Prediabetes is determined through blood tests that measure how your body handles sugar, with three main tests used: the A1C test, the fasting blood sugar test, and the oral glucose tolerance test. Any one of these can flag prediabetes, and the process typically starts with your doctor ordering bloodwork based on your age, weight, or other risk factors. Because prediabetes rarely causes noticeable symptoms, testing is the only reliable way to catch it.
The Three Blood Tests Used
Each test measures blood sugar differently, and your doctor may use one or a combination depending on your situation.
The A1C test is the most convenient because it doesn’t require fasting. It reflects your average blood sugar over the past two to three months. 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 fasting blood sugar test measures your blood sugar after an overnight fast. A result between 100 and 125 mg/dL signals prediabetes. Normal is below 100, and 126 or higher on two separate tests points to diabetes.
The oral glucose tolerance test is a two-step process. Your blood is drawn after fasting, then you drink a sugary liquid and have your blood drawn again two hours later. A two-hour result between 140 and 199 mg/dL indicates prediabetes, while 200 or above suggests diabetes.
If your result lands close to a diagnostic cutoff, your doctor will likely repeat the test in three to six months rather than making an immediate call. A single borderline result doesn’t lock in a diagnosis.
Who Should Get Tested
The American Diabetes Association recommends screening for all adults starting at age 35, regardless of weight. Before 35, screening makes sense if you’re overweight or have additional risk factors: a parent or sibling with type 2 diabetes, a history of gestational diabetes, a sedentary lifestyle, high blood pressure, or polycystic ovary syndrome.
Ethnicity plays a role in risk as well. Research from the Multi-Ethnic Study of Atherosclerosis found that the relationship between body size and diabetes risk varies significantly by race. People of Chinese descent faced the highest diabetes risk per unit of body fat, meaning they developed problems at smaller waist sizes. At the upper end of body measurements, Hispanic and Black participants had the highest absolute rates of new diabetes cases, at roughly 5.3 and 5.0 cases per 100 person-years respectively. This is one reason screening guidelines encourage testing earlier for people from higher-risk ethnic backgrounds, including Black, Hispanic, Asian American, and Native American populations.
Women who had gestational diabetes deserve particular attention. About 50% of women with gestational diabetes eventually develop type 2 diabetes, so lifelong screening at least every three years is recommended for this group.
Physical Signs That Suggest Testing
Prediabetes itself is mostly silent, which is why blood tests matter so much. But one visible clue worth knowing about is a skin condition called acanthosis nigricans: dark, velvety patches that appear in body creases like the neck, armpits, or groin. These patches sometimes show up on the hands, elbows, or knees too. They signal insulin resistance, the underlying problem in prediabetes, and are especially common in people carrying extra weight. If you notice these patches, it’s a strong reason to ask for blood sugar testing even if you’re not due for routine screening.
Are At-Home A1C Kits Reliable?
Over-the-counter A1C test kits are available at pharmacies and online, and the idea of checking your blood sugar at home is appealing. Unfortunately, their accuracy is inconsistent. A study evaluating three commercial home A1C kits found that none met the national standardization goal of having at least 90% of results fall within 5% of a laboratory reference value. The best-performing kit hit 82%, while the other two came in at just 46% and 29%.
In practical terms, this means a home kit might read 5.9% when your actual value is 5.5% (normal) or 6.3% (high end of prediabetes). That margin of error matters when the entire prediabetes range spans less than one percentage point. A home kit can be useful as a rough check, but a lab-drawn blood test is necessary for anything close to a real diagnosis.
How to Prepare for Testing
If your doctor orders an A1C test, no preparation is needed. You can eat and drink normally beforehand. For a fasting blood sugar test or an oral glucose tolerance test, you’ll need to fast overnight, typically 8 to 12 hours. Water is fine during the fast, but food, coffee, and other drinks should wait until after your blood is drawn.
What a Prediabetes Result Means
A prediabetes diagnosis is not a diabetes diagnosis. It’s a warning that your blood sugar is elevated but hasn’t crossed the threshold into full diabetes. The trajectory from here isn’t fixed. A large pooled analysis of 19 studies found that within 10 years, about 12.5% of people with prediabetes progressed to type 2 diabetes, while 36.1% reverted to normal blood sugar levels. Roughly 5 to 10% of people with prediabetes convert to diabetes each year without intervention.
Those numbers shift dramatically with lifestyle changes. Losing 5 to 7% of your body weight (about 10 to 14 pounds for someone weighing 200 pounds) and getting 150 minutes of moderate physical activity per week can cut the risk of progressing to diabetes by more than half. These were the core findings of the Diabetes Prevention Program, the landmark trial that most current guidelines are built on.
Follow-Up Testing Schedule
Once prediabetes is identified, both the ADA and the U.S. Preventive Services Task Force recommend annual retesting. This yearly check lets you and your doctor track whether your blood sugar is trending back toward normal, holding steady, or creeping toward diabetes. The test used for follow-up is usually the same one that caught the prediabetes in the first place, though your doctor may switch to A1C for convenience if you started with a fasting test.
For people with normal initial results and no major risk factors, retesting every three years is sufficient. If your results were on the higher end of normal, or you have several risk factors, your doctor may recommend checking sooner.