A fasting blood sugar between 100 and 125 mg/dL falls in the prediabetes range. Below 100 is normal, and 126 or higher on two separate tests indicates type 2 diabetes. But fasting glucose is only one of three tests used to identify prediabetes, and each measures something slightly different.
The Three Prediabetes Tests and Their Ranges
Doctors can diagnose prediabetes using any of three standard blood tests. Each has its own cutoff values, set by the American Diabetes Association.
Fasting plasma glucose (FPG) measures your blood sugar after at least eight hours without eating. A result of 100 to 125 mg/dL is prediabetic. This is the most common screening test because it’s simple and inexpensive.
A1C reflects your average blood sugar over the past two to three months, expressed as a percentage. An A1C between 5.7% and 6.4% means prediabetes. Below 5.7% is normal, and 6.5% or higher indicates diabetes. Because it captures a longer window, A1C can catch blood sugar problems that a single fasting test might miss on a good day.
Oral glucose tolerance test (OGTT) measures how your body handles a large dose of sugar. You drink a sugary solution, then have your blood drawn two hours later. A reading of 140 to 199 mg/dL at the two-hour mark signals prediabetes. Anything 200 or above points to diabetes. This test is more sensitive at detecting early glucose problems but takes longer and is used less often in routine screening.
These three tests don’t always agree. You might have a normal fasting glucose but an elevated A1C, or vice versa. If one test comes back in the prediabetic range, your doctor will typically repeat it or run a second type of test to confirm.
What Normal Levels Look Like
For comparison, here’s what healthy blood sugar numbers look like across all three tests:
- Fasting glucose: below 100 mg/dL
- A1C: below 5.7%
- Two-hour glucose tolerance: below 140 mg/dL
If your numbers sit right at the lower edge of the prediabetes range (say, a fasting glucose of 101 or an A1C of 5.7%), the situation is very different from someone at the upper end (125 mg/dL fasting or 6.4% A1C). Higher numbers within the prediabetic range carry a greater risk of progressing to diabetes.
Why These Numbers Matter
Prediabetes means your body is losing its ability to manage blood sugar efficiently. In most people, this involves one of two problems, or both working together.
If you carry excess weight, the primary issue is usually insulin resistance. Your cells stop responding well to insulin, so your pancreas has to produce more and more of it to keep blood sugar in check. Eventually the pancreas can’t keep up, and glucose levels start creeping upward. In leaner individuals, research published in the Journal of the Endocrine Society found a different pattern: the pancreas gradually produces less insulin, even without significant resistance. The decline in insulin production was actually the bigger driver of rising blood sugar in lean people with prediabetes.
Either way, prediabetic glucose levels represent a middle zone where damage is beginning but hasn’t become severe. Blood vessels, nerves, and kidneys that are fine at normal glucose levels start experiencing low-grade stress in this range.
How Likely Is Progression to Diabetes
About 5 to 10% of people with prediabetes develop type 2 diabetes each year. That means without any changes, roughly half of people with prediabetes will become diabetic within a decade. But that progression isn’t inevitable. A large pooled analysis of 19 prospective studies found that many people with prediabetes return to normal blood sugar levels, particularly those who make early lifestyle changes.
The factors that accelerate progression are predictable: weight gain, physical inactivity, a diet high in refined carbohydrates, and a family history of diabetes. The factors that slow or reverse it are equally predictable: losing 5 to 7% of your body weight (about 10 to 14 pounds for someone who weighs 200), getting 150 minutes of moderate activity per week, and shifting toward meals built around vegetables, whole grains, and lean protein.
Why Most People Don’t Know They Have It
Prediabetes almost never causes noticeable symptoms. Most people find out through routine blood work, not because something felt wrong. An estimated 98 million American adults have prediabetes, and the majority don’t know it.
There is one physical sign worth knowing about: darkened, velvety patches of skin called acanthosis nigricans. These tend to appear on the neck, armpits, or groin and are a visible marker of insulin resistance. Skin tags in these areas can also be associated with elevated insulin levels. But most people with prediabetes have no outward signs at all, which is why blood testing is the only reliable way to catch it.
Which Test to Ask For
If you’re getting screened for the first time, a fasting glucose or A1C test is the standard starting point. Both are simple blood draws. The A1C has the advantage of not requiring you to fast, making it easier to fit into a regular appointment. The oral glucose tolerance test is more involved but can detect problems earlier, especially in people whose fasting numbers still look normal.
If your results land in the prediabetic range, retesting in three to six months gives you a clear picture of whether your numbers are stable, improving, or getting worse. That trajectory matters more than any single reading.