Type 2 diabetes is diagnosed through blood glucose (sugar) testing, typically performed in a healthcare provider's office or clinical laboratory.

This chronic condition occurs when the body cannot properly use insulin, resulting in high blood sugar levels. Over time, elevated blood sugar can lead to serious health issues, including heart disease, kidney failure, vision loss, and nerve damage.

In the United States, type 2 diabetes accounts for the majority of diagnosed diabetes cases.

Getting a Diabetes Diagnosis

Healthcare providers can detect both type 2 diabetes and its early stage—prediabetes—by measuring blood sugar levels. These tests may be part of a routine physical or performed when evaluating another health concern.

If your blood sugar is higher than normal, your provider may order one or more of the following tests to confirm a diagnosis.

A1C Test

Also known as glycated hemoglobin or HbA1C, the A1C test measures your average blood sugar over the past two to three months. It works by assessing how much of your hemoglobin (the oxygen-carrying protein in red blood cells) is coated with sugar.

A1C levels and what they mean:

  • Normal: Less than 5.7%

  • Prediabetes: 5.7% to 6.4%

  • Diabetes: 6.5% or higher

The A1C test may not be suitable for everyone, such as individuals who are pregnant, have anemia, or have undergone recent blood transfusions or dialysis. Additionally, some population groups may require alternative testing.

Oral Glucose Tolerance Test (OGTT)

The OGTT assesses how well your body processes sugar. You’ll first give a fasting blood sample, then drink a sugary beverage containing about 75 grams of glucose. After two hours, another blood sample is taken to measure your blood sugar response.

Results of OGTT:

  • Normal: Less than 140 mg/dL

  • Prediabetes: 140 to 199 mg/dL

  • Diabetes: 200 mg/dL or more

Fasting Plasma Glucose (FPG) Test

This test measures blood sugar after fasting for at least 8 hours (usually overnight).

FPG levels:

  • Normal: Less than 100 mg/dL

  • Prediabetes: 100 to 125 mg/dL

  • Diabetes: 126 mg/dL or higher

Random Plasma Glucose Test

Also known as a casual glucose test, this test can be done any time of day without fasting. It's usually used if you show clear symptoms of diabetes. A reading of 200 mg/dL or higher may indicate diabetes.

Progression of Type 2 Diabetes

Most people develop prediabetes before progressing to type 2 diabetes. Gestational diabetes, which occurs during pregnancy, is another form that may increase future risk of type 2 diabetes.

Prediabetes

Prediabetes is diagnosed when blood sugar is elevated but not high enough to qualify as diabetes. It usually presents no clear symptoms.

Risk factors include:

  • High cholesterol

  • Age over 45

  • Family history of diabetes

  • High blood pressure

  • History of heart disease or stroke

  • Obesity

  • Physical inactivity

A prediabetes diagnosis means you may develop type 2 diabetes in the future without lifestyle changes.

Type 2 Diabetes

In type 2 diabetes, blood sugar levels are consistently high. Common symptoms include:

  • Frequent urination

  • Excessive thirst or hunger

  • Fatigue

  • Blurry vision

  • Slow-healing wounds

  • Tingling or numbness in extremities

If you experience any of these symptoms, your provider will confirm the diagnosis with one or more of the standard tests.

Gestational Diabetes

This form of diabetes can develop during pregnancy, typically between weeks 24–28. It is diagnosed through a modified oral glucose test and increases the risk of developing type 2 diabetes after childbirth.

One-step process:

  • Fast overnight

  • Drink a 75g glucose drink

  • Blood samples are taken at fasting, 1 hour, and 2 hours

Diagnosis is likely if levels are:

  • ≥ 92 mg/dL at fasting

  • ≥ 180 mg/dL after 1 hour

  • ≥ 153 mg/dL after 2 hours

Two-step process:

  • Step 1: Drink a 50g glucose drink; if 1-hour result is ≥ 140 mg/dL, move to step 2

  • Step 2: Fast overnight, drink a 100g glucose drink; blood is tested four times over three hours
    Diagnosis is made if two or more readings are elevated.

Diabetes Comorbidities

Type 2 diabetes often co-occurs with other chronic health issues due to shared risk factors. Common comorbid conditions include:

  • Heart disease

  • Hypertension (high blood pressure)

  • Disorders of lipid metabolism

  • Osteoarthritis

Mental health issues, especially depression, are also common and can interfere with diabetes management. Healthcare providers should monitor mental health as part of diabetes care.

Screening for Related Conditions

Certain diseases and medications can mimic or contribute to diabetes. These include:

  • Iron overload disorders

  • Pancreatic diseases (e.g., pancreatitis, cystic fibrosis)

  • Endocrine disorders (e.g., thyroid imbalances)

  • Genetic issues with pancreatic function

  • Infections

  • Metabolic syndromes

  • Use of specific medications (e.g., corticosteroids, neuroleptics)

It's important to inform your healthcare provider of any existing conditions or medications you’re taking during diabetes testing.

A Quick Review

Type 2 diabetes is a common chronic disease caused by insulin dysfunction, resulting in elevated blood sugar levels. Diagnosis is made through one or more blood sugar tests, including the A1C, FPG, OGTT, and random glucose tests.

Early detection and diagnosis are key to managing the condition and preventing complications. If you're at risk, speak with a healthcare provider about getting tested.