Type 2 diabetes is diagnosed with a blood test that measures how much sugar is in your blood. There are several tests that can do this, and each has a specific threshold that separates normal blood sugar from prediabetes and diabetes. In most cases, your doctor will need two abnormal results before confirming a diagnosis.
The Three Main Diagnostic Tests
Any of three blood tests can be used to diagnose type 2 diabetes. Your doctor may choose one based on convenience, your health history, or whether certain conditions could affect accuracy.
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. An A1C below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes. This test doesn’t require fasting, so it can be done at any time of day.
The fasting blood sugar test measures your blood glucose after you haven’t eaten for at least eight hours. A result under 100 mg/dL is normal, 100 to 125 mg/dL falls in the prediabetes range, and 126 mg/dL or higher indicates diabetes.
The oral glucose tolerance test checks how your body handles a large dose of sugar. You drink a sweet liquid containing 75 grams of glucose, then have your blood drawn two hours later. A reading under 140 mg/dL is normal, 140 to 199 mg/dL suggests prediabetes, and 200 mg/dL or higher points to diabetes. This test takes longer and is less commonly used for routine screening, but it catches some cases that fasting tests miss.
Why You Usually Need Two Tests
A single abnormal result isn’t enough for a diagnosis. Guidelines require two abnormal results, either from the same blood sample using different tests or from two separate blood draws. If your doctor runs two different tests and the results conflict (one says diabetes, one doesn’t), the test that came back above the diagnostic threshold gets repeated. The diagnosis is based on whichever result can be confirmed.
There is one exception. If your blood sugar is 200 mg/dL or higher on a random (non-fasting) test and you already have classic symptoms of diabetes, like frequent urination, extreme thirst, unexplained weight loss, or blurred vision, that combination can be enough for a diagnosis without a second test.
How to Prepare for Testing
If your test requires fasting, you’ll need to avoid all food and drinks (except plain water) for 8 to 12 hours beforehand. Plain water is fine and encouraged, but skip flavored water, coffee, juice, or anything with sugar or lemon. You should also avoid smoking, chewing gum, and exercise during the fasting window, as all of these can affect your results.
Most prescribed medications are safe to continue taking before a fasting test, but check with your doctor if you’re unsure, especially about over-the-counter supplements or vitamins. The A1C test requires no preparation at all.
When A1C Results May Not Be Reliable
The A1C test relies on red blood cells behaving normally, so anything that changes how long your red blood cells live or how they carry oxygen can throw off the results. Conditions like sickle cell disease or other hemoglobin variants, significant blood loss, recent blood transfusions, iron deficiency anemia, and kidney disease can all make A1C readings inaccurate. Pregnancy also affects A1C reliability, particularly in the second and third trimesters. If any of these apply to you, your doctor will likely use a fasting glucose test or oral glucose tolerance test instead.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends that adults aged 35 to 70 who are overweight or obese get screened for type 2 diabetes and prediabetes. But several factors can push that timeline earlier. If you’re American Indian, Alaska Native, Black, Hispanic or Latino, or Native Hawaiian or Pacific Islander, screening at a younger age makes sense because these populations have significantly higher rates of type 2 diabetes.
Body weight thresholds also vary by ethnicity. For most adults, a BMI of 25 or higher is considered overweight. For Asian Americans, research shows that diabetes risk rises at a lower BMI, so a cutoff of 23 is more appropriate. If you have a family history of diabetes, a history of gestational diabetes, or polycystic ovary syndrome, those are additional reasons to get tested earlier and more frequently.
What Happens After a Prediabetes Result
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 yet diabetic, but your blood sugar is higher than it should be. Prediabetes means your body is starting to lose the ability to manage glucose efficiently. Without changes, roughly 15% to 30% of people with prediabetes develop type 2 diabetes within five years.
The practical value of catching prediabetes is that it’s often reversible. Losing 5% to 7% of your body weight and getting about 150 minutes of moderate physical activity per week (like brisk walking) can cut the risk of progression by more than half. Your doctor will typically recheck your blood sugar every one to three years to track whether your levels are improving, holding steady, or moving toward diabetes.