Doctors diagnose diabetes with blood tests that measure how much sugar is in your blood, either at a single moment or averaged over the past few months. Three main tests are used, each with a specific cutoff number that separates normal blood sugar from prediabetes and diabetes. In most cases, you’ll need two abnormal results before a diagnosis is confirmed.
The Three Main Diagnostic Tests
Any of the following tests can be used to diagnose diabetes. Your doctor will choose based on your situation, but the A1C and fasting glucose tests are the most common starting points.
A1C test. This blood draw 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. No fasting is required. An A1C below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
Fasting plasma glucose test. You fast overnight (at least eight hours), then have your blood drawn. A result of 99 mg/dL or below is normal, 100 to 125 mg/dL falls in the prediabetes range, and 126 mg/dL or higher indicates diabetes.
Oral glucose tolerance test (OGTT). After fasting, you drink a sugary solution containing 75 grams of glucose, and your blood is drawn two hours later. A reading of 200 mg/dL or higher at the two-hour mark indicates diabetes. This test is more time-consuming, so it’s used less often for routine screening, but it’s particularly useful for detecting gestational diabetes during pregnancy.
Why Doctors Usually Require Two Results
A single abnormal result generally isn’t enough. Diagnosis requires two abnormal test results, either from the same blood sample using two different tests or from repeating the same test on a separate occasion. Blood sugar naturally fluctuates day to day, and someone in the early stages of diabetes may not test above the threshold every time. Requiring confirmation reduces the chance of a false diagnosis.
If you get two different tests and the results conflict (say, your A1C is in the diabetes range but your fasting glucose is normal), your doctor will repeat whichever test came back above the cutoff. The diagnosis is based on the confirmed test.
There is one exception. If your blood sugar is 200 mg/dL or higher on a random (non-fasting) blood draw and you already have classic symptoms of diabetes, like excessive thirst, frequent urination, and unexplained weight loss, a single result is enough to confirm the diagnosis. This scenario is most common with type 1 diabetes in children and young adults, where symptoms tend to appear suddenly.
When the A1C Test Can Be Unreliable
The A1C test works by measuring sugar attached to hemoglobin, the protein inside red blood cells. Anything that changes the lifespan of your red blood cells or alters your hemoglobin can throw off the result.
Conditions that may cause inaccurate A1C readings include sickle cell disease, iron-deficiency anemia, recent blood loss or transfusion, kidney failure, and liver disease. Iron deficiency, specifically, can produce a falsely high A1C, potentially leading to an incorrect diabetes diagnosis.
People of African, Mediterranean, or Southeast Asian descent are more likely to carry hemoglobin variants (different forms of the hemoglobin protein) that can interfere with certain A1C tests. If you fall into one of these groups, your doctor may rely on fasting glucose or the oral glucose tolerance test instead.
How Type 1 and Type 2 Are Told Apart
The blood sugar tests described above can diagnose diabetes, 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.
Type 1 diabetes is an autoimmune disease where your immune system attacks the cells in your pancreas that produce insulin. To confirm this, doctors test for specific antibodies in your blood. Five antibodies are commonly checked: GAD, insulin antibodies, IA-2, islet cell antibodies, and ZnT8. Testing positive for two or more of these antibodies means there’s a greater than 90% chance the diabetes is type 1.
Most doctors start by testing for GAD antibodies, the most common marker, and then check for additional antibodies if that first test is positive. This testing is especially helpful in adults, where type 1 can be harder to identify. Some adults with type 1 are initially misdiagnosed with type 2 because they develop symptoms more gradually than children typically do. Updated guidelines now specifically recommend antibody testing and a clearer process for investigating suspected type 1 diabetes in adults.
One practical note: if you’ve already been taking insulin for more than two weeks, the insulin antibody test becomes unreliable, since the test can’t distinguish between antibodies against your own insulin and antibodies triggered by the injected insulin.
Screening for Gestational Diabetes
Gestational diabetes develops during pregnancy in someone who didn’t have diabetes before. Screening typically happens between 24 and 28 weeks of pregnancy, using a two-step process that’s standard in the United States.
In the first step, you drink a 50-gram glucose solution and have your blood drawn one hour later. If your blood sugar is at or above the screening threshold (usually 135 or 140 mg/dL, depending on your doctor’s practice), you move to the second step: a longer test where you drink a 100-gram glucose solution and have your blood drawn at multiple intervals over three hours. Results from that second test determine whether you have gestational diabetes.
What Prediabetes Numbers Look Like
Prediabetes sits in the gap between normal and diabetic blood sugar levels. It means your blood sugar is elevated but not high enough for a diabetes diagnosis. Across all three major tests, the prediabetes ranges are:
- A1C: 5.7% to 6.4%
- Fasting glucose: 100 to 125 mg/dL
- Oral glucose tolerance test: 140 to 199 mg/dL at the two-hour mark
A prediabetes result doesn’t mean you’ll inevitably develop diabetes, but it does signal that your body is already struggling to manage blood sugar effectively. People with results in this range, particularly those at the higher end, have a significantly increased risk of progressing to type 2 diabetes. Lifestyle changes at this stage, especially weight loss and regular physical activity, can slow or reverse that progression.
It’s also worth knowing that you can get conflicting results from different tests. Your A1C might fall in the prediabetes range while your fasting glucose comes back normal. This can happen in the early stages, when blood sugar hasn’t risen consistently enough to show on every test. Your doctor will use the abnormal result to guide next steps.