How is prediabetes diagnosed?

Prediabetes is diagnosed with a blood test. The American Diabetes Association recognises three: an A1c of 5.7 to 6.4 percent, a fasting plasma glucose of 100 to 125 mg/dL, or a 2-hour oral glucose tolerance test result of 140 to 199 mg/dL. Any one of these can indicate prediabetes.

Why can different glucose tests give different results?

The A1c, fasting plasma glucose, and oral glucose tolerance test each measure glucose in a different way and over a different timeframe, so a person can be flagged by one test and not another. A clinician interprets the results together and may repeat testing.

Does a single test confirm prediabetes?

Clinicians often confirm an abnormal result with repeat testing, particularly when a value is close to a threshold or when tests disagree. The American Diabetes Association describes confirmation testing as part of standard practice, and a healthcare provider decides how to proceed.

Prediabetes · diagnosis

How Prediabetes Is Diagnosed

7 min read · Updated July 2026

Prediabetes is diagnosed with a blood test, not with symptoms. The American Diabetes Association (ADA) recognises three standard tests — A1c, fasting plasma glucose, and the oral glucose tolerance test — each with its own cutoff. Any one of them can identify prediabetes, and understanding how they differ helps make sense of results that do not always agree.

Three tests, one goal

All three tests answer the same question from different angles: is glucose regulation drifting above normal but staying below the diabetes threshold? Because prediabetes is usually silent, testing is the only reliable way to find it. For context on what the condition is, see the prediabetes guide and our explainer on what prediabetes is.

1. A1c (HbA1c)

The A1c test reflects average blood glucose over roughly the previous two to three months, so it does not require fasting and is not thrown off by a single meal. Per the ADA, an A1c of 5.7–6.4% is the prediabetes range; below 5.7% is considered normal, and 6.5% or higher is in the diabetes range. Our detailed article on the prediabetes A1c range focuses on this test, and the HbA1c calculator shows how A1c relates to average glucose.

2. Fasting plasma glucose (FPG)

The fasting plasma glucose test measures blood sugar after an overnight fast, typically at least eight hours without eating. Per the ADA, an FPG of 100–125 mg/dL is called impaired fasting glucose and falls in the prediabetes range; below 100 mg/dL is normal, and 126 mg/dL or higher is in the diabetes range. See impaired fasting glucose for a closer look.

3. Oral glucose tolerance test (OGTT)

The OGTT measures how the body handles a standardised glucose drink. Blood sugar is checked before and two hours after drinking it. Per the ADA, a 2-hour value of 140–199 mg/dL is called impaired glucose tolerance and indicates prediabetes; below 140 mg/dL is normal, and 200 mg/dL or higher is in the diabetes range. See impaired glucose tolerance for detail.

Standard ADA cutoffs. Prediabetes occupies the middle band on each test. Thresholds are attributed to the American Diabetes Association and may be updated over time.

Why results can vary between tests

Because the three tests measure glucose differently and over different timeframes, it is entirely possible to be flagged by one and not another. The A1c captures a two-to-three-month average, the FPG captures a single fasting snapshot, and the OGTT captures the body's response to a glucose load. A person might, for example, have a normal fasting glucose but an OGTT in the impaired range. Individual factors can also affect A1c specifically, which is one reason clinicians do not rely on any single number in isolation. All of this sits under the broader idea of dysglycemia.

Confirmation testing

A single abnormal result does not automatically settle the picture. The ADA describes confirming abnormal results with repeat testing, especially when a value sits close to a threshold or when two tests disagree. In practice a clinician may repeat the same test on a separate day, or order a different test, before drawing conclusions. This is a strength of the process, not a delay — it reduces the chance of acting on a one-off reading.

Who should be tested, and what comes next

Screening is generally guided by risk rather than symptoms. Factors such as age, excess weight, family history, physical inactivity, and a history of gestational diabetes all raise risk; see prediabetes risk factors for the full list as described by the CDC and ADA. If testing points to prediabetes, the encouraging news is that the earlier end of the spectrum often responds to everyday changes in diet and physical activity, and the broader question of whether prediabetes can be reversed is worth exploring with a clinician. Tools such as continuous glucose monitoring can add day-to-day context under professional guidance.

Endobits is clinical decision-support software used under clinician oversight. It helps put glucose data in context against the standard ranges; it does not itself diagnose prediabetes or replace a clinician's judgement.

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Sources

American Diabetes Association — Understanding Diagnosis (A1c, FPG, and OGTT criteria). Centers for Disease Control and Prevention — Diabetes Testing. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) — Diabetes Tests & Diagnosis.

This article is educational and not medical advice. Diagnostic thresholds are attributed to the American Diabetes Association and may be updated over time. Talk to a qualified healthcare provider about your health. Endobits is clinical decision-support software used under clinician oversight, not a diagnostic device.