A fasting blood sugar of 126 mg/dL or higher, or an A1C of 6.5% or higher, is considered diabetic. These are the two most common tests used for diagnosis, but there are several ways to measure blood sugar, each with its own threshold. Here’s what every number means and how the tests work together.
The Four Tests and Their Cutoffs
There are four blood tests that can diagnose diabetes, and each measures something slightly different. The numbers below reflect the current American Diabetes Association standards, which have remained unchanged in the most recent 2026 guidelines.
- Fasting blood sugar: 126 mg/dL or higher. You must fast for at least 8 hours beforehand (water is fine). A normal result is below 100 mg/dL.
- A1C (hemoglobin A1C): 6.5% or higher. This reflects your average blood sugar over the past two to three months. No fasting required. Normal is below 5.7%.
- Oral glucose tolerance test (OGTT): 200 mg/dL or higher two hours after drinking a sugary solution. Normal is below 140 mg/dL.
- Random blood sugar: 200 mg/dL or higher, but only when combined with classic symptoms like excessive thirst, frequent urination, or unexplained weight loss. This test can be taken at any time of day regardless of when you last ate.
Any one of these results, when confirmed, is enough to establish a diagnosis. Your doctor will choose which test to use based on your situation, though fasting glucose and A1C are the most commonly ordered in routine checkups.
The Prediabetes Range
Between normal and diabetic, there’s a middle zone called prediabetes. These numbers signal that your blood sugar is elevated but hasn’t crossed the diabetes threshold yet.
- Fasting blood sugar: 100 to 125 mg/dL
- A1C: 5.7% to 6.4%
- Oral glucose tolerance test: 140 to 199 mg/dL at the two-hour mark
Prediabetes is significant because it’s the stage where lifestyle changes, like losing a modest amount of weight or increasing physical activity, can prevent or delay progression to type 2 diabetes. Many people with prediabetes have no symptoms at all, which is why routine screening matters.
Why These Specific Numbers?
The cutoffs aren’t arbitrary. They’re based on the blood sugar level where the risk of serious complications starts to climb sharply. One key study found that people with a fasting glucose at or above 126 mg/dL (7.0 mmol/L) had roughly 3.6 times the risk of developing retinopathy, a type of eye damage, compared to those with lower levels. The same pattern held for A1C: at 6.5% or above, the risk of retinopathy was about 3.4 times higher. These thresholds mark the point where high blood sugar begins to damage small blood vessels, particularly in the eyes, kidneys, and nerves.
Why You Usually Need Two Tests
A single abnormal result generally isn’t enough for a definitive diagnosis. Blood sugar fluctuates throughout the day based on stress, illness, meals, and sleep, so one high reading could be a fluke. The standard protocol requires two abnormal results, either from the same blood sample using two different tests or from repeating the same test on a separate occasion. If both come back in the diabetic range, the diagnosis is confirmed.
The exception is if you already have obvious symptoms of very high blood sugar, like extreme thirst, frequent urination, and unexplained weight loss. In that case, a single random blood sugar of 200 mg/dL or higher is enough. People in this situation often feel noticeably unwell, which is what distinguishes this scenario from a routine screening.
If two different tests give conflicting results (say your A1C is 6.5% but your fasting glucose is 118 mg/dL), the test that came back above the threshold gets repeated. The diagnosis is based on whichever test is confirmed.
When A1C May Not Be Accurate
The A1C test works by measuring how much sugar has attached to your red blood cells over their lifespan. Anything that changes how long your red blood cells live, or alters the hemoglobin inside them, can throw off the result.
Certain inherited hemoglobin variants, most common in people of African, Mediterranean, or Asian descent, can cause A1C results to read falsely high or low depending on the lab method used. The four most common variants are hemoglobin S (the sickle cell trait), hemoglobin C, hemoglobin D, and hemoglobin E. Conditions like significant kidney disease, liver failure, and certain types of anemia can also affect A1C accuracy by shortening or lengthening the life of red blood cells.
If any of these apply to you, your doctor may rely on fasting glucose or the oral glucose tolerance test instead, since those measure your blood sugar directly rather than through an indirect marker.
Gestational Diabetes Uses Different Numbers
Pregnant women are screened for gestational diabetes using a modified version of the glucose tolerance test, and the thresholds are lower than those used for type 2 diabetes. This is because even moderately elevated blood sugar during pregnancy can affect the baby’s development. If you’re pregnant, your provider will walk you through the specific screening process, which typically happens between weeks 24 and 28. The numbers and testing steps differ enough from standard diabetes diagnosis that they’re treated as a separate category entirely.
What Your Number Means in Practice
If your fasting glucose comes back at, say, 115 mg/dL, you’re in the prediabetes range. That’s not a diabetes diagnosis, but it’s a clear signal your body is struggling to manage blood sugar efficiently. At 130 mg/dL, you’ve crossed the threshold, and a repeat test will likely confirm diabetes if the first wasn’t a one-time spike from illness or stress.
A1C gives you a broader picture. Because it averages your blood sugar over two to three months, it’s less sensitive to day-to-day variation. An A1C of 6.0% means your average blood sugar has been in the prediabetic range. At 6.5%, you’ve hit the diabetes cutoff, and at 7.0% or above, blood sugar has been consistently elevated enough that treatment becomes important to prevent complications.
These numbers exist on a continuum. There’s no biological switch that flips at exactly 126 mg/dL or 6.5%. But these thresholds represent the point where the risk of organ damage rises meaningfully, making them the line where monitoring shifts to active management.