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How to diagnose diabetes

Are you on the road to type 2 Diabetes?

Are you on the road to type 2 Diabetes?

You’ll need to get your blood sugar tested to find out for sure if you have prediabetes or type 1, type 2, or gestational diabetes. Testing is simple, and results are usually available quickly.

Tests for Type 1 Diabetes, Type 2 Diabetes, and Prediabetes

Your doctor will have you take one or more of the following blood tests to confirm the diagnosis:

A1C Test

The A1C test measures your average blood sugar level over the past 2 or 3 months. An A1C below 5.7% is normal, between 5.7 and 6.4% indicates you have prediabetes, and 6.5% or higher indicates you have diabetes.

Fasting Blood Sugar Test

This measures your blood sugar after an overnight fast (not eating). A fasting blood sugar level of 99 mg/dL or lower is normal, 100 to 125 mg/dL indicates you have prediabetes, and 126 mg/dL or higher indicates you have diabetes.

Glucose Tolerance Test

This measures your blood sugar before and after you drink a liquid that contains glucose. You’ll fast (not eat) overnight before the test and have your blood drawn to determine your fasting blood sugar level. Then you’ll drink the liquid and have your blood sugar level checked 1 hour, 2 hours, and possibly 3 hours afterward. At 2 hours, a blood sugar level of 140 mg/dL or lower is considered normal, 140 to 199 mg/dL indicates you have prediabetes, and 200 mg/dL or higher indicates you have diabetes.

Random Blood Sugar Test

This measures your blood sugar at the time you’re tested. You can take this test at any time and don’t need to fast (not eat) first. A blood sugar level of 200 mg/dL or higher indicates you have diabetes.

Random Blood Sugar Test

Result* A1C Test Fasting Blood Sugar Test Glucose Tolerance Test Random Blood Sugar Test
Diabetes 6.5% or above 126 mg/dL or above 200 mg/dL or above 200 mg/dL or above
Prediabetes 5.7 – 6.4% 100 – 125 mg/dL 140 – 199 mg/dL N/A
Normal Below 5.7% 99 mg/dL or below 140 mg/dL or below N/A

*Results for gestational diabetes can differ. Ask your health care provider what your results mean if you’re being tested for gestational diabetes.
Source: American Diabetes Association

If your doctor thinks you have type 1 diabetes, your blood may also tested for autoantibodies (substances that indicate your body is attacking itself) that are often present in type 1 diabetes but not in type 2 diabetes. You may have your urine tested for ketones (produced when your body burns fat for energy), which also indicate type 1 diabetes instead of type 2 diabetes.

Tests for Gestational Diabetes

Gestational diabetes is diagnosed using blood tests. You’ll probably be tested between 24 and 28 weeks of pregnancy. If your risk is higher for getting gestational diabetes (due to having more risk factors), your doctor may test you earlier. Blood sugar that’s higher than normal early in your pregnancy may indicate you have type 1 or type 2 diabetes rather than gestational diabetes.

Glucose Screening Test

This measures your blood sugar at the time you’re tested. You’ll drink a liquid that contains glucose, and then 1 hour later your blood will be drawn to check your blood sugar level. A normal result is 140 mg/dL or lower. If your level is higher than 140 mg/dL, you’ll need to take a glucose tolerance test.

Glucose Tolerance Test

This measures your blood sugar before and after you drink a liquid that contains glucose. You’ll fast (not eat) overnight before the test and have your blood drawn to determine your fasting blood sugar level. Then you’ll drink the liquid and have your blood sugar level checked 1 hour, 2 hours, and possibly 3 hours afterward. Results can differ depending on the size of the glucose drink and how often your blood sugar is tested. Ask your doctor what your test results mean.

Prevent Type 2 Diabetes

If your test results show you have prediabetes, ask your doctor or nurse if there is a lifestyle change program offered through the CDC-led National Diabetes Prevention Program in your community. You can also search for an online or in-person program. Having prediabetes puts you at greater risk for developing type 2 diabetes, but participating in the program can lower your risk by as much as 58% (71% if you’re over age 60).

