Getting tested for diabetes is straightforward: you visit a primary care provider and get a blood test. The three standard tests are a fasting blood sugar test, an A1C test, or an oral glucose tolerance test. Most people get results within a day or two, and in many cases your doctor can order the test during a routine checkup with no special referral needed.
The Three Main Diabetes Tests
Each test measures blood sugar in a slightly different way, and any one of them can be used to diagnose diabetes or prediabetes.
Fasting blood sugar test. This is the simplest option. You fast for 8 to 12 hours (water is fine), then have your blood drawn. A result under 100 mg/dL is normal. Between 100 and 125 mg/dL indicates prediabetes. A reading of 126 mg/dL or higher points to diabetes.
A1C test. This measures your average blood sugar over the previous two to three months. No fasting is required, which makes it convenient. A normal A1C is below 5.7%. Between 5.7% and 6.4% is considered prediabetes, and 6.5% or higher indicates diabetes.
Oral glucose tolerance test (OGTT). You fast overnight, have your blood drawn, then drink a sugary liquid containing 75 grams of glucose. Your blood is drawn again two hours later. A two-hour reading under 140 mg/dL is normal, 140 to 199 mg/dL suggests prediabetes, and 200 mg/dL or higher means diabetes. This test takes longer but is especially sensitive at catching early blood sugar problems that other tests might miss.
There’s also a random blood sugar test, which can be done at any time without fasting. If it comes back at 200 mg/dL or above and you’re experiencing symptoms like excessive thirst, frequent urination, or unexplained weight loss, that alone can lead to a diabetes diagnosis.
How to Prepare
For the A1C test, you don’t need to do anything special. Eat and drink normally beforehand. For the fasting blood sugar test or the oral glucose tolerance test, you’ll need to avoid all food and beverages other than water for 8 to 12 hours before the blood draw. Most people schedule these for first thing in the morning so the fasting period happens overnight.
Let your provider know about any medications you’re taking, any recent illness, and your general activity level. All of these factors can shift your results. Your provider will give you specific instructions, but in general, just follow your normal routine in the days leading up to the test.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends routine diabetes screening for adults aged 35 to 70 who are overweight or obese (a BMI of 25 or higher). If you’re Asian American, screening is recommended at a lower BMI threshold of 23 or higher, because the risk of diabetes rises at lower body weights in this population.
Earlier screening is also recommended for people who are Black, Hispanic or Latino, American Indian or Alaska Native, or Native Hawaiian or Pacific Islander, since these groups face a disproportionately higher risk. Beyond ethnicity and weight, other reasons to get tested sooner include a family history of diabetes, a history of gestational diabetes, high blood pressure, or abnormal cholesterol levels.
If none of these risk factors apply to you but you’re noticing symptoms like increased thirst, frequent urination, blurred vision, fatigue, or slow-healing cuts, bring those up with your doctor. A blood test can rule diabetes in or out quickly.
What Happens After an Abnormal Result
A single abnormal result on a fasting blood sugar or A1C test typically needs to be confirmed with a second test. Your provider may repeat the same test on a different day or use a different test to cross-check. The exception is when you have a random blood sugar of 200 mg/dL or higher along with clear symptoms. In that case, one test is usually enough for a diagnosis.
If your results fall in the prediabetes range, that’s actually useful information. Prediabetes is reversible in many cases through changes in diet, physical activity, and weight management. Your provider will likely recommend retesting every one to three years to monitor whether your blood sugar stays stable or trends upward.
Testing for Type 1 vs. Type 2
The blood sugar tests above identify diabetes but don’t automatically tell you which type. Type 2 accounts for the vast majority of cases and is driven by insulin resistance. Type 1 is an autoimmune condition where the body attacks the cells that produce insulin, and it can develop at any age, not just in childhood.
If your provider suspects type 1, they’ll order an autoantibody panel. This checks for immune markers that attack insulin-producing cells. A combined panel testing four specific autoantibodies detects autoimmune diabetes in about 98% of newly diagnosed type 1 cases. Fewer than 3% of people with type 2 diabetes test positive for these antibodies, so the panel is a reliable way to distinguish between the two types. The more antibodies that come back positive, the stronger the likelihood of type 1.
Testing During Pregnancy
Gestational diabetes is screened separately, typically between 24 and 28 weeks of pregnancy. The most common approach is a two-step process. First, you drink a smaller glucose solution and have your blood drawn one hour later. No fasting is needed for this initial screen. If your blood sugar comes back high, you return for a longer, more involved test.
The follow-up requires fasting for 8 to 14 hours. You drink a solution containing 100 grams of glucose, and your blood is drawn four times: before the drink and then at one, two, and three hours after. The whole appointment takes about three and a half hours. Some providers use a one-step approach instead, which involves a 75-gram glucose drink and takes about two hours with three blood draws.
Where to Get Tested and What It Costs
Your primary care provider can order any of these tests, and the blood draw is typically done at their office or at a nearby lab. You don’t need to see a specialist to get screened.
Most health insurance plans cover diabetes screening with no out-of-pocket cost, especially if you have risk factors. Medicare Part B covers up to two diabetes screenings per year at no cost to you, as long as your provider identifies at least one risk factor like high blood pressure, obesity, abnormal cholesterol, a history of high blood sugar, or a family history of diabetes. If you’re 65 or older and overweight with a family history, you also qualify.
Community health centers and free clinics often provide low-cost or free screenings as well, particularly during awareness events. If cost is a barrier, these can be a practical option.
At-Home Tests: Useful but Limited
You can buy A1C test kits over the counter at most pharmacies. These involve a finger prick and give you a result at home. They can be a helpful way to get a general sense of where your blood sugar stands, but they aren’t accurate enough to diagnose diabetes. Point-of-care A1C devices miss roughly 18% of cases that a lab test would catch. The FDA has stated that over-the-counter A1C tests should not be used for diagnosis.
Home blood glucose meters, the kind people with diabetes use daily, can tell you what your blood sugar is at a given moment, but a single reading doesn’t carry the same diagnostic weight as a formal lab test. If a home test gives you a concerning number, treat it as a reason to schedule a proper lab test rather than a diagnosis in itself.