Three main blood tests are used to diagnose diabetes: the A1C test, the fasting plasma glucose test, and the oral glucose tolerance test. Each measures blood sugar differently, and any one of them can confirm a diagnosis of prediabetes or diabetes. A fourth option, the random plasma glucose test, can diagnose diabetes without fasting but is only used when symptoms are already present.
A1C (Hemoglobin A1C) Test
The A1C test measures the percentage of your red blood cells that have glucose stuck to their surface protein, hemoglobin. Because red blood cells live about three months, the test captures your average blood sugar over that entire window rather than a single moment in time. No fasting is required.
The results break down like this:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
The A1C is the most convenient option since you can eat and drink normally beforehand. But it has limitations. Conditions that change how long your red blood cells survive, such as sickle cell disease, certain anemias, significant kidney disease, or liver failure, can push the result falsely high or low. Some inherited hemoglobin variants also interfere with certain lab methods used to run the test. If any of these apply to you, your doctor will likely rely on a glucose-based test instead.
Fasting Plasma Glucose Test
This is the most straightforward glucose measurement. You fast overnight (no food, though water is fine), then have your blood drawn in the morning. The result tells you exactly how much glucose is circulating when your body hasn’t had to process a recent meal.
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
A single elevated reading isn’t usually enough for a diagnosis on its own. Doctors typically confirm with a second test on a different day, unless the result is very high and you already have symptoms like excessive thirst or frequent urination.
Oral Glucose Tolerance Test (OGTT)
The OGTT is the most involved of the three standard tests. You fast overnight, give a fasting blood sample, then drink a solution containing 75 grams of sugar. Your blood is drawn again one hour later and two hours later. The two-hour reading is the one used for diagnosis.
- Normal (at two hours): below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
This test is especially useful for catching prediabetes and early diabetes because it shows how well your body handles a sugar load in real time. Some people have a normal fasting number but a clearly abnormal response after drinking the glucose solution. The trade-off is that it takes about two and a half hours in a lab, and the sugary drink can cause nausea.
Random Plasma Glucose Test
A random blood glucose test can be taken at any time of day, regardless of when you last ate. It’s not used for routine screening. Instead, it’s ordered when someone shows up with classic diabetes symptoms: frequent urination, unexplained weight change, blurred vision, confusion, or fainting. A reading of 200 mg/dL or higher, combined with those symptoms, is enough for a diabetes diagnosis without a follow-up test.
Gestational Diabetes Screening
Pregnant women go through a separate screening process, typically between weeks 24 and 28. Most practices in the U.S. use a two-step approach. The first step is a one-hour glucose challenge where you drink a 50-gram sugar solution and have your blood drawn an hour later. If that result is elevated, you move on to a full three-hour tolerance test using a 100-gram glucose solution, with blood drawn at fasting, one hour, two hours, and three hours.
A one-step approach also exists, using a 75-gram solution with measurements at fasting, one hour, and two hours. In the one-step method, just one elevated value is enough for a gestational diabetes diagnosis. In the two-step method, two or more values need to be at or above the thresholds. The specific cutoffs differ slightly depending on which criteria your provider follows, but common fasting thresholds are 92 mg/dL (one-step) or 95 mg/dL (two-step).
Tests That Tell Type 1 From Type 2
The tests above confirm that diabetes is present, but they don’t tell you which type. When the answer isn’t obvious from age and clinical picture alone, two additional blood tests help sort it out.
Autoantibody Panel
Type 1 diabetes is an autoimmune condition where the immune system attacks insulin-producing cells in the pancreas. That attack leaves a fingerprint: specific antibodies circulating in the blood. Labs typically check for at least two of the five major antibodies (known by shorthand as GAD, IA-2, IAA, ICA, and ZnT8). Testing positive for two or more of these antibodies means there’s a greater than 90% chance of type 1 diabetes. A single positive result warrants checking for the others.
C-Peptide Test
C-peptide is a byproduct that your pancreas releases in equal amounts alongside insulin. Measuring it reveals how much insulin your body is actually producing. A low C-peptide level points toward type 1 diabetes, where the pancreas has lost much of its ability to make insulin. A normal or high level is more consistent with type 2 diabetes, where the body produces insulin but doesn’t use it efficiently. This test is also useful years after diagnosis to track how much insulin-producing function remains.
Fructosamine Test
In situations where an A1C result would be unreliable (because of anemia, hemoglobin variants, or recent blood loss), a fructosamine test offers an alternative. It works on a similar principle, measuring glucose attached to proteins in the blood, but reflects a shorter window of two to three weeks instead of three months. It’s not commonly used for initial diagnosis, but it’s a practical option when A1C can’t be trusted or when doctors need to see how blood sugar control has changed over a shorter period.
Which Test You’ll Likely Get First
For routine screening, most adults get either an A1C or a fasting glucose test during a standard blood draw at their annual checkup. These are quick, inexpensive, and widely available. The OGTT is more sensitive for detecting early prediabetes but is reserved for situations where the simpler tests are borderline or when a doctor wants a clearer picture. If your result falls in the prediabetes or diabetes range, expect to repeat the same test or a different one on a separate day to confirm. The exception is if you have clear symptoms and a blood sugar of 200 mg/dL or higher, which counts as a confirmed diagnosis on its own.