Diabetes Treatment Plan

If your test results show you have type 1, type 2, or gestational diabetes, talk with your doctor or nurse about a detailed treatment plan—including diabetes self-management education and support services—and specific steps you can take to be your healthiest.

How to diagnose diabetes

Diagnosis for both type 1 and type 2 diabetes can occur in a number of different ways.

Usually type 2 is diagnosed by diabetes symptoms, such as polyuria (excessive urination) and polydipsia (excessive thirst).

Otherwise, diabetes is picked up through screening, hyperglycaemia when doctor investigates a complication, or signs and symptoms prompted by diabetes

What is a diabetes screening test?

A screening test determines whether a person has diabetes, and how serious it is. Depending on where you are and what your circumstances are, the screening test will vary.

Tests include:

  • Random blood glucose tests – commonly used to test for type 1 diabetes
  • Urine glucose test
  • Fasting plasma glucose tests (FPG tests)
  • HbA1c test
  • Oral glucose tolerance tests

Additional diagnostic tests, such as urine ketone tests, GAD autoantibodies tests or C-peptide tests may also be used, as part of the diagnosis, to distinguish between type 1 and type 2 diabetes.

For adults aged between 40 and 50, screening should be considered. For people who have higher risk factors (ethnicity, family history, obesity) screening should be conducted beforehand.

If you have recently been diagnosed with diabetes by your healthcare team, please see our guide for newly diagnosed

Measuring hemoglobin A1c (HbA1c) can help diagnose cases of prediabetes and type 2 diabetes.

According to diagnostic guidelines set out by the World Health Organisation (WHO), a HbA1c value of:

  • 6% (below 42 mmol/mol) is considered non-diabetic
  • 6-6.4% (42 to 47 mmol/mol) indicates impaired fasting glucose regulation and is considered prediabetes
  • 6.5% or more (48 mmol/mol and above) indicates the presence of type 2 diabetes

Diabetes screening is strongly recommended for adults aged between 40 and 50 years, or earlier for any adults with one or more risk factors ( ethnicity, family history, obesity, etc).

How to diagnose diabetes

A diabetes diagnosis must come from a doctor or other qualified medical professional because of the nature of the illness. Diabetes is a disease in which, because of problems with the hormone insulin, glucose (sugar created during digestion) can’t enter the cells of the body but instead stays in the bloodstream. Blood sugar builds to unhealthy levels, a condition called hyperglycemia. The only way to know your blood sugar levels is through blood tests in a medical lab ordered by your doctor. Let’s look at the types of tests used for a diabetes diagnosis.

Types of Tests Used to Diagnose Diabetes

A common question is, How do they test for diabetes? Doctors order different types of lab tests to diagnose type 1 and type 2 diabetes (“What Is the Difference Between Type 1 and Type 2 Diabetes?”). These include:

  • Glycated hemoglobin test (more commonly referred to as the A1C)
  • Random (or casual) plasma glucose test
  • Fasting plasma glucose test (FPG)
  • Oral glucose tolerance test (OGTT)

While all tests measure hyperglycemia, each one has a slightly different procedure and diabetes diagnosis criteria, or an acceptable range of blood sugar levels.

Diabetes Diagnostic Tests Explained

Knowing about these tests and diabetes diagnostic criteria can help you mentally prepare for diabetes testing and thus reduce anxiety about the process.

The A1C examines how much blood glucose has been attached to hemoglobin over the past two to three months. This is different from other tests. Most tests measure the amount of glucose in the bloodstream during the test. An A1C test identifies trends by looking at how much glucose is attached to hemoglobin, a protein in red blood cells. The result is measured in percentages.

