The only way to confirm whether you have diabetes is through a blood test ordered by a healthcare provider or, as a first step, an at-home screening kit. You can’t diagnose diabetes from symptoms alone, but recognizing the warning signs can tell you it’s time to get tested. Three standard blood tests can give you a clear answer, and most require little more than skipping breakfast.
Symptoms That Should Prompt Testing
Diabetes often develops gradually, and many people have elevated blood sugar for months or years before noticing anything wrong. When symptoms do appear, the most common ones are excessive thirst, frequent urination, and unusual fatigue. These happen because your body can’t move sugar from your blood into your cells efficiently, so it tries to flush the excess sugar out through urine, pulling water with it and leaving you dehydrated and drained.
Other signs worth paying attention to:
- Unexplained weight loss, especially if you haven’t changed your diet or exercise habits
- Blurry vision, caused by high blood sugar pulling fluid from the lenses of your eyes
- Slow-healing cuts or sores, particularly on the feet
- Tingling, numbness, or burning pain in the hands and feet
- Frequent infections, including yeast infections and gum infections
You don’t need all of these to warrant testing. Even one or two, especially the thirst-and-urination combination, is reason enough to schedule a blood draw.
Who Should Get Screened Without Symptoms
You don’t have to feel sick to have diabetes. The U.S. Preventive Services Task Force recommends screening for adults aged 35 to 70 who are overweight (BMI of 25 or higher) or obese, even if they feel perfectly fine. If you’re Asian American, screening is recommended at a lower BMI threshold of 23, because type 2 diabetes tends to develop at a lower body weight in this population.
Earlier screening is also recommended if you’re American Indian, Alaska Native, Black, Hispanic or Latino, or Native Hawaiian/Pacific Islander, all groups with a disproportionately higher prevalence of diabetes. A family history of diabetes, a personal history of gestational diabetes, or a diagnosis of polycystic ovary syndrome are additional reasons to test sooner rather than later.
The Three Diagnostic Blood Tests
Doctors use three tests to diagnose diabetes, and any one of them can give a definitive answer. Each measures blood sugar in a slightly different way.
A1C (Hemoglobin A1C)
This is often the most convenient test because it doesn’t require fasting. 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. A normal result is below 5.7%. A reading of 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
One important caveat: A1C results can be inaccurate if you have certain hemoglobin variants, including sickle cell trait. Some lab methods produce falsely high or low readings in these cases, which could lead to a missed diagnosis or an incorrect one. If you carry sickle cell trait or another hemoglobin variant, your provider will likely rely on one of the other two tests instead.
Fasting Plasma Glucose
This test measures your blood sugar after you haven’t eaten for at least 8 hours (up to 12 hours, depending on the lab). You can drink plain water during the fast, but skip coffee, juice, soda, and flavored water, as these can affect results. A normal fasting level is below 100 mg/dL. Between 100 and 125 mg/dL falls in the prediabetes range. A result of 126 mg/dL or higher indicates diabetes.
Most people schedule this test first thing in the morning so the fasting period overlaps with sleep.
Oral Glucose Tolerance Test
This test is more involved but gives a detailed picture of how your body handles sugar in real time. You fast for eight hours, then have your blood drawn. After that, you drink a sweet solution containing 75 grams of sugar. Your blood is drawn again at the one-hour and two-hour marks. A two-hour result below 140 mg/dL is normal. Between 140 and 199 mg/dL signals prediabetes, and 200 mg/dL or higher confirms diabetes.
The oral glucose tolerance test is used less often for routine screening because it takes about two and a half hours in the clinic. It’s more commonly used during pregnancy to check for gestational diabetes, where a longer three-hour version with a higher sugar load is standard.
What Happens After an Abnormal Result
A single abnormal reading doesn’t always seal the diagnosis. Unless your blood sugar is very high and you already have classic symptoms, your provider will typically repeat the test or run a second type of test to confirm. This is because blood sugar can fluctuate from stress, illness, or other temporary factors.
Once diabetes is confirmed, the next question is what type you have. Most adults with diabetes have type 2, where the body still produces insulin but doesn’t use it efficiently. Type 1 diabetes, where the immune system destroys the insulin-producing cells in the pancreas, is less common and can appear at any age. A small number of adults develop a slow-progressing form of type 1 that’s sometimes called LADA (latent autoimmune diabetes in adults), which can initially look like type 2.
To tell the difference, your provider may order additional blood work. A C-peptide test measures how much insulin your pancreas is producing. Low C-peptide levels suggest type 1. Antibody tests can detect whether your immune system is attacking your own pancreas, which is the hallmark of type 1 and LADA. These tests are done alongside a glucose measurement and are especially important if you’re younger, lean, or not responding as expected to standard type 2 treatments.
At-Home A1C Kits: Helpful but Limited
Over-the-counter A1C test kits are available at most pharmacies and can give you a general sense of where your blood sugar stands. However, their accuracy varies significantly. A University of Florida study of 219 people with diabetes found that only one of three tested home kits (Home Access) came close to lab-grade accuracy, with 82% of its samples falling within 5% of a standard lab draw. The other two kits hit that accuracy benchmark in only 46% and 29% of samples.
A home kit can be a reasonable first step if you’re curious or nervous about visiting a doctor, but it’s not a substitute for a lab test. If your home result comes back in the prediabetes or diabetes range, follow up with a clinical blood draw. And if your result is normal but you have symptoms, don’t assume you’re in the clear.
Prediabetes: The In-Between Zone
If your numbers land in the prediabetes range (A1C of 5.7% to 6.4%, fasting glucose of 100 to 125 mg/dL, or a two-hour glucose tolerance result of 140 to 199 mg/dL), you’re not diabetic yet, but your blood sugar is higher than it should be. Roughly 80% of people with prediabetes don’t know they have it.
Prediabetes is not a death sentence for your pancreas. It’s the stage where lifestyle changes have the most impact. Losing even 5% to 7% of your body weight and getting about 150 minutes of moderate activity per week can reduce the risk of progressing to type 2 diabetes by more than half. Your provider will likely recommend retesting every one to three years to track whether your numbers are improving, holding steady, or climbing.