- Prediabetes is diagnosed with a blood test: HbA1c, fasting glucose or a 2-hour glucose tolerance test.
- Prediabetes ranges: HbA1c 5.7–6.4%, fasting glucose 100–125 mg/dL, or 2-hour glucose 140–199 mg/dL.
- A result in the prediabetes range is usually confirmed with a repeat test.
- Adults 35 and older, and younger adults with risk factors, should be tested even without symptoms.
Because prediabetes rarely causes symptoms, a blood test is the only reliable way to know. The good news is that testing is quick, widely available and usually inexpensive. There are three main tests. Each looks at blood sugar from a slightly different angle, and your doctor may use one or more of them.
The three tests at a glance
| Test | What it measures | Fasting? | Good to know |
|---|---|---|---|
| HbA1c | Average blood sugar over the past 2–3 months | No | Convenient; less accurate with anemia, some hemoglobin variants, pregnancy or recent blood loss |
| Fasting plasma glucose | Blood sugar after at least 8 hours without food | Yes | Simple and cheap; shows how well the liver controls sugar overnight |
| Oral glucose tolerance test (OGTT) | Blood sugar 2 hours after a 75 g sugar drink | Yes | Most sensitive for after-meal problems; takes about 2 hours |
How each test works
HbA1c measures the percentage of hemoglobin, the oxygen-carrying protein in red blood cells, that has glucose stuck to it. Red blood cells live for about three months, so HbA1c reflects your average blood sugar over that time. One day of unusual eating hardly changes it, which makes it a stable, convenient test.
Fasting plasma glucose is a snapshot of your blood sugar first thing in the morning. Overnight, your liver releases glucose to keep you going; if the liver is insulin resistant, it releases too much, and fasting glucose creeps up.
The oral glucose tolerance test is a stress test for your blood sugar system. After a fasting blood sample, you drink a sweet solution containing 75 grams of glucose, then have blood taken again two hours later. It picks up people whose fasting sugar is normal but whose bodies struggle after meals, which is often the earliest change.
The ranges that matter
American Diabetes Association diagnostic ranges
Markers show example results in the prediabetes range. The WHO and UK use a fasting range of 110–125 mg/dL (6.1–6.9 mmol/L) and an HbA1c range of 6.0–6.4% (42–47 mmol/mol).
You only need one test in the prediabetes range to be diagnosed, but doctors usually confirm it with a second test, either the same test on another day or a different test. It is quite common for the tests to disagree slightly: HbA1c might be normal while fasting glucose is raised, or the other way round. That is because each measures a different part of the picture.
How to prepare
Getting an accurate fasting or OGTT result
- Tell the lab if you are unwell, recently had surgery or a blood transfusion, or are pregnant, as these affect results.
- Steroid tablets, some diuretics and some antipsychotic medicines can raise blood sugar.
- For HbA1c you do not need to fast, but anemia, kidney disease and certain hemoglobin traits can make it less reliable.
Who should get tested?
Test at least every 3 years if results are normal.
At any age: family history, high blood pressure, low HDL or high triglycerides, inactivity, or high-risk ethnicity.
Every 1–3 years for life, because risk stays high.
Re-test every year to see how you are doing.
Not sure if you need a test? Our Diabetes Risk Check takes about a minute and tells you whether testing is recommended.
What happens after a prediabetes result
A prediabetes result is not a diagnosis of disease; it is a chance to act. Your doctor may also check your blood pressure, cholesterol, kidney function and liver enzymes, because these often change together. Many health systems offer structured lifestyle programs, such as the National Diabetes Prevention Program in the US or the NHS Diabetes Prevention Programme in England. Ask whether you can be referred.
You can also track progress yourself: a repeat HbA1c after 3–6 months of changes shows whether your average blood sugar is moving in the right direction. See Treating Prediabetes for what to focus on.
MythA home glucose meter can tell me if I have prediabetes.
FactFingerstick meters are not accurate enough for diagnosis. Use a lab test ordered by your doctor.
Common questions
My HbA1c is normal but my fasting glucose is high. Which is right?
Both can be right. They measure different things. Your doctor will usually repeat the abnormal test or add an OGTT to decide.
Can I have a test at a pharmacy?
Some pharmacies offer HbA1c screening. It is a useful first check, but results in the prediabetes or diabetes range should be confirmed by your doctor.
How much does testing cost?
HbA1c and fasting glucose are among the cheapest routine blood tests, and screening is covered by many insurance plans and public health systems for people at risk. Ask your clinic what applies to you.
Should children be tested?
Children and teens who are overweight and have other risk factors, such as a family history of diabetes, may be tested from puberty or age 10. Ask your child’s doctor.
Already diagnosed with type 2 diabetes? Read How Type 2 Diabetes Is Diagnosed in our 18-part Type 2 Diabetes Basics series.
- HbA1c, fasting glucose and the OGTT can all diagnose prediabetes.
- Prediabetes: HbA1c 5.7–6.4%, fasting glucose 100–125 mg/dL, 2-hour glucose 140–199 mg/dL.
- Results are usually confirmed with a second test.
- Test from age 35, or earlier with risk factors, and yearly once you have prediabetes.
- Next: What happens to prediabetes over time?
Sources
- American Diabetes Association Professional Practice Committee. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2026. Diabetes Care. 2026;49(Suppl 1).
- US Preventive Services Task Force. Screening for prediabetes and type 2 diabetes: US Preventive Services Task Force recommendation statement. JAMA. 2021;326(8):736–743.
- International Expert Committee. International Expert Committee report on the role of the A1C assay in the diagnosis of diabetes. Diabetes Care. 2009;32(7):1327–1334.
- Tabák AG, Herder C, Rathmann W, Brunner EJ, Kivimäki M. Prediabetes: a high-risk state for diabetes development. Lancet. 2012;379(9833):2279–2290.
- National Institute for Health and Care Excellence. Type 2 diabetes: prevention in people at high risk (PH38).
This article is for general education and is not a substitute for advice from your own doctor or healthcare team.