Diabetes is tested through blood tests that measure how much sugar is in your blood, either at a single moment or averaged over several months. The most common tests are the A1C, fasting blood sugar, and oral glucose tolerance test, each with specific cutoff numbers that separate normal blood sugar from prediabetes and diabetes. In most cases, you’ll need two abnormal results before receiving a formal diagnosis.
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 how much sugar has attached to your red blood cells. A higher percentage means your blood sugar has been running higher over that period. No fasting is required, so you can eat and drink normally before the test.
The diagnostic ranges are straightforward:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
The A1C is popular because it’s convenient and gives a longer-term picture of blood sugar control rather than a single snapshot. However, it isn’t reliable for everyone. People with certain blood conditions, including sickle cell trait and other hemoglobin variants, can get falsely high or low A1C readings. If your A1C doesn’t match what your day-to-day blood sugar readings suggest, your doctor may suspect the test is being thrown off and switch to a different method.
Fasting Blood Sugar Test
The fasting plasma glucose test measures your blood sugar after you haven’t eaten or had anything to drink (besides water) for at least eight hours. Most people schedule this as an early morning blood draw so the fasting period happens overnight.
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Type 2 diabetes: 126 mg/dL or above
Results from a fasting glucose test typically come back within 24 hours, since glucose is part of a standard blood panel that most labs process quickly. If your result comes back in the diabetes range, you’ll need a second abnormal result to confirm the diagnosis, either by repeating the same test or taking a different one.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) checks how well your body handles a large dose of sugar. You fast overnight, then drink a syrupy solution containing 75 grams of sugar at the lab. Two hours later, your blood is drawn again.
Your two-hour blood sugar level determines the result:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or above
This test is more time-consuming than the others since you need to sit at the lab for the full two hours. It’s especially useful for catching cases where fasting blood sugar looks borderline but the body struggles to clear sugar after a meal. The OGTT is also the standard screening test for gestational diabetes during pregnancy, typically performed between weeks 24 and 28.
Random Blood Sugar Test
A random blood sugar test can be done at any time, without fasting. It’s usually ordered when someone is showing obvious symptoms of diabetes, like excessive thirst, frequent urination, unexplained weight loss, or blurred vision. A result of 200 mg/dL or above, combined with symptoms, points to diabetes. This test can’t diagnose prediabetes, and it isn’t used for routine screening. It’s a fast way to confirm what symptoms already suggest.
Why Two Tests Are Required
Blood sugar levels fluctuate naturally throughout the day and can be affected by stress, illness, medications, and what you ate the night before. A single elevated reading doesn’t guarantee diabetes. Because of this variability, clinical guidelines require two abnormal test results before making a diagnosis.
Those two results can come from the same blood sample (for example, if your doctor orders both a fasting glucose and an A1C from one draw and both come back above the threshold) or from two separate visits. If you take two different tests and they give conflicting results, the test that came back above the diagnostic cutoff gets repeated. The diagnosis is confirmed based on that repeated test.
The one exception is when a random blood sugar test shows 200 mg/dL or higher and you’re already experiencing classic diabetes symptoms. In that situation, a single test may be sufficient.
Testing for Type 1 vs. Type 2
The blood sugar tests described above can tell you that you have diabetes, but they don’t tell you which type. Type 1 diabetes is an autoimmune condition where the immune system destroys the cells in the pancreas that produce insulin. Type 2 is a metabolic condition where the body either doesn’t make enough insulin or can’t use it effectively.
When Type 1 is suspected, doctors order autoantibody tests. These look for specific immune proteins that signal the body is attacking its own insulin-producing cells. Four key autoantibodies are used as markers: islet cell antibodies, antibodies targeting an enzyme called GAD-65, insulin autoantibodies, and IA-2A antibodies. If one or more of these are present, it confirms an autoimmune process and points toward Type 1. Autoantibody testing is especially important in adults who are initially diagnosed with Type 2 but don’t respond to typical treatments, since some adults develop a slower-onset form of Type 1.
Gestational Diabetes Screening
Pregnant women are routinely screened for gestational diabetes between 24 and 28 weeks. There are two main approaches. The two-step method starts with a smaller sugar drink (50 grams), and if the one-hour result is elevated, you return for a longer three-hour test with a 100-gram sugar drink. The one-step method skips the initial screening and goes straight to a 75-gram OGTT, the same test used for diagnosing Type 2.
Both approaches produce similar pregnancy outcomes. The two-step method is more common in the United States, while the one-step method tends to flag more cases and results in higher use of follow-up care. Your ob-gyn’s office will typically use whichever protocol their practice has standardized.
When the A1C Isn’t Accurate
For most people, the A1C is the simplest and most reliable test. But certain conditions can make it unreliable. Hemoglobin variants (common in people of African, Mediterranean, or Southeast Asian descent) can interfere with the lab method used to measure A1C. Other conditions that affect red blood cell turnover, like iron-deficiency anemia, recent blood transfusions, or kidney disease, can also skew results.
When A1C isn’t trustworthy, doctors can use a fructosamine test instead. This test measures sugar attached to proteins in the blood (mostly albumin) and reflects average blood sugar over the past two to three weeks rather than two to three months. It’s a shorter window, but it sidesteps the issues that affect A1C. Fructosamine testing has its own limitations: conditions that alter protein levels, like advanced liver disease or kidney disease that causes protein loss, can make fructosamine readings less reliable too.
Who Should Get Tested
Routine screening for Type 2 diabetes is recommended starting at age 35 for all adults. Testing should happen earlier if you have risk factors like being overweight, having a family history of diabetes, a history of gestational diabetes, or belonging to a racial or ethnic group with higher diabetes rates (including Black, Hispanic, Native American, Asian American, and Pacific Islander populations). If your results come back normal, repeat testing every three years is a reasonable interval, or sooner if your risk factors change.
For Type 1, there’s no routine screening program for the general public. Testing is typically prompted by symptoms: sudden weight loss, extreme thirst, frequent urination, fatigue, and sometimes nausea or vomiting. Type 1 can appear at any age, though it’s most commonly diagnosed in children and young adults. Autoantibody screening is sometimes offered to close relatives of people with Type 1, since the risk is higher in families with an existing case.