How Is Type 2 Diabetes Diagnosed?
Medically reviewed by Kelly Wood, MD
Type 2 diabetes is diagnosed by testing your blood glucose (sugar) levels. These tests are usually done in a healthcare provider's office or clinical laboratory.
Type 2 diabetes is a chronic disease that causes abnormal insulin processing in the body, leading to high blood sugar levels. Diabetes affects more than 30 million Americans, and type 2 diabetes accounts for about 90% of diagnosed cases.
Your pancreas produces insulin. This hormone is responsible for letting sugar into your cells to use for energy. If you have diabetes, your pancreas cannot produce insulin, or your cells do not respond to insulin. This results in high blood sugar, which can lead to several health risks, including heart disease, blindness, kidney failure, and limb amputation, among many others.
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Related: How Is Thyroid Disease Diagnosed?
Getting a Diabetes Diagnosis
A healthcare provider can diagnose type 2 diabetes and its initial stage, prediabetes, by measuring your blood sugar levels. They will do this during your standard yearly physical or during check-ups for another health concern.
If your blood sugar levels are higher than average, a healthcare provider will order several tests to confirm a diabetes diagnosis. The following are the typical tests that healthcare providers use to diagnose diabetes.
A1C Test
An A1C test measures your average blood sugar for the past two to three months. This test is also called a glycated hemoglobin, glycosylated hemoglobin, hemoglobin A1C, or HbA1C test.
An A1C test measures how much of your blood cells are covered with sugar. Sugar grabs onto hemoglobin, or the protein in your blood that carries oxygen, when it enters the bloodstream. Sugar attaching to hemoglobin is a normal biological process. A high amount of sugar-coated hemoglobin puts people at a high risk of prediabetes and diabetes.
Your results are reported as a percentage. A healthcare provider will make an assessment of your diagnosis based on the following criteria:
Normal: Less than 5.7%
Prediabetes: Between 5.7% to 6.4%
Diabetes: 6.5% or higher
It's important to know the A1C test is not appropriate for everyone. A healthcare provider might choose other tests to make a diagnosis if you are pregnant, have been previously diagnosed with anemia, or have recently undergone some other blood-related procedure like hemodialysis or blood transfusion. Anemia occurs if your blood does not have enough healthy red blood cells. The A1C test has also not been well-researched for diagnoses in non-White populations.
Oral Glucose Tolerance Test
A healthcare provider will measure your blood sugar before and after drinking a sugary beverage during the oral glucose test. The oral glucose test is meant to test how your body is processing sugar.
You'll provide an initial blood sample to a healthcare provider. They will give you a sugary drink containing about 75 grams (g) of sugar. You'll wait two hours and provide a second blood sample. The healthcare provider will then measure your sugar tolerance levels.
The blood sugar diagnostic measurements of the oral glucose tolerance test are:
Normal: Less than 140 milligrams per deciliter (mg/dl)
Prediabetes: 140 to 199 mg/dl
Diabetes: 200 mg/dl or higher
Fasting Plasma Glucose
The fasting plasma glucose test measures your blood sugar levels after fasting for at least eight hours. You'll take this test first thing in the morning and come into a healthcare provider's office or lab prior to eating breakfast.
The criteria for the fasting plasma glucose test are:
Normal: Less than 100 mg/dl
Prediabetes: 100 to 125 mg/dl
Diabetes: 126 mg/dl or higher
Random Plasma Glucose Test
A random glucose test, or a casual plasma glucose test, is a blood sugar test you can take at any time. Healthcare providers usually call for this test if you demonstrate clear and severe diabetes symptoms. They will diagnose diabetes if your blood sugar is greater than or equal to 200 mg/dl.
Progression of Type 2 Diabetes
People typically develop prediabetes before the disease progress to type 2 diabetes. Gestational diabetes, or diabetes during pregnancy, is another type of diabetes.
Prediabetes
Prediabetes occurs prior to a diabetes diagnosis. Your blood sugar levels are higher than average but have not yet reached the levels to be diagnosed with type 2 diabetes.
This type of diabetes is diagnosed with the standard diabetes diagnostic tests. There are no clear symptoms of prediabetes.
