How Type 2 Diabetes Is Diagnosed: The Tests and the Numbers

5 min readType 2 Diabetes BasicsPart 4 of 18

In brief

Type 2 diabetes is diagnosed with a blood test: A1C, fasting plasma glucose, a 2-hour oral glucose tolerance test, or a random glucose test with symptoms.
Diabetes is diagnosed at an A1C of 6.5% or more, a fasting glucose of 126 mg/dL (7.0 mmol/L) or more, or a 2-hour value of 200 mg/dL (11.1 mmol/L) or more.
Unless symptoms are obvious, a result is confirmed with a second test before diagnosis.

You cannot diagnose diabetes from how you feel or from a home glucose meter. It takes a laboratory blood test, and doctors use the same small set of tests and thresholds worldwide. Knowing what each test measures helps you understand your results and ask the right questions.

The same tests also identify prediabetes. We cover the prediabetes ranges in detail in How Prediabetes Is Diagnosed. Here we focus on the diabetes range and what happens next.

The four tests at a glance

Where do your numbers fall?

A1C (HbA1c)no fasting needed

Normalbelow 5.7%Pre5.7–6.4Diabetes6.5% or higher

Fasting plasma glucoseafter 8+ hours without food

Normalunder 100 mg/dLPre100–125Diabetes126+ (7.0 mmol/L)

2-hour glucose tolerance testafter a 75 g glucose drink

Normalunder 140 mg/dLPre140–199Diabetes200+ (11.1 mmol/L)

Markers show example results only. Random glucose of 200 mg/dL (11.1 mmol/L) or more with classic symptoms also confirms diabetes.

1. A1C (HbA1c)

A1C measures how much glucose has stuck to hemoglobin in your red blood cells. Because red cells live about three months, it reflects your average blood sugar over the past two to three months. It needs no fasting and can be done at any time of day, which is why it is the most common test. An A1C of 6.5% (48 mmol/mol) or higher means diabetes.

A1C can be misleading in some situations, including anemia, recent blood loss or transfusion, pregnancy, advanced kidney disease and some inherited hemoglobin variants that are more common in people of African, Mediterranean and South or Southeast Asian heritage. Your doctor may then rely on glucose tests instead. For more on what A1C tells you, see A1C vs Blood Glucose.

2. Fasting plasma glucose

This measures glucose in your blood after at least eight hours without food or caloric drinks, usually first thing in the morning. A result of 126 mg/dL (7.0 mmol/L) or higher means diabetes.

3. Oral glucose tolerance test (OGTT)

After a fasting sample, you drink a solution containing 75 grams of glucose, and blood is taken again two hours later. A 2-hour result of 200 mg/dL (11.1 mmol/L) or higher means diabetes. It is the most sensitive test, but also the most time-consuming, so it is used mainly in pregnancy and when other results are borderline.

4. Random plasma glucose

If you have classic symptoms of high blood sugar, such as thirst, passing a lot of urine and weight loss, a single glucose of 200 mg/dL (11.1 mmol/L) or higher at any time of day is enough to diagnose diabetes.

A standard blood sample is all that is needed for A1C and glucose tests.

Why a second test is usually needed

From first result to diagnosis

First testA1C or glucose in the diabetes range

ConfirmRepeat, or two different tests from the same sample

DiagnosisBoth results in the diabetes range

PlanBaseline checks and a treatment plan

Lab results can vary from day to day. So unless you have obvious symptoms with a clearly high glucose, the American Diabetes Association recommends confirming a diagnosis with two abnormal results. These can be the same test repeated on another day, or two different tests (for example A1C and fasting glucose) from the same blood sample. If two different tests disagree, the one in the diabetes range is repeated.

Who should be tested, and how often?

All adults from age 35.
Adults of any age with overweight or obesity plus at least one other risk factor, such as a close relative with diabetes, high blood pressure, abnormal cholesterol, inactivity, PCOS, or higher-risk ethnic background.
Anyone with prediabetes (every year), and women with a history of gestational diabetes (at least every three years).
If results are normal, repeat at least every three years, or sooner if risk factors change.

After the diagnosis: what happens next

A diagnosis is the start of a plan, not a verdict. At or soon after diagnosis, most people have a set of baseline checks so the care team knows where things stand:

Blood pressure, weight and waist measurement
Cholesterol and triglycerides
Kidney tests: a blood test (eGFR) and a urine test for protein (albumin-to-creatinine ratio)
A dilated eye exam or retinal photograph
A foot check for sensation and circulation
Liver tests, and a referral to diabetes education

Occasionally a person diagnosed as an adult actually has type 1 diabetes or another form. If you are young, lean, losing weight quickly, or not responding to usual treatment, your doctor may order antibody tests to check.

Common questions

Do I need to fast for an A1C test?

No. A1C reflects the past two to three months, so it is not affected by what you ate that morning. You do need to fast for at least eight hours before a fasting plasma glucose test, and before an oral glucose tolerance test. Water is fine.

My A1C and fasting glucose disagree. Which one is right?

Each test measures something slightly different, so they sometimes disagree. Guidelines say that if one test is in the diabetes range and the other is not, the abnormal test should be repeated. Your doctor may also consider conditions that make A1C less reliable, such as anemia or a hemoglobin variant.

Can stress or illness cause a false high result?

Serious illness, surgery, some medicines (especially steroids) and major stress can raise blood glucose temporarily. That is one reason results are confirmed with a second test when a person is well. A1C is less affected by short-term illness than glucose tests.

Key takeaways

A1C 6.5% or more, fasting glucose 126 mg/dL or more, or 2-hour OGTT 200 mg/dL or more means diabetes.
Results are usually confirmed with a second test.
Test from age 35, or earlier if you have risk factors.
Next: A1C vs blood glucose: what each number tells you.

Type 2 Diabetes Basics: all 18 articles

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).
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.
US Preventive Services Task Force. Screening for prediabetes and type 2 diabetes: recommendation statement. JAMA. 2021;326(8):736–743.
National Institute of Diabetes and Digestive and Kidney Diseases. Diabetes Tests & Diagnosis.
National Institute of Diabetes and Digestive and Kidney Diseases. The A1C Test & Diabetes.

This article is for general education and is not a substitute for advice from your own doctor, diabetes educator or pharmacist.

How we review: Every Vitablizz article is checked against the sources it cites and updated when guidance changes. This article is for education and is not a substitute for personal medical advice. Read our Editorial Policy.