Prediabetes is detected through a simple blood test, and there are three standard options your doctor can use. Each measures blood sugar in a different way, and all can be done at a routine office visit or lab appointment. Most people with prediabetes have no symptoms, so testing is the only reliable way to catch it.
The Three Standard Blood Tests
Any of the following tests can identify prediabetes. Your doctor may order one or a combination depending on your situation.
A1C test. This measures your average blood sugar over the past two to three months. It works by looking at the percentage of your red blood cells that have sugar attached to them. A result between 5.7% and 6.4% falls in the prediabetes range. Below 5.7% is normal, and 6.5% or higher indicates diabetes. The A1C doesn’t require fasting, so it can be drawn at any time of day.
Fasting plasma glucose (FPG) test. This measures your blood sugar after you’ve gone at least 8 hours without eating or drinking anything other than water. It’s typically done first thing in the morning before breakfast. A fasting level between 100 and 125 mg/dL signals prediabetes. Below 100 is normal, and 126 or above points to diabetes.
Oral glucose tolerance test (OGTT). This is a two-part test. Your blood is drawn after an overnight fast, then you drink a standardized sugary liquid and have your blood drawn again two hours later. A two-hour reading between 140 and 199 mg/dL indicates prediabetes. This test is more involved but can catch blood sugar problems the other two tests miss, particularly in people whose fasting numbers look normal but whose bodies struggle to process a sugar load.
How to Prepare for Each Test
The A1C requires no preparation at all. You can eat normally beforehand, and the blood draw takes just a few minutes.
Both the fasting glucose test and the oral glucose tolerance test require at least 8 hours of fasting beforehand. Water is fine during the fast. For the OGTT specifically, you should eat at least 150 grams of carbohydrates per day for the three days leading up to the test. That’s roughly your normal diet if you eat bread, pasta, rice, or fruit regularly, but if you’ve been on a very low-carb diet, your results could be artificially high. Your doctor’s office should mention this, but it’s worth asking about if you eat low-carb.
The OGTT also takes longer. Plan for about two and a half hours at the lab, since you’ll need to wait between blood draws.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends screening for prediabetes in adults aged 35 to 70 who are overweight or obese (a BMI of 25 or higher). Because the task force gives this a “B” grade, most insurance plans cover the test as a preventive service with no copay or cost-sharing for people who meet those criteria.
Earlier screening is recommended for people from populations with higher diabetes rates, including American Indian/Alaska Native, Black, Hispanic/Latino, and Native Hawaiian/Pacific Islander communities. For Asian Americans, screening is recommended at a lower BMI threshold of 23 or above, because metabolic risk rises at a lower body weight in this group.
If you have a family history of type 2 diabetes, a history of gestational diabetes, or polycystic ovary syndrome, testing earlier or more frequently is reasonable even if you don’t meet the standard screening criteria.
How Often to Retest
If your results come back in the prediabetes range, expect to be retested every one to three years. More frequent monitoring helps track whether your blood sugar is stable, improving, or trending toward diabetes.
If your initial screening is normal but your doctor still considers you high risk, retesting within a year is reasonable. Your doctor might also confirm the result with a different test. For example, if your A1C is normal but your risk factors are strong, a fasting glucose or OGTT can provide a second perspective, since the tests measure slightly different things.
At-Home A1C Kits
Over-the-counter A1C test kits are available at most pharmacies for around $30 to $50. They use a finger-prick blood sample and give results in a few minutes. While they can offer a rough snapshot, they’re notably less reliable than laboratory testing.
Lab-based A1C tests are certified through the National Glycohemoglobin Standardization Program, which holds them to tight accuracy standards. Point-of-care and at-home devices don’t face the same requirements. Studies comparing the two have found that home-style devices missed about 18% of people whose lab results showed elevated A1C levels. The devices tend to be most accurate in the 6% to 8% range, which means they’re less reliable right around the prediabetes cutoff where precision matters most.
An at-home kit can be a useful first step if you’re curious, but a result in the normal range doesn’t rule out prediabetes. A lab test through your doctor remains the standard for diagnosis.
Factors That Can Affect Your Results
The A1C test relies on red blood cells, so anything that changes your red blood cell turnover can skew the number. Iron-deficiency anemia, sickle cell trait, recent blood loss or transfusion, and certain hemoglobin variants can all push your A1C reading artificially higher or lower. If you have any of these conditions, your doctor may rely more heavily on fasting glucose or the OGTT instead.
For the fasting tests, stress, illness, and certain medications (particularly steroids) can temporarily raise blood sugar. If a result seems inconsistent with your overall health picture, your doctor will typically repeat the test on a different day or order a second type of test to confirm.
What a Prediabetes Result Means in Practice
Prediabetes means your blood sugar is higher than normal but not yet in the diabetes range. It’s not a diagnosis you’re stuck with. The landmark Diabetes Prevention Program trial found that moderate lifestyle changes (losing 5% to 7% of body weight and getting about 150 minutes of physical activity per week) reduced the risk of progressing to type 2 diabetes by 58%.
If you’re diagnosed with prediabetes, your doctor will likely recommend changes to diet and activity levels, and possibly refer you to a structured diabetes prevention program. Many of these programs are covered by insurance. Retesting every year or so will show whether those changes are moving your numbers in the right direction.