Prediabetes is defined by three possible blood tests, each with its own number range. A fasting blood sugar of 100 to 125 mg/dL, an A1C of 5.7% to 6.4%, or a two-hour glucose tolerance result of 140 to 199 mg/dL all fall in the prediabetic range. Any one of these results is enough for a diagnosis. More than 115 million American adults have prediabetes, and 8 in 10 of them don’t know it.
The Three Tests and Their Ranges
Doctors use three different blood tests to check for prediabetes. You only need one abnormal result to qualify, though your doctor may order more than one to confirm.
- Fasting blood sugar (fasting plasma glucose): You fast for at least 8 hours, then have your blood drawn. Normal is 99 mg/dL or below. Prediabetes is 100 to 125 mg/dL. At 126 mg/dL or above, it’s type 2 diabetes.
- A1C (hemoglobin A1C): This measures your average blood sugar over the past two to three months, expressed as a percentage. Normal is below 5.7%. Prediabetes is 5.7% to 6.4%. At 6.5% or above, it’s diabetes.
- Oral glucose tolerance test (OGTT): You drink a sugary solution containing 75 grams of glucose, and your blood is drawn two hours later. Normal is 139 mg/dL or below. Prediabetes is 140 to 199 mg/dL. At 200 mg/dL or above, it’s diabetes.
The fasting blood sugar test and A1C are the most commonly ordered because they’re simple and convenient. The oral glucose tolerance test takes longer and is used less often in routine screening, but it can catch problems with how your body processes sugar that the other tests miss.
What Each Test Actually Tells You
These three tests measure different things, which is why your results on one test don’t always match the others perfectly. Fasting blood sugar is a snapshot of what’s happening right now, after your body has gone without food overnight. It reflects how well your liver manages glucose production while you sleep and how sensitive your cells are to insulin at baseline.
A1C gives you the big picture. Because it tracks how much sugar has been sticking to your red blood cells over two to three months, it captures patterns that a single fasting test could miss. Someone who spikes after meals but has normal fasting numbers might still show an elevated A1C. Certain conditions like anemia or sickle cell trait can affect A1C accuracy, so your doctor may rely more on the other tests in those cases.
The glucose tolerance test specifically measures how your body handles a sugar load. It’s the most sensitive test for catching the early stages of insulin resistance, when your body is struggling to clear sugar from the bloodstream after eating but hasn’t yet lost control of fasting levels.
Where You Fall in the Range Matters
Not all prediabetes is the same. Someone with a fasting glucose of 101 mg/dL is in a very different position than someone at 124 mg/dL, even though both technically have prediabetes. The closer your numbers are to the diabetes threshold, the higher your risk of progressing. Research tracking people with prediabetes found a diabetes incidence rate of roughly 40 per 1,000 person-years, but that risk concentrates heavily in people at the upper end of the prediabetic range.
If your numbers are at the lower end of the prediabetic range, you have more metabolic runway. Your body is showing early signs of insulin resistance, but the system isn’t under severe strain yet. This is when lifestyle changes have the biggest payoff.
How Prediabetes Is Different in Children
The same adult cutoffs are currently used to diagnose prediabetes in children and adolescents: A1C of 5.7% to 6.4%, fasting glucose of 100 to 125 mg/dL, and the same glucose tolerance ranges. However, these thresholds were developed from adult studies and have never been validated through long-term research in younger populations. Some pediatric studies suggest these adult cutoffs may underestimate how many children actually have prediabetes or diabetes. The right diagnostic thresholds for kids remain an open question.
Prediabetes Can Be Reversed
A prediabetes diagnosis is not a locked-in path to type 2 diabetes. The landmark Diabetes Prevention Program trial showed that people who made moderate lifestyle changes reduced their risk of developing type 2 diabetes by 58% compared to those who didn’t. For people 60 and older, the reduction was even larger, at 71%. These results were nearly twice as effective as taking medication, which reduced risk by 31%.
The changes that drove those results were not extreme. Losing just 5% of your body weight, if you’re overweight, can be enough to shift your blood sugar back toward normal. For someone who weighs 200 pounds, that’s 10 pounds. Combining modest weight loss with about 150 minutes of physical activity per week (a 30-minute walk five days a week qualifies) is the approach that produced the strongest results in clinical trials.
If you’ve gotten blood work back with numbers in the prediabetic range, the most useful next step is to know which test was used and where in the range you fall. That context shapes how urgently you need to act and how much room you have to course-correct through everyday changes like diet and movement.