Finding out if you have diabetes starts with a blood test, usually ordered by your doctor based on symptoms or routine screening. There is no single “diabetes test.” Instead, doctors use one of several blood tests, each measuring blood sugar in a different way, and each with specific cutoff numbers that separate normal results from prediabetes and diabetes.
Symptoms That Should Prompt Testing
Many people with type 2 diabetes have no obvious symptoms for years, which is why routine screening matters. But certain warning signs are strong reasons to get tested sooner rather than later:
- Increased thirst and frequent urination. When blood sugar is high, your kidneys work harder to filter the excess, pulling more water from your body.
- Hunger that persists after eating. Your cells can’t efficiently use glucose for energy, so your body signals for more food.
- Unexplained weight loss. This is especially common in type 1 diabetes, where the body starts burning fat and muscle for fuel.
- Blurred vision. High blood sugar causes fluid shifts in the lens of the eye.
- Fatigue. Without proper glucose uptake, energy levels drop.
- Slow-healing sores or frequent infections. Recurring yeast infections, urinary tract infections, or skin infections can all be early clues.
Type 2 diabetes sometimes reveals itself through complications that have already developed: numbness or tingling in the feet, vision loss, or sexual problems. Type 1 diabetes tends to come on faster and more dramatically, occasionally showing up as a medical emergency called diabetic ketoacidosis, which causes fruity-smelling breath, abdominal pain, vomiting, and difficulty breathing.
The A1C Test
The A1C test (also called hemoglobin A1C) is one of the most common ways to screen for diabetes. It measures your average blood sugar over the previous two to three months by looking at how much sugar has attached to your red blood cells. You don’t need to fast beforehand, which makes it convenient.
The results break down into three categories:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or above: Diabetes
A result in the diabetes range is typically confirmed with a second test on a different day before a formal diagnosis is made.
Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten for 8 to 12 hours, usually overnight. You’ll have blood drawn first thing in the morning. A fasting blood sugar of 126 mg/dL or above indicates diabetes. Your doctor will tell you exactly how long to fast before the appointment.
Because it requires fasting, this test captures your baseline blood sugar level, the number your body settles at when it hasn’t had to process any food. If that baseline is already elevated, it means your body isn’t managing glucose properly even at rest.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) is more involved but gives your doctor a detailed picture of how your body handles sugar in real time. You fast overnight, have your blood drawn, then drink a solution containing 75 grams of glucose (roughly equivalent to the sugar in two cans of soda). Your blood is drawn again two hours later.
The two-hour reading determines where you fall:
- Below 140 mg/dL: Normal
- 140 to 199 mg/dL: Prediabetes (sometimes called impaired glucose tolerance)
- 200 mg/dL or above: Diabetes
This test is particularly useful for catching cases where fasting levels look borderline but the body struggles to process a sugar load. It takes longer than other tests, so it’s used less often for routine screening but remains a definitive diagnostic tool.
Random Blood Sugar Test
If you’re showing obvious symptoms of diabetes, your doctor may not wait for a fasting appointment. A random blood sugar test can be done at any time, regardless of when you last ate. A result of 200 mg/dL or above, combined with classic symptoms like excessive thirst and frequent urination, is enough to diagnose diabetes on the spot.
How Type 1 and Type 2 Are Distinguished
The blood sugar tests above can tell you that you have diabetes, but they can’t tell you which type. That distinction matters because type 1 and type 2 have different causes and require different treatment approaches.
Type 1 diabetes is an autoimmune condition where your immune system attacks the cells in the pancreas that produce insulin. To confirm type 1, doctors order autoantibody blood tests that look for specific immune markers. The most commonly tested are antibodies against GAD65, IA-2, insulin itself, and a protein called zinc transporter 8. If one or more of these autoantibodies is present, the diagnosis points toward type 1. Most commercial labs can run these tests.
Type 2 diabetes, which accounts for roughly 90 to 95% of all diabetes cases, involves the body becoming resistant to insulin rather than destroying the cells that make it. It’s diagnosed primarily through the blood sugar tests described above, and the absence of autoantibodies helps confirm it’s not type 1. Age, weight, and family history also factor into the clinical picture.
Screening During Pregnancy
Gestational diabetes develops during pregnancy and is screened for separately. Most pregnant women are tested between 24 and 28 weeks. Women with higher risk factors, such as obesity or a family history of diabetes, may be screened at their first prenatal visit using the same criteria as the general population (fasting glucose of 126 mg/dL or above, or an A1C of 6.5% or above).
The standard screening in the United States uses a two-step process. First, you drink a smaller sugar solution (50 grams) and have your blood drawn one hour later. If your blood sugar comes back at 140 mg/dL or above, you move to the second step: a longer three-hour test with a larger sugar load (100 grams) and blood draws at one, two, and three hours. Two or more elevated readings on this second test confirm gestational diabetes. Symptoms of gestational diabetes tend to be mild, often just increased thirst or more frequent urination, so the screening test is the primary way it’s caught.
Home Test Kits and Their Limits
You can buy home A1C test kits over the counter, and they may seem like an easy way to check yourself without a doctor’s visit. But their accuracy falls well short of lab standards. A 2022 study evaluating three commercial home A1C kits found that the best-performing brand only matched lab results within an acceptable margin 82% of the time. The other two matched just 46% and 29% of the time. None of the three met the national standardization goal of 90% accuracy.
That means a home kit could show you a normal result when your actual A1C is in the prediabetes or diabetes range, or vice versa. Home glucose meters (the kind people with diabetes use daily) are useful for monitoring but aren’t designed for initial diagnosis either. If you’re concerned about diabetes, a lab-based blood test is the only reliable way to get an answer.
Who Should Get Screened
You don’t need symptoms to get tested. The American Diabetes Association recommends screening for all adults starting at age 35, repeated every three years if results are normal. Testing should start earlier if you have risk factors: being overweight, having a parent or sibling with diabetes, a history of gestational diabetes, or belonging to a racial or ethnic group with higher rates of type 2 diabetes (including Black, Hispanic, Native American, Asian American, and Pacific Islander populations).
Children and teenagers who are overweight and have additional risk factors like a family history of type 2 diabetes should also be screened, starting at age 10 or at the onset of puberty. For type 1 diabetes, which can appear at any age but often starts in childhood, testing is typically triggered by symptoms rather than routine screening, though autoantibody testing is increasingly used to identify children at risk before symptoms develop.