There are three main blood tests used to diagnose diabetes, and any of them can confirm a diagnosis: the A1C test, the fasting blood sugar test, and the oral glucose tolerance test. Each measures blood sugar in a different way, and your doctor will choose one based on your situation. A random blood sugar test can also diagnose diabetes if you already have obvious symptoms.
The A1C Test
The A1C test is one of the most common ways to screen for and diagnose diabetes. It measures your average blood sugar over the past two to three months by looking at how much sugar has attached to your red blood cells. Unlike other tests, it doesn’t require fasting, so you can eat and drink normally beforehand.
The results come back as a percentage:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
The A1C test is convenient, but it isn’t always accurate for everyone. Conditions that affect red blood cells, like sickle cell trait or certain types of anemia, can throw off the results. If you have one of these conditions, your doctor will likely use a different test instead.
The Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten for at least 8 hours (water is fine). You’ll typically schedule a morning blood draw so the fasting period happens overnight while you sleep. A result of 126 mg/dL or higher indicates diabetes.
The fasting test gives a snapshot of your blood sugar at a single point in time, which makes it more sensitive to day-to-day variation than the A1C. That’s why doctors usually repeat an abnormal result on a separate day before making a formal diagnosis. If you ate something by mistake or had an unusually stressful night, it can nudge your numbers higher than they’d normally be.
The Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) checks how well your body handles a large dose of sugar. You fast for at least 8 hours, then drink a solution containing 75 grams of glucose. Two hours later, your blood is drawn again.
The two-hour results break down like this:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
This test is more time-consuming than the others since you’ll need to stay at the lab for at least two hours. It’s particularly useful for catching prediabetes and early diabetes that other tests might miss, because it directly challenges your body’s ability to clear sugar from the blood.
The Random Blood Sugar Test
If you’re already showing classic symptoms of high blood sugar, like frequent urination, extreme thirst, and unexplained weight loss, a doctor can draw your blood at any time of day regardless of when you last ate. A result of 200 mg/dL or higher, combined with those symptoms, is enough to diagnose diabetes on the spot. This test isn’t used for routine screening. It’s reserved for situations where symptoms are already pointing strongly toward diabetes.
How Gestational Diabetes Is Tested
Diabetes testing during pregnancy follows a different process. Most pregnant people are screened between 24 and 28 weeks with one of two approaches.
The more common method in the U.S. uses two steps. First, you drink a smaller glucose solution and have your blood drawn one hour later. If your blood sugar is elevated, you come back for a longer, more detailed test. For this second test, you fast overnight, drink a solution with 100 grams of glucose, and have your blood drawn at one hour, two hours, and three hours. Values are considered abnormal if they hit or exceed these thresholds: 95 mg/dL fasting, 180 mg/dL at one hour, 155 mg/dL at two hours, or 140 mg/dL at three hours. Meeting or exceeding two or more of these values confirms gestational diabetes.
Some providers use a one-step approach instead, which involves a single 75-gram glucose drink with blood draws at fasting, one hour, and two hours. The cutoffs are slightly different: 92 mg/dL fasting, 180 mg/dL at one hour, or 153 mg/dL at two hours. With this method, hitting just one abnormal value is enough for a diagnosis.
Tests That Tell Type 1 From Type 2
The blood sugar tests above can confirm that you have diabetes, but they don’t tell you which type. That distinction matters because type 1 and type 2 diabetes have different causes and require different treatment.
Type 1 diabetes is an autoimmune condition where the immune system attacks the cells in the pancreas that make insulin. To check for this, doctors can order autoantibody blood tests. These look for specific immune proteins that target the pancreas. There are five main ones, and the presence of multiple autoantibodies strongly suggests type 1 diabetes. This testing is especially helpful in adults, where the line between type 1 and type 2 can sometimes be unclear. If no autoantibodies are found, type 2 diabetes is the more likely diagnosis.
Urine Tests and Ketones
Urine tests don’t diagnose diabetes on their own, but they play a supporting role. The most important urine test checks for ketones, which are chemicals your body produces when it burns fat instead of sugar for energy. Small amounts of ketones are normal, but high levels signal a dangerous condition called diabetic ketoacidosis (DKA), which happens most often in type 1 diabetes.
Early warning signs of high ketones include excessive thirst, frequent urination, and dehydration. As levels climb, symptoms get more serious: nausea, belly pain, confusion, difficulty breathing, and a distinctive fruity smell on the breath. If you’ve already been diagnosed with diabetes and develop these symptoms, a urine ketone test can quickly confirm whether ketoacidosis is developing.
How to Prepare for Your Test
If your test requires fasting, you’ll need to avoid all food and drinks except plain water for 8 to 12 hours beforehand. Your doctor’s office will tell you the exact timeframe. The easiest approach is to schedule a morning appointment and stop eating after dinner the night before. The A1C and random blood sugar tests require no preparation at all.
One abnormal result usually isn’t enough for a diabetes diagnosis on its own (unless it’s a random test with clear symptoms). In most cases, your doctor will repeat the same test on a different day or use a second type of test to confirm the result. If your numbers fall in the prediabetes range, that’s valuable information too, since lifestyle changes at that stage can significantly delay or prevent type 2 diabetes from developing.