Diabetes mellitus – Diagnostics – Overview of Information and Clinical Research

Diagnosing diabetes involves a series of simple blood tests that measure how well your body manages sugar. Early detection can help prevent serious complications and guide treatment decisions, making timely testing essential for anyone experiencing symptoms or at risk.

Introduction: Who Should Get Tested and When

Understanding when to seek diabetes testing is an important step in protecting your health. If you notice unusual symptoms like feeling thirstier than normal, needing to urinate more often, losing weight without trying, feeling tired and weak, or having blurry vision, it’s time to talk to your doctor about testing.^[1]^[6] These warning signs can appear suddenly, especially in type 1 diabetes, but in type 2 diabetes they may develop so slowly and be so mild that you might not notice them for years.^[7]

You don’t need to wait for symptoms to appear before getting tested. Many people discover they have diabetes through routine screening, even when they feel perfectly fine. The American Diabetes Association recommends that all adults aged 35 or older should have routine tests for type 2 diabetes.^[15] This is especially important because diabetes can silently damage your body’s blood vessels, nerves, kidneys, and eyes long before you experience any noticeable problems.^[6]

Certain people should consider testing at a younger age or more frequently. If you have prediabetes—a condition where blood sugar levels are higher than normal but not yet high enough to be called diabetes—you should be monitored regularly since this often progresses to type 2 diabetes unless you take preventive steps.^[2] Being overweight, having a family history of diabetes, or belonging to certain racial or ethnic groups also increases your risk and makes earlier testing advisable.^[8]

⚠️ Important

More than 1 in 3 adults in the United States has prediabetes, but more than 8 in 10 of them don’t even know it.^[2] This means millions of people are walking around with higher-than-normal blood sugar without realizing they’re at risk for developing full diabetes. Getting tested is the only way to know where you stand and what steps you can take to protect your health.

Pregnant women should also be screened for gestational diabetes, which develops during pregnancy. This type usually goes away after the baby is born, but it increases your risk of developing type 2 diabetes later in life, so follow-up testing is important.^[2] Children and teenagers are not immune either—while type 1 diabetes typically appears in young people, more children are now being diagnosed with type 2 diabetes as well, often related to weight and lifestyle factors.^[2]

Diagnostic Methods for Identifying Diabetes

Doctors rely on several different blood tests to diagnose diabetes and distinguish it from other conditions. Each test measures sugar (glucose) in your blood in a slightly different way, and your doctor will choose the most appropriate one based on your situation. Understanding these tests can help you feel more prepared and less anxious about the diagnostic process.

The A1C Test

The most common test used to diagnose type 2 diabetes is called the glycated hemoglobin test, or A1C test.^[15] This test is particularly useful because it doesn’t require fasting—you can have it done at any time of day, regardless of when you last ate. The A1C measures your average blood sugar level over the past two to three months by checking how much sugar has attached to a protein in your red blood cells called hemoglobin.^[10]

The results are given as a percentage. An A1C below 5.7 percent is considered healthy and normal. If your result falls between 5.7 and 6.4 percent, you have prediabetes, which is a warning sign that you should take action to prevent full diabetes from developing. An A1C of 6.5 percent or higher on two separate tests means you have diabetes.^[15] Your doctor might order a second test to confirm the diagnosis because making sure the results are accurate is important before starting treatment.

However, the A1C test isn’t perfect for everyone. Certain health conditions can interfere with its accuracy. For instance, if you have certain types of anemia, are pregnant, or have other medical conditions that affect your red blood cells, your doctor may need to use a different testing method to get reliable results.^[10]

Fasting Blood Sugar Test

If the A1C test isn’t suitable for you, your doctor might use a fasting blood sugar test. This test requires you to avoid eating overnight before giving a blood sample, typically in the morning.^[15] The test measures how much glucose is in your blood after this period of not eating, when your levels should naturally be lower.

Results are measured in milligrams of sugar per deciliter of blood, abbreviated as mg/dL. A fasting blood sugar level less than 100 mg/dL is healthy. A level between 100 and 125 mg/dL indicates prediabetes. If your fasting blood sugar is 126 mg/dL or higher on two separate tests, this suggests diabetes.^[15] Because blood sugar can vary from day to day, repeating the test helps ensure an accurate diagnosis.

Random Blood Sugar Test

Sometimes doctors need to check your blood sugar immediately, without