Prediabetes is detected through a simple blood test, and three standard options exist: an A1C test, a fasting blood glucose test, or an oral glucose tolerance test. Any of these can confirm whether your blood sugar levels fall in the prediabetes range, and your doctor may use one or a combination depending on your situation. Most adults with a BMI of 25 or higher should begin screening at age 35.
The Three Blood Tests Used
Each test measures blood sugar differently, and each has its own prediabetes range. You only need one abnormal result to receive a prediabetes diagnosis, though your doctor may order a second test to confirm.
A1C test: This measures your average blood sugar over the past two to three months by looking at the percentage of your red blood cells coated with sugar. A result between 5.7% and 6.4% indicates prediabetes. No fasting is required, so it can be done at any time of day. That convenience makes it one of the most commonly ordered screening tests.
Fasting plasma glucose (FPG): This is a single blood draw taken after you’ve fasted for 8 to 12 hours. A result between 100 and 125 mg/dL falls in the prediabetes range. During the fasting period, you can drink plain water but should avoid coffee, juice, soda, flavored water, gum, smoking, and exercise, all of which can affect results.
Oral glucose tolerance test (OGTT): This is the most involved option. After fasting overnight, you have your blood drawn, then drink a syrupy solution containing 75 grams of sugar. Your blood is drawn again at one hour and two hours. A two-hour reading between 140 and 199 mg/dL indicates prediabetes. Because it measures how your body handles a sugar load in real time, some clinicians consider it the most sensitive of the three tests.
Which Test Is Right for You
For most people, the A1C or fasting glucose test is the starting point. They’re quick, widely available, and inexpensive. The oral glucose tolerance test takes about two and a half hours in the clinic and is less commonly used for routine screening, though it can catch cases that the other two tests miss.
The A1C test does have some blind spots. Conditions like severe anemia, kidney failure, liver disease, sickle cell anemia, thalassemia, recent blood loss or transfusions, and pregnancy can all push the result falsely high or low. If you have any of these, your doctor will likely rely on one of the glucose-based tests instead.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends screening for prediabetes in adults aged 35 to 70 who are overweight or obese (BMI of 25 or higher). If you’re Asian American, screening is recommended at a lower BMI threshold of 23. For people who are American Indian, Alaska Native, Black, Hispanic/Latino, or Native Hawaiian/Pacific Islander, screening may be appropriate before age 35 because these groups face a disproportionately higher risk of developing diabetes.
Beyond age and weight, other factors that raise your risk include a family history of diabetes, high blood pressure, a history of gestational diabetes, and a physically inactive lifestyle. The CDC offers a free online risk assessment tool that scores you on these factors and can help you decide whether to ask your doctor for testing.
Children and adolescents can develop prediabetes too. The American Diabetes Association recommends risk-based screening for kids who are at least 10 years old or have started puberty, are overweight (BMI at or above the 85th percentile for their age), and have at least one additional risk factor such as a family history of type 2 diabetes.
How Often to Retest
If your results come back normal, the standard recommendation is to rescreen every three years. Women who had gestational diabetes during a pregnancy should also be tested every three years going forward, since their risk of developing type 2 diabetes remains elevated long after delivery. If your results fall in the prediabetes range, your doctor will likely want to monitor more frequently and discuss lifestyle changes that can bring your numbers down.
At-Home A1C Kits
Over-the-counter A1C test kits are available at most pharmacies and online. These use a finger-prick blood sample and deliver results in minutes. In a study evaluating one widely sold home kit, 93.2% of user results fell within acceptable accuracy margins compared to laboratory reference values, with a strong statistical correlation (R² = 0.93) between home and lab results.
That said, a home test is a reasonable first step but not a substitute for clinical testing. The accuracy margin on these kits is plus or minus about 13.5%, which means a true A1C of 5.7% (the low end of prediabetes) could read as normal. If a home result concerns you, or even if it looks fine but you have multiple risk factors, follow up with a lab-based test.
Cost and Insurance Coverage
Prediabetes screening qualifies as a preventive service under most health insurance plans, including marketplace plans. That means it’s typically covered at no cost to you, with no copay or coinsurance, as long as you use an in-network provider. This applies even if you haven’t met your deductible. If you’re uninsured, community health centers and local health departments often offer low-cost blood sugar screenings, and at-home A1C kits generally cost between $30 and $50.