Several blood tests can show whether you have diabetes, but the three most common are the A1C test, the fasting blood sugar test, and the oral glucose tolerance test. Each measures blood sugar in a different way, and any one of them can be used to diagnose diabetes or prediabetes. A doctor will typically confirm the result with a second test before making a formal diagnosis.
The A1C Test
The A1C test (sometimes called hemoglobin A1C or HbA1C) is one of the most widely used tests for diagnosing diabetes because it doesn’t require fasting and gives a bigger-picture view of your blood sugar. Instead of measuring what your blood sugar is right now, it measures how much sugar has attached to your red blood cells over the past three months. Red blood cells live about 90 days, so the sugar coating on them reflects your average blood sugar during that window.
The results come back as a percentage:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
One important caveat: the A1C test can give misleading results if you have certain blood conditions. Iron-deficiency anemia, chronic kidney disease, sickle cell disease, or anything that changes how long your red blood cells survive can throw off the reading. If you have sickle cell disease (specifically HbSS, HbCC, or HbSC), the A1C test should not be used at all. In those cases, your doctor will rely on one of the other blood sugar tests instead.
Fasting Blood Sugar Test
The fasting plasma glucose test measures your blood sugar after you haven’t eaten for a set period. You’ll need to fast for 8 to 12 hours beforehand. Plain water is fine during the fast, but skip flavored or carbonated water, which can contain sugars or sweeteners that affect the result.
The thresholds are straightforward:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
Because this test requires fasting, it’s usually scheduled as a morning appointment. It’s a single blood draw and results typically come back quickly. If your number falls in the diabetes range, your doctor will usually repeat the test on a different day to confirm.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) checks how well your body handles a large dose of sugar. You fast overnight, have your blood drawn, then drink a solution containing 75 grams of glucose (roughly the sugar equivalent of two cans of soda). Two hours later, your blood is drawn again.
Your two-hour result tells the story:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
This test takes longer and is less convenient than the others, so it’s not used as often for routine screening. It does, however, catch some cases of diabetes that a fasting test alone might miss, because it reveals how your body processes sugar in real time rather than just measuring a resting level.
Random Blood Sugar Test
If you’re already showing symptoms of diabetes, like excessive thirst, frequent urination, or unexplained weight loss, your doctor may skip the fasting requirement and order a random blood sugar test. This can be taken at any time of day regardless of when you last ate. A result of 200 mg/dL or higher, combined with symptoms, points to diabetes. This test is most useful in urgent situations where waiting for a fasting test isn’t practical.
Screening for Gestational Diabetes
Pregnant women undergo a slightly different version of glucose testing, usually between 24 and 28 weeks of pregnancy. The process typically happens in two steps. First, you drink a smaller glucose solution and have your blood drawn one hour later. If that result is 140 mg/dL or higher, you return for a longer version of the oral glucose tolerance test, this time with blood draws at fasting, one hour, two hours, and three hours. High readings at two or more of those time points confirm gestational diabetes.
Women with risk factors like obesity, a family history of diabetes, or a previous pregnancy with gestational diabetes may be screened earlier than 24 weeks.
Tests That Tell Type 1 From Type 2
The tests above confirm that diabetes is present, but they don’t distinguish between type 1 and type 2. That distinction matters because the two conditions have different causes and require different treatment approaches. Type 1 diabetes is an autoimmune disease where the immune system destroys the cells in the pancreas that make insulin. Type 2 is a metabolic condition where the body still makes insulin but can’t use it effectively.
To tell them apart, doctors use two additional tools. The first is an autoantibody test, which looks for immune proteins that attack insulin-producing cells. The most commonly measured is called GAD antibody. If autoantibodies are present, it strongly suggests type 1 or an autoimmune form of diabetes. The second tool is a C-peptide test, which measures how much insulin your pancreas is actually producing. Very low C-peptide levels suggest type 1 diabetes, while normal or high levels point toward type 2. These tests are especially useful when the diagnosis isn’t obvious, such as in adults who develop diabetes later in life but don’t fit the typical type 2 profile.
Urine Tests and Their Role
Urine tests don’t diagnose diabetes on their own, but they play a supporting role. A urine ketone test measures whether your body is breaking down fat for energy instead of using blood sugar. This happens when your cells can’t access glucose, either because there isn’t enough insulin (common in type 1 diabetes) or during periods of very high blood sugar.
High ketone levels can signal diabetic ketoacidosis, a serious and potentially life-threatening complication. Early symptoms include extreme thirst, frequent urination, and dehydration. More severe signs include nausea, abdominal pain, confusion, and fruity-smelling breath. Ketone testing is primarily a monitoring tool for people already diagnosed with diabetes, particularly type 1, rather than a way to detect diabetes for the first time.
Which Test You’re Most Likely to Get
For routine screening, most doctors start with either the A1C or the fasting blood sugar test. Both are simple, require just one blood draw, and have well-established cutoff numbers. The A1C is often preferred because you don’t need to fast, making it easier to fit into a regular office visit. If you have a condition that makes A1C unreliable, your doctor will default to a fasting or glucose tolerance test instead.
Regardless of which test is used first, an abnormal result almost always gets confirmed with a second test. That second test can be the same one repeated on a different day, or a different test entirely. Two separate abnormal results are the standard for a formal diabetes diagnosis.