Diabetes is diagnosed through blood tests that measure how much sugar is in your blood, either at a single point in time or averaged over the past few months. The most common tests are the A1C test, the fasting blood sugar test, and the oral glucose tolerance test. Each has specific cutoff numbers that separate normal blood sugar from prediabetes and diabetes, and in most cases, an abnormal result needs to be confirmed with a second test before a formal diagnosis is made.
The A1C Test
The A1C test (also called hemoglobin A1C) measures your average blood sugar over the past two to three months. It works by checking what percentage of your hemoglobin, the oxygen-carrying protein in red blood cells, has sugar attached to it. The higher your blood sugar has been running, the more sugar-coated hemoglobin you’ll have.
The results break down into three ranges:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
The biggest advantage of the A1C test is convenience. You don’t need to fast beforehand and it can be drawn at any time of day, which makes it easier to fit into a routine appointment. It also captures a longer window of blood sugar behavior rather than a single snapshot, so one unusually good or bad day won’t skew the result.
That said, the A1C test isn’t reliable for everyone. Several conditions can throw off the results by changing how long your red blood cells survive. Sickle cell anemia, thalassemia, kidney failure, liver disease, recent blood transfusions, and iron deficiency anemia can all produce misleadingly high or low A1C readings. Pregnancy and significant blood loss can also interfere. If any of these apply to you, your doctor will rely on direct blood sugar measurements instead.
The Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten for at least eight hours, typically overnight. It gives a clean reading of your baseline blood sugar without the influence of a recent meal.
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
If your fasting blood sugar comes back at 126 mg/dL or above, the test needs to be repeated on a separate day to confirm the result. A single elevated reading isn’t enough for a diagnosis unless you also have obvious symptoms of diabetes, like excessive thirst, frequent urination, and unexplained weight loss.
The Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) measures how efficiently your body processes sugar. You fast overnight, have your blood drawn, then drink a liquid containing 75 grams of glucose (roughly the sugar equivalent of two cans of soda). Your blood sugar is measured again two hours later.
At the two-hour mark, a reading below 140 mg/dL is normal. Between 140 and 199 mg/dL indicates prediabetes. A result of 200 mg/dL or higher means diabetes. This test is more time-consuming than the others, since you need to stay at the lab for the full two hours, but it’s particularly good at catching people whose fasting numbers look fine but whose bodies struggle to bring blood sugar back down after eating.
Random Blood Sugar Test
When someone shows up with classic diabetes symptoms (constant thirst, urinating far more than usual, blurred vision, unexplained weight loss), a doctor may draw blood regardless of when the person last ate. If that random blood sugar comes back at 200 mg/dL or higher and symptoms are present, that’s enough for a diabetes diagnosis without a second confirmatory test. This scenario is more common with type 1 diabetes, where symptoms tend to develop rapidly and be hard to miss.
Why Confirmation Matters
For people without symptoms, a single abnormal blood test does not equal a diabetes diagnosis. The standard practice is to repeat the same test on a different day or to run a second type of test to confirm the result. If your A1C comes back at 7.0% and a repeat test shows 6.8%, the diagnosis is confirmed because both readings are in the diabetes range.
If two different tests give conflicting results (say, an A1C in the diabetes range but a fasting blood sugar in the prediabetes range), the test that came back abnormal is typically repeated. This built-in redundancy exists because blood sugar naturally fluctuates, lab errors happen, and a diagnosis of diabetes carries lifelong implications for treatment and insurance.
How Gestational Diabetes Is Screened
Pregnant women are screened for gestational diabetes between 24 and 28 weeks of pregnancy using a slightly different process. Most practices in the United States use a two-step approach. First, you drink a smaller sugar solution (50 grams) without needing to fast, and your blood is drawn one hour later. If that result is 135 mg/dL or higher, you move on to the full diagnostic test.
The second step involves fasting overnight and then drinking a larger 100-gram glucose solution. Blood is drawn at fasting, one hour, two hours, and three hours. Gestational diabetes is diagnosed if two or more of these readings exceed the thresholds: 95 mg/dL fasting, 180 mg/dL at one hour, 155 mg/dL at two hours, or 140 mg/dL at three hours.
Some providers use a one-step method instead, with a 75-gram glucose load and lower thresholds. Only one abnormal value is needed for diagnosis with this approach, so it catches more cases. The one-step method uses cutoffs of 92 mg/dL fasting, 180 mg/dL at one hour, and 153 mg/dL at two hours. Which method your provider uses depends on their practice guidelines.
Telling Type 1 From Type 2
The blood sugar tests described above diagnose diabetes but don’t distinguish between types. In most adults, especially those who are overweight and have a gradual onset of symptoms, type 2 diabetes is assumed. But when the picture is less clear, particularly in younger, leaner patients or those whose blood sugar deteriorates quickly despite treatment, additional blood tests can check for autoantibodies.
Type 1 diabetes is an autoimmune condition where the immune system attacks the insulin-producing cells in the pancreas. That immune attack leaves detectable markers in the blood. The most commonly tested autoantibodies target specific proteins in the pancreas, and finding one or more of them strongly suggests type 1. This distinction matters because type 1 diabetes requires insulin from the start, while type 2 can often be managed initially with lifestyle changes and other medications.
Getting this classification right is especially important for adults diagnosed after age 30 who may be misclassified as type 2. A subgroup sometimes called latent autoimmune diabetes in adults (LADA) has the same autoimmune process as type 1 but progresses more slowly, and these patients will eventually need insulin regardless of other treatments.
Preparing for Your Test
If you’re getting an A1C test, no preparation is needed. You can eat and drink normally beforehand.
For a fasting blood sugar test or oral glucose tolerance test, you’ll need to fast for at least eight hours before the blood draw. Water is fine during the fast, but skip coffee, juice, and anything else with calories or sweeteners. Most people schedule these tests first thing in the morning to make the fasting period easier. If you’re taking medications, ask your provider whether to take them before the test, since some can affect blood sugar readings.
For the oral glucose tolerance test, plan to be at the lab for about two and a half hours total. Bring something to read or do, because you’ll be sitting and waiting between blood draws. The glucose drink is very sweet and can cause nausea in some people, but this is temporary.