If you’re at higher risk due to weight, family history, or other factors, screening for prediabetes should start now, at any age. For everyone else, the American Diabetes Association recommends beginning at age 35. The timing of the test itself matters too: most prediabetes blood tests require an 8- to 12-hour fast and are best done first thing in the morning.
Who Should Get Screened and When
The 2025 ADA guidelines break screening into two groups. The first includes adults of any age who are overweight or obese (a BMI of 25 or higher, or 23 or higher for people of Asian descent) and have at least one additional risk factor. Those risk factors include a parent or sibling with diabetes, a history of cardiovascular disease, high blood pressure (130/80 or above), low HDL cholesterol (below 35), high triglycerides (above 250), polycystic ovary syndrome, physical inactivity, or belonging to a higher-risk ethnic group such as African American, Latino, Native American, or Asian American.
If none of those apply, screening should begin at age 35. After a normal result, repeat testing every three years is reasonable. If your results come back in the prediabetic range, your doctor will likely recheck more frequently, typically once a year.
Screening After Gestational Diabetes
Women who developed diabetes during pregnancy carry a significantly higher long-term risk. The ADA recommends a glucose tolerance test between 4 and 12 weeks after delivery. If that result comes back normal, follow-up screening should happen every one to three years going forward. This is one of the most commonly missed screening windows, since the postpartum period is busy and the pregnancy complication can feel like it’s “over.”
The Three Tests Used to Diagnose Prediabetes
Prediabetes is identified through one of three lab tests, each measuring blood sugar in a different way.
Fasting plasma glucose (FPG) is the most common. You fast for at least 8 hours (water is fine), then have your blood drawn. A normal result is below 100 mg/dL. Prediabetes falls between 100 and 125 mg/dL. Anything at 126 or above points to diabetes.
A1C measures your average blood sugar over the past two to three months. It doesn’t require fasting, which makes it convenient. Prediabetes is defined as an A1C between 5.7% and 6.4%.
Oral glucose tolerance test (OGTT) is more involved but catches some cases the other two miss. After fasting for 10 to 16 hours, you drink a solution containing 75 grams of glucose dissolved in about 10 ounces of water. Your blood is drawn two hours later. A result between 140 and 199 mg/dL at the two-hour mark indicates prediabetes.
The OGTT has specific conditions that affect accuracy. It should be done between 7 and 9 a.m., after at least three days of eating your normal diet (at least 150 grams of carbohydrates daily). You need to stay seated during the test, and smoking is off-limits. Being bedridden or acutely ill can impair glucose tolerance and skew results.
Why Morning Testing Matters
Fasting tests are almost always scheduled first thing in the morning for a practical reason: most of your fasting hours happen while you sleep. But morning timing also intersects with something called the dawn phenomenon, an early-morning rise in blood sugar that typically occurs between 4 and 8 a.m. Your body naturally releases hormones like cortisol, growth hormone, and glucagon during the predawn hours. These hormones increase insulin resistance, which pushes blood sugar up.
For people with prediabetes, this morning surge can be more pronounced. If your fasting numbers seem inconsistently high when you test at home, the dawn phenomenon could be a factor. Checking your blood sugar once during the early-morning hours (around 3 or 4 a.m.) for a few consecutive nights can help determine whether this hormonal pattern is inflating your morning readings.
Home Meters vs. Lab Tests
A home glucometer is useful for tracking patterns, but it’s not a diagnostic tool. International standards allow home meters to be off by up to 15 mg/dL when your actual blood sugar is below 100, and up to 15% when it’s 100 or above. That margin of error is fine for daily monitoring, but it’s wide enough to blur the line between normal and prediabetic. A fasting reading of 105 on your home meter could reflect a true value anywhere from 90 to 120. Diagnosis should always be confirmed through a lab test.
If you’re using a home meter to track trends between lab visits, consistency helps. Test at the same time each morning, use the same hand-washing routine before pricking your finger, and avoid squeezing the fingertip hard, which can dilute the blood sample with tissue fluid.
Continuous Glucose Monitors for Early Detection
Continuous glucose monitors, small sensors worn on the skin that check blood sugar every few minutes, are primarily designed for people with diabetes. But there’s growing interest in using them for people at risk of prediabetes. A CGM can reveal post-meal spikes and overnight patterns that a single fasting test would miss entirely.
One small study found that sedentary, overweight individuals who wore a CGM for 10 days alongside an activity tracker felt more motivated to exercise after seeing how physical activity affected their blood sugar in real time. CGMs may also catch prediabetes or diabetes earlier in people with strong family histories or those taking medications that raise blood sugar, like certain steroids. They’re not yet standard for prediabetes screening, but they offer a level of detail no single blood draw can match.
How Often to Recheck
If your initial screening is normal and you have no risk factors, retesting every three years is the standard recommendation. If you do have risk factors or your numbers are creeping toward the prediabetic range (a fasting glucose in the mid-90s, for example, or an A1C of 5.5 to 5.6%), annual testing makes more sense. Once prediabetes is confirmed, yearly monitoring helps track whether lifestyle changes are working or whether you’re progressing toward diabetes. About 70% of people with prediabetes eventually develop type 2 diabetes, but that trajectory isn’t inevitable. Weight loss of even 5 to 7% of body weight and regular physical activity can cut that risk nearly in half.