The only reliable way to find out if you have diabetes is through a blood test ordered by your doctor. You cannot diagnose diabetes from symptoms alone, and many people with type 2 diabetes have no noticeable symptoms at all. Three standard blood tests can give you a definitive answer, and most require little more than a blood draw at a lab or doctor’s office.
Symptoms That Should Prompt Testing
Diabetes raises blood sugar levels, and when those levels stay high, your body starts showing predictable signs. The most common are frequent urination, increased thirst, and increased hunger. These three tend to travel together: excess sugar in your blood pulls water from your tissues (making you thirsty), your kidneys work overtime to filter that sugar out (making you urinate more), and your cells struggle to absorb energy from food (making you hungrier).
Other warning signs include unexplained weight loss, blurry vision, slow-healing cuts or sores, tingling or numbness in your hands or feet, and fatigue that doesn’t improve with rest. Type 1 diabetes tends to produce symptoms quickly, sometimes within weeks. Type 2 diabetes is far more gradual. Blood sugar can creep upward for years without symptoms obvious enough to notice, which is why screening matters even when you feel fine.
Who Should Get Screened Without Symptoms
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). This threshold drops to a BMI of 23 for Asian Americans, who face higher risk at lower body weights. Screening should also start earlier than 35 if you are American Indian, Alaska Native, Black, Hispanic/Latino, or Native Hawaiian/Pacific Islander, since these groups have disproportionately higher rates of diabetes.
Other risk factors that warrant earlier or more frequent screening include a family history of diabetes, a history of gestational diabetes, polycystic ovary syndrome, or a sedentary lifestyle. If any of these apply to you, bring it up with your doctor rather than waiting for symptoms.
The Three Main Diagnostic Tests
Doctors use three primary blood tests to diagnose diabetes. Any one of them can confirm a diagnosis, and each measures blood sugar in a slightly different way.
A1C Test
The A1C test measures your average blood sugar over the past three months. It works by checking how much sugar has attached to your red blood cells, which gives a longer-term picture than a single blood sugar reading. No fasting is required. You can eat and drink normally beforehand. An A1C of 6.5% or higher indicates diabetes. Results between 5.7% and 6.4% fall into the prediabetes range.
The A1C test is not always accurate for everyone. Certain blood conditions, some types of anemia, and hemoglobin variants can skew results. Pregnancy (particularly the second and third trimesters) can also affect accuracy. If any of these apply to you, your doctor will likely use one of the other tests instead.
Fasting Plasma Glucose Test
This test measures your blood sugar at a single point in time, after you’ve fasted for at least eight hours. Fasting means nothing to eat or drink except sips of water, so the test is typically done first thing in the morning. A result of 126 mg/dL or higher indicates diabetes. Results between 100 and 125 mg/dL indicate prediabetes.
Oral Glucose Tolerance Test
The oral glucose tolerance test checks how your body handles a sugar load. After fasting, you drink a liquid containing a measured amount of glucose, then have your blood drawn two hours later. A two-hour result of 200 mg/dL or higher indicates diabetes. Results between 140 and 199 mg/dL indicate prediabetes. This test is more involved than the others and is used less often for routine screening, though it’s standard during pregnancy.
There’s also a random plasma glucose test, which can be done at any time without fasting. A result of 200 mg/dL or higher, combined with classic symptoms like excessive thirst and frequent urination, is enough to diagnose diabetes.
Why One Test Usually Isn’t Enough
A single abnormal result typically needs to be confirmed before you receive a formal diagnosis. The standard protocol requires two abnormal results, either from the same blood sample tested two different ways or from two separate samples on different days. If you get one high result, your doctor will either repeat that same test or run a different one. If both come back above the diagnostic threshold, the diagnosis is confirmed.
The exception is when your blood sugar is very high and you already have obvious symptoms. In that case, your doctor may diagnose diabetes based on a single test.
What About At-Home Test Kits?
Over-the-counter A1C test kits exist, but their accuracy is mixed. A University of Florida Health study tested three popular home A1C kits against lab results from 219 people with diabetes. The benchmark for accuracy was that at least 90% of samples should fall within 5% of the lab result. Only one kit (Home Access) came close, with 82% of samples meeting that standard. The other two kits hit the mark in just 46% and 29% of samples.
An inaccurately high reading could cause unnecessary alarm, while a falsely low result could give you a dangerous sense of security. Home kits can offer a rough sense of where you stand, but they are not a substitute for a lab test. If a home test shows a concerning result, follow up with your doctor for proper testing.
Screening During Pregnancy
Gestational diabetes develops during pregnancy and is screened separately from type 1 or type 2. Most pregnant women are tested between 24 and 28 weeks. The process involves drinking a glucose solution (either 75 or 100 grams, depending on the testing approach) and then having blood drawn to see how your body processes the sugar. Your OB will typically include this as part of routine prenatal care, so you don’t need to request it.
What a Prediabetes Result Means
If your results land in the prediabetes range, that’s not a diabetes diagnosis, but it is a clear warning. Prediabetes means your blood sugar is higher than normal and trending in the wrong direction. The good news: lifestyle changes can cut your risk of progressing to type 2 diabetes by 58%, and by 71% if you’re over 60.
The changes involved are modest but meaningful. Losing 5% to 7% of your body weight (10 to 14 pounds for someone who weighs 200) and getting 150 minutes of brisk walking or similar activity per week is enough to make a significant difference. The CDC runs a Diabetes Prevention Program specifically designed around these targets, available in person and online across the country.
Type 1 vs. Type 2: Further Testing
The standard blood tests diagnose diabetes but don’t always make clear which type you have. For most adults diagnosed later in life with risk factors like excess weight, type 2 is assumed. But when the picture isn’t obvious, particularly in younger or leaner patients, doctors can run autoantibody tests to check for an immune system attack on the pancreas, which is the hallmark of type 1 diabetes.
These tests look for specific antibodies that target insulin-producing cells. The presence of multiple antibodies strongly suggests type 1 diabetes or a high likelihood of developing it. Negative results don’t completely rule out type 1, but they make type 2 far more likely. Getting the type right matters because the treatment approaches differ significantly.