Checking for prediabetes requires a simple blood test, and there are three standard options your doctor can order. More than 115 million American adults have prediabetes, but 8 in 10 don’t know it, largely because the condition rarely causes obvious symptoms. A blood test is the only reliable way to find out where you stand.
Three Blood Tests That Detect Prediabetes
Any of the following tests can identify prediabetes. Your doctor may choose one based on convenience, cost, or your specific situation.
A1C test. This measures your average blood sugar over the past two to three months. It doesn’t require fasting, so you can have it done at any time of day regardless of when you last ate. A normal A1C is below 5.7%. Prediabetes falls between 5.7% and 6.4%. An A1C of 6.5% or higher indicates diabetes.
Fasting plasma glucose (FPG) test. This checks your blood sugar after you haven’t eaten for at least eight hours, typically overnight. A normal fasting level is below 100 mg/dL. Prediabetes is 100 to 125 mg/dL. A result of 126 mg/dL or higher points to diabetes.
Oral glucose tolerance test (OGTT). This is the most involved option. You fast overnight, then drink a syrupy solution containing 75 grams of sugar at the lab or doctor’s office. Your blood is drawn one hour and two hours after you finish the drink. A two-hour reading below 140 mg/dL is normal. Prediabetes falls between 140 and 199 mg/dL, and 200 mg/dL or above indicates diabetes.
The A1C test is the most convenient since it requires no fasting or timed blood draws. However, some conditions like anemia or certain blood disorders can affect its accuracy. Your doctor can advise which test makes the most sense for you.
How to Prepare for Each Test
For an A1C test, there’s nothing to do. Eat and drink normally beforehand. For a fasting glucose test, stop eating at least eight hours before your appointment. Water is fine. Most people schedule it first thing in the morning after an overnight fast.
The oral glucose tolerance test requires the same overnight fast, plus about two and a half hours at the lab or clinic. You’ll drink the glucose solution on site and wait while your blood is drawn at timed intervals. The drink is very sweet, and some people find it mildly nauseating, but the process is straightforward.
Who Should Get Tested
The American Diabetes Association recommends that all adults begin screening at age 35, even without risk factors. But if you’re overweight or obese and have at least one additional risk factor, testing makes sense at any age. Those risk factors include:
- A parent or sibling with type 2 diabetes
- African American, Latino, Native American, Asian American, or Pacific Islander heritage
- History of heart disease
- High blood pressure (130/80 or above, or currently on blood pressure medication)
- Low HDL (“good”) cholesterol or high triglycerides
- Polycystic ovary syndrome (PCOS)
- A physically inactive lifestyle
- A history of gestational diabetes during pregnancy
The BMI threshold is lower for Asian Americans. Screening is recommended at a BMI of 23 or above, compared to 25 for the general population, because insulin resistance tends to develop at lower body weights in this group.
If your results come back normal, repeating the test every three years is reasonable. If you gain significant weight or develop new risk factors in the meantime, earlier retesting makes sense. People already diagnosed with prediabetes should be tested yearly to track whether their levels are improving, holding steady, or progressing toward diabetes.
What Happens if Your Results Are Borderline
Prediabetes itself is a borderline zone, but your results can fall anywhere within that range. Someone with an A1C of 5.7% is in a very different position than someone at 6.3%. If your numbers land near the upper end of the prediabetes range, your doctor may recommend retesting in three to six months rather than waiting a full year.
For a diabetes diagnosis, guidelines require two abnormal test results to confirm. This can be two results from the same blood sample using different tests, or a repeat of the same test on a second sample. Prediabetes doesn’t require the same formal confirmation, but a single borderline result is often rechecked to make sure it wasn’t a fluke.
Physical Signs Worth Paying Attention To
Prediabetes is often called a “silent” condition because most people feel completely fine. There’s no pain, no fatigue dramatic enough to notice, no obvious warning. That’s exactly why so many people have it without knowing.
One physical sign that can point to insulin resistance is a skin condition called acanthosis nigricans: patches of dark, thick, velvety skin that typically appear on the back of the neck, in the armpits, or in the groin. The skin may feel different to the touch, sometimes itchy, and small skin tags can develop in the same areas. This happens because excess insulin in the blood stimulates skin cells to reproduce faster than normal. Not everyone with prediabetes develops these patches, but if you notice them, they’re worth mentioning to your doctor.
Where to Get Tested
Your primary care doctor can order any of these tests during a routine visit. Many people discover their prediabetes through standard bloodwork done at an annual physical, since fasting glucose and A1C are commonly included in basic metabolic panels or wellness screenings. If you don’t have a primary care doctor, walk-in labs and some pharmacies offer A1C testing without a prescription, though the cost varies.
If you’re at higher risk based on the factors above, you don’t need to wait for your next scheduled checkup. Call your doctor’s office and ask specifically for a prediabetes screening. The test itself takes minutes, results typically come back within a day or two, and catching prediabetes early opens a window where lifestyle changes like losing 5% to 7% of your body weight and getting 150 minutes of moderate activity per week can meaningfully reduce the chance of progressing to type 2 diabetes.