Expect to spend about two and a half hours at the clinic. The glucose drink tastes like flat, very sweet soda. Some people feel nauseous afterward. You’ll sit in the waiting room until your two-hour blood draw is complete. For the three-hour version used in pregnancy screening, add another hour.
If your first test comes back in the diabetes range, your doctor will usually order a repeat of the same test or a different test on another day to confirm. The exception is if you already have classic symptoms and a random glucose of 200 mg/dL or higher, which is diagnostic on its own.