There are several risk factors that can lead to prediabetes, including:
Abnormal cholesterol levels
Age (i.e., 45 or older)
Family history of diabetes
History of heart disease or stroke
Prolonged physical inactivity
About 96 million people in the United States adults have prediabetes. A diagnosis of prediabetes is an indicator that you could develop type 2 diabetes over time.
Type 2 Diabetes
Type 2 diabetes is characterized by highly elevated blood sugar levels. You might start presenting more obvious diabetes symptoms, such as:
An urge to urinate more frequently
Feeling very thirsty or very hungry (even after you have eaten)
Extreme fatigue
Wounds that are slow to heal
Tingling, pain, or numbness in the hands or feet
A healthcare provider will use standard blood sugar tests to confirm your diagnosis if you have any of these symptoms or risk factors. They can also get you started on a treatment plan.
Gestational Diabetes
Gestational diabetes is a type of diabetes that can occur during pregnancy. This type of diabetes happens when your body doesn't process insulin correctly, resulting in increased blood sugar. A diagnosis of gestational diabetes can put you at risk of type 2 diabetes after you've given birth.
Gestational diabetes is usually diagnosed 24–28 weeks into pregnancy during standard prenatal check-ups with a healthcare provider. This type of diabetes is diagnosed using a one- or two-step process that involves modified versions of the oral glucose tolerance test.
You'll have to fast overnight during the one-step process. You'll go to a healthcare provider's office the next day and drink a sugary beverage with 75 g of sugar. They will collect blood samples three times: at fasting and after one and two hours of drinking the sugary beverage. A healthcare provider will assess all three measurements and provide a diagnosis.
You'll likely be diagnosed with gestational diabetes if your blood samples meet the following criteria:
Blood sugar levels higher or equal to 92 mg/dl at fasting
Blood sugar levels higher or equal to 180 mg/dl after one hour
Blood sugar levels higher or equal to 153 mg/dl after two hours
The two-step process involves the following:
Step one: You'll drink a beverage containing 50 g of sugar. A healthcare provider will move you to step two if your blood sugar levels exceed 140 mg/dl after one hour.
Step two: You'll fast overnight and then drink a beverage containing 100 g of sugar. A healthcare provider will measure your blood sugar levels four times: after fasting and at one, two, and three hours after drinking the beverage. It's likely that you'll be diagnosed with gestational diabetes if your blood samples are considered high on two of the four samples tested.
Diabetes Comorbidities
Conditions that co-occur are known as comorbidities. Diabetes commonly occurs alongside several other health conditions. The disease shares risk factors with many other chronic health conditions. These include heart disease, hypertension (high blood pressure), lipid metabolism disorders, and osteoarthritis (OA).
There has been an increase in the rate of mental health conditions diagnosed alongside diabetes. A study published in 2019 found high levels of depression in people with diabetes. The researchers noted that healthcare providers ought to closely monitor mental health conditions in people with diabetes. These conditions can be a barrier for those who need to manage diabetes.
Screening for Related Conditions
Healthcare providers use blood tests to confirm a diabetes diagnosis. There are a variety of other health conditions that present signs and symptoms similar to those of diabetes.
Conditions or medications that can result in a differential diabetes diagnosis include:
Complications of iron overload
Diseases that affect the pancreas (e.g., pancreatitis or cystic fibrosis)
Endocrinopathies, or diseases of the endocrine gland (e.g., an overactive or underactive thyroid)
Genetic irregularities in pancreatic cell function
Infections
Other metabolic syndromes
Treatment with certain medications (e.g., corticosteroids, neuroleptics, or pentamidine)
It's important to tell a healthcare provider whether you have been previously diagnosed with any of these conditions if you are undergoing testing for diabetes. Let them know if you are actively taking these medications.
Related: How Is Heart Disease Diagnosed?
A Quick Review
Type 2 diabetes is one of the most common health conditions affecting people worldwide. This chronic disease causes abnormal insulin processing, leading to high blood sugar levels. Regularly testing your blood sugar levels will give a healthcare provider a clear picture of your health status and allow them to provide you with an accurate diagnosis.
If your blood sugar levels are higher than average, a healthcare provider may order tests, including an A1C test, an oral glucose test, and a random plasma glucose test. Talk to a healthcare provider about a potential diabetes screening if you think you may be at risk.
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