Diabetes is diagnosed through blood tests that measure how much sugar is in your blood, either at a single moment or averaged over several months. There are four main tests doctors use, and in most cases, you’ll need two abnormal results before receiving a formal diagnosis. Which test your doctor orders depends on your symptoms, your risk factors, and whether you’ve been fasting.
The A1C Test
The A1C test is one of the most common diabetes screening tools because it doesn’t require fasting or any special preparation. It measures your average blood sugar over the past two to three months by looking at the percentage of your red blood cells that have sugar attached to them.
The results break down into three categories:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
Because it reflects a long-term average rather than a single snapshot, the A1C is less affected by what you ate the night before or how stressed you felt that morning. The test needs to be run in a certified lab to count toward a diagnosis. At-home A1C kits exist, but they aren’t considered diagnostic on their own.
Fasting Plasma Glucose Test
This is a straightforward blood draw taken after you haven’t eaten for at least 8 hours. Most people schedule it first thing in the morning so the fasting period overlaps with sleep. During the fast, you can drink plain water but nothing else: no coffee, juice, flavored water, or gum. You should also avoid smoking and exercise before the test, since both can shift your blood sugar.
The thresholds are:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
If your result lands right near a cutoff, your doctor will typically repeat the test in three to six months rather than diagnose you based on a borderline number.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) is more involved. You fast overnight, then have your blood drawn. After that first draw, you drink a sugary solution containing 75 grams of glucose, roughly the equivalent of drinking two cans of soda at once. Two hours later, your blood is drawn again to see how efficiently your body processed all that sugar.
A two-hour result of 200 mg/dL or higher indicates diabetes. Results between 140 and 199 mg/dL fall into the prediabetes range. This test is especially common during pregnancy to screen for gestational diabetes, though the specific glucose amounts and timing differ slightly in that context.
Because the OGTT requires multiple blood draws spread over two hours, it’s less convenient than the A1C or fasting glucose test. Doctors usually reserve it for situations where other test results are inconclusive or when screening during pregnancy.
Random Plasma Glucose Test
If you’re already showing obvious symptoms of diabetes, like frequent urination, extreme thirst, unexplained weight loss, or blurred vision, your doctor can diagnose you with a random blood sugar test taken at any time of day, regardless of when you last ate. A result of 200 mg/dL or higher, combined with those classic symptoms, is enough for a diagnosis without a second confirmatory test.
This is the test most likely to be used in urgent care or emergency settings, where waiting for an overnight fast isn’t practical and symptoms make the diagnosis fairly clear.
When Confirmation Is Required
Outside of that emergency scenario, a single abnormal test result isn’t enough. The American Diabetes Association’s 2025 guidelines require two abnormal results before diagnosing diabetes in someone without obvious symptoms. Those two results can come from different tests done on the same day (for example, an A1C and a fasting glucose drawn at the same visit) or from the same test repeated on two different days.
If your initial screening comes back normal, repeat testing every three years is generally reasonable. But if you gain weight, develop new risk factors, or notice symptoms like increased thirst or frequent urination, earlier retesting makes sense.
Who Should Get Tested
Current guidelines recommend that all adults begin screening at age 35, even without symptoms. Testing should happen earlier, at any age, if you’re overweight or obese and have at least one additional risk factor such as a family history of diabetes, a history of gestational diabetes, high blood pressure, or physical inactivity. For children and adolescents, risk-based screening is recommended after age 10 or after puberty begins, whichever comes first, if they are overweight or obese with additional risk factors.
Tests That Distinguish Type 1 From Type 2
The tests above tell you whether you have diabetes, but not which type. That distinction matters because type 1 and type 2 diabetes have different causes and require different treatment approaches. Two additional tests help sort this out.
Autoantibody Tests
Type 1 diabetes is an autoimmune condition where the immune system attacks the insulin-producing cells in the pancreas. Autoantibody blood tests detect the specific immune proteins responsible for that attack. Doctors typically test for at least two autoantibodies at once, because finding multiple positive results raises the likelihood of type 1 diabetes to above 90%. The most commonly tested antibody targets an enzyme called GAD, usually paired with one or more others.
These tests are particularly useful when the diagnosis isn’t obvious, such as an adult who develops diabetes without the typical risk profile for type 2, or when someone initially diagnosed with type 2 isn’t responding to standard treatment. A form of type 1 diabetes called LADA (latent autoimmune diabetes in adults) can look like type 2 at first but progresses differently, and autoantibody testing is the main way to catch it.
C-Peptide Test
C-peptide is a molecule your pancreas releases in equal amounts alongside insulin. Measuring it tells your doctor how much insulin your body is still producing on its own. Low C-peptide levels suggest the pancreas isn’t making enough insulin, which is the hallmark of type 1 diabetes. High levels point toward type 2, where the body produces plenty of insulin (sometimes too much) but can’t use it effectively. Together with autoantibody results, the C-peptide test gives a clear picture of which type of diabetes you’re dealing with.
Urine Tests and Ketone Monitoring
Urine tests don’t diagnose diabetes directly, but they play an important role in detecting a dangerous complication called diabetic ketoacidosis, or DKA. When your body can’t get enough energy from glucose, it starts breaking down fat at a rapid rate, producing acids called ketones. Small amounts of ketones are normal during fasting or intense exercise, but in someone with uncontrolled diabetes, ketone levels can climb to life-threatening levels.
Urine ketone tests are most often used for people already diagnosed with type 1 diabetes, though DKA can also occur in type 2. Early warning signs include excessive thirst, frequent urination, and headache. If ketone levels keep rising, symptoms escalate to nausea, abdominal pain, confusion, difficulty breathing, and a distinctive fruity smell on the breath. Those severe symptoms require immediate medical attention.