In the A1C test to diagnose diabetes:

  • A sample of your blood is drawn
  • You don’t have to fast (go without eating or drinking)
  • You don’t have to drink a glucose solution
  • If the result is 6.5 percent or higher after two tests, a diabetes diagnosis is given
  • A result of 5.7 and 6.4 percent indicates prediabetes
  • Anything below 5.7 percent is considered normal

The random plasma glucose test measures the amount of glucose in the blood at the time of the test. This test:

  • Involves the drawing of a blood sample
  • Is done any time of day
  • Is random in that it isn’t planned around the timing of meals.
  • Diagnoses diabetes in the blood sugar level is 200 mg/dL (milligrams per deciliter) or higher

The fasting plasma glucose test is another test that measures blood sugar levels. In this test,

  • Blood is drawn after fasting (usually first thing in the morning)
  • If the blood test shows that the blood glucose level is 126 mg/DL or higher, a diabetes diagnosis is given
  • A reading between 100 mg/dL and 125 ml/dL indicates prediabetes
  • Lower than 100 mg/dL is normal

The oral glucose tolerance test is another test to determine if hyperglycemia is present and high enough for a diagnosis of diabetes. In the OGTT,

  • After 8 hours of fasting, usually overnight, blood is drawn to measure the fasting blood sugar level
  • The person being tested drinks a glucose solution
  • Blood is drawn periodically over the next two hours to see how blood glucose levels react
  • At the end of the test, a blood glucose level of 200 ml/dL leads to a diabetes diagnosis
  • Blood sugar levels between 140 and 199 mg/dL indicate prediabetes
  • Healthy blood sugar levels are under 140 mg/dL

The glucose challenge test and glucose tolerance test are used to diagnose gestational diabetes, or diabetes that a woman develops during pregnancy and ends within six weeks after delivery. This test is routinely given during a prenatal check-up, typically during weeks 24-28 of pregnancy.

  • The glucose challenge test happens first
  • The woman drinks a glucose solution
  • Blood is drawn one hour later to measure blood sugar levels
  • If the reading is above 126 mg/dL, a follow-up test is needed
  • The follow-up glucose tolerance test is the same as before but with a solution containing a higher concentration of glucose
  • Blood sugar levels are checked hourly for three hours
  • If at least two of the hourly readings are above 126, gestational diabetes is diagnosed

There is one more important answer to the question How do you diagnose diabetes? The answer? More than once. As with any type of medical testing, there can be interferences that affect the results. Some prescription medications, being under extreme stress, and even trauma can influence test results. Your doctor will likely verify your prescription medications as well as talk with you about lifestyle factors before ordering blood glucose tests.

In many cases, if a test indicates hyperglycemia, it’s repeated on a second day to ensure that a positive result for diabetes is consistent and wasn’t an error. Then, if you are given a diabetes diagnosis, you can begin to treat diabetes immediately and take charge of your health and wellbeing (“What Are Diabetes Treatment Guidelines?”).

How to diagnose diabetes

Nearly 33 million American adults have Type 2 diabetes — and many don’t know it. Type 1 diabetes often remains undiagnosed until symptoms become so severe that hospitalization is required.

Both of these facts speak to a larger truth: Left untreated, diabetes can cause many health complications. That’s why it’s crucial to know the warning signs and to see a healthcare professional regularly for routine wellness screenings.

Symptoms

Prediabetes has no clear symptoms.

Those with Type 1 or Type 2 diabetes also may have no symptoms — or such mild symptoms that they go unnoticed for quite some time.

But since some people experience diabetes warning signs, it’s important to know the symptoms below:

Prediabetes Type 1 Diabetes Type 2 Diabetes
No clear symptoms Increased or extreme thirst Increased thirst
Increased appetite Increased appetite
Increased fatigue Fatigue
Increased or frequent urination Increased urination, especially at night
Unusual weight loss Weight loss
Blurred vision Blurred vision
Fruity odor or breath Sores that do not heal
In some cases, no symptoms In some cases, no symptoms

If you have any of these symptoms, see your health care professional right away for a check-up and proper diagnosis.

Who should be tested for prediabetes and diabetes?

The U.S. Department of Health and Human Services recommends that you should be tested if you are:

  • Over age 45
  • Overweight, younger than 45 and have one or more additional risk factors, such as:
    • High blood pressure
    • High cholesterol
    • A family history of diabetes
    • Are African-American, Asian-American, Latino/Hispanic-American, Native American or of Pacific Islander descent
    • Have a history of gestational diabetes (diabetes during pregnancy) or delivering a baby more than nine pounds

If your blood glucose levels are normal, you should be tested about every three years. If you have prediabetes, you should be checked for diabetes every one to two years after that diagnosis.

Tests for Diagnosing Prediabetes and Diabetes

Three tests can help health care professionals make a diagnosis of prediabetes and diabetes:

HbA1C (A1C or glycosylated hemoglobin test)

The A1C test can diagnose prediabetes and diabetes. It measures your average blood glucose control for the past two to three months. Blood sugar is measured by the amount of glycosylated hemoglobin (A1C) in your blood. This test is more convenient because no fasting is required.

An A1C of 5.7% to 6.4% means that you have prediabetes, and you’re at high risk for developing diabetes. Diabetes is diagnosed when the A1C is 6.5% or higher.

Fasting Plasma Glucose Test

A fasting plasma glucose test requires fasting (nothing to eat or drink except water) for eight hours before the test.

You will have your blood drawn for this test. Then the plasma (the fluid part of the blood) is combined with other substances to determine the amount of glucose in the plasma. Blood glucose is measure in milligrams per deciliter (mg/dL).

This chart contains the FPG test’s blood glucose ranges for prediabetes and diabetes, and describes what each diagnosis means:

Blood Glucose Range Diagnosis What It Means
100 to 125 mg/dL Prediabetes (also called impaired fasting glucose) Blood glucose levels are higher than normal, but not high enough to be diagnosed as diabetes. This condition increases risk for developing Type 2 diabetes, heart disease and stroke.
126 mg/dL or more Diabetes mellitus (Type 2 diabetes) Type 2 diabetes develops when your body doesn’t make enough insulin or develops “insulin resistance” and can’t efficiently use the insulin it makes. It greatly increases your risk of heart disease and stroke.

Oral Glucose Tolerance Test

This test measures how well the body handles a standard amount of glucose.

To do this test, your health care professional will draw blood before and two hours after you drink a large, premeasured beverage containing glucose. Then, your doctor can compare the before-and-after glucose levels contained in your plasma to see how well your body processed the sugar. These levels are measured in mg/dL.

The chart below contains the oral glucose tolerance test ranges for prediabetes and diabetes and what each diagnosis means:

Blood Glucose Range Diagnosis What It Means
140 to 199 mg/dL Prediabetes (also called impaired fasting glucose) Blood glucose levels are higher than normal, but not high enough to be diagnosed as diabetes. This condition increases risk for developing Type 2 diabetes, heart disease and stroke.
200 mg/dL or higher Diabetes mellitus (Type 2 diabetes) Type 2 diabetes develops when your body doesn’t make enough insulin or develops “insulin resistance” and can’t efficiently use the insulin it makes. It greatly increases your risk of heart disease and stroke.

Tests for Monitoring Diabetes

If you’re diagnosed with Type 2 diabetes, you can monitor your blood sugar level several ways to evaluate how well your treatment plan is working:

HbA1C

Your health care team will do an HbA1C test at least twice a year.

Home monitoring

Easy-to-use home monitors allow people to test their blood sugar on their own. A variety of these devices are available.

If you’re managing diabetes with the help of a home monitor, talk to your doctor to learn what to do when your results are too high or low.

Tests to Measure Heart Health

People with diabetes are at increased risk for a range of health complications, including cardiovascular disease.

Learn more about tests health care professionals may use to check your heart health.

Written by American Heart Association editorial staff and reviewed by science and medicine advisers. See our editorial policies and staff.