Prediabetes is diagnosed through one of three blood tests: an A1C test, a fasting plasma glucose test, or an oral glucose tolerance test. Each measures blood sugar differently, and any one of them can identify prediabetes. A result in the prediabetes range typically needs to be confirmed with a second test on a different day before the diagnosis is official.
The Three Tests Used to Diagnose Prediabetes
All three tests are standard blood draws, but they measure different things and require different preparation. Your doctor may choose one based on convenience, your health history, or whether certain conditions could affect accuracy.
A1C Test
The A1C test measures your average blood sugar over the past two to three months. It does this by looking at how much sugar has attached to your red blood cells. A result between 5.7% and 6.4% indicates prediabetes. Below 5.7% is normal, and 6.5% or higher points to type 2 diabetes.
The main advantage of the A1C test is that you don’t need to fast beforehand. It can be done at any time of day, which makes it the most convenient option. Because it reflects a longer window of blood sugar behavior rather than a single moment, it’s less affected by what you ate the night before or how stressed you were that morning.
Fasting Plasma Glucose Test
This test measures your blood sugar at a single point in time after you’ve fasted for at least 8 hours (usually overnight). A fasting glucose level of 100 to 125 mg/dL falls in the prediabetes range. Below 100 is normal, and 126 or higher suggests diabetes.
Because it captures a snapshot rather than an average, this test can be more sensitive to short-term fluctuations. That’s one reason a confirmatory test is important.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) is the most involved of the three. You fast overnight, have your blood drawn, then drink a sugary liquid containing 75 grams of glucose. Two hours later, your blood is drawn again. A two-hour reading of 140 to 199 mg/dL indicates prediabetes. A result of 200 mg/dL or higher indicates diabetes.
This test shows how efficiently your body processes a large dose of sugar, which can catch problems that a fasting test alone might miss. It’s less commonly used in routine screening because it takes longer, but it’s considered especially useful for detecting prediabetes in certain populations.
Why a Second Test Is Required
A single abnormal result isn’t enough for a diagnosis. Blood sugar levels fluctuate naturally, and one borderline reading could reflect a bad night’s sleep, an illness, or normal variation. To confirm prediabetes, you’ll need a repeat test on a different day. This can be the same test again or a different one. For example, if your A1C came back at 5.8%, your doctor might repeat the A1C or follow up with a fasting glucose test.
The only exception is when someone already has clear symptoms of high blood sugar, like frequent urination, excessive thirst, and unexplained weight loss. In that case, a single test may be sufficient, though those symptoms more commonly point to diabetes rather than prediabetes.
When A1C Results May Be Unreliable
The A1C test works by measuring sugar attached to the protein in red blood cells. Anything that changes how your red blood cells behave or how long they survive can throw off the result. Several conditions can falsely raise or lower your A1C:
- Severe anemia
- Kidney failure
- Liver disease
- Blood disorders like sickle cell anemia or thalassemia
- Certain medications, including opioids and some HIV drugs
- Recent blood loss or blood transfusions
- Pregnancy (particularly early or late stages)
If any of these apply to you, your doctor will likely use a fasting glucose or oral glucose tolerance test instead. This is particularly relevant for people of African, Mediterranean, or Southeast Asian descent, who have higher rates of hemoglobin variants that can interfere with A1C accuracy.
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 plasma glucose test or an oral glucose tolerance test, you’ll need to fast for 8 to 12 hours before the blood draw. Most people schedule these for first thing in the morning and skip breakfast. Water is fine during the fasting period. Your doctor’s office will tell you the exact fasting window based on the test ordered.
For the oral glucose tolerance test, plan to be at the lab for about two and a half hours total. You’ll drink the glucose solution after your first blood draw, then wait two hours for the second draw. Bring something to read or do, because you’ll need to stay at the facility.
Diagnosis in Children and Teens
The same three tests can be used to screen for prediabetes in children and adolescents, and the American Diabetes Association supports using A1C for diagnosing type 2 diabetes in young people. However, the evidence behind A1C in pediatric populations is more limited than in adults. Some research suggests that fasting glucose or the oral glucose tolerance test may be more reliable for children, particularly among certain ethnic groups. If your child is being screened, the doctor may prefer one test over another depending on the situation.
What a Prediabetes Diagnosis Means
Prediabetes means your blood sugar is higher than normal but not yet high enough to qualify as type 2 diabetes. It’s a warning sign, not a guarantee. Without changes, many people with prediabetes develop type 2 diabetes within 5 years. But that progression isn’t inevitable. Moderate weight loss (even 5 to 7% of body weight) and regular physical activity can significantly reduce that risk, and some people bring their blood sugar back to normal levels entirely.
Once diagnosed, your doctor will typically recheck your blood sugar every one to three years to track whether your levels are improving, holding steady, or moving toward diabetes. The specific test and frequency depend on your initial results and your overall risk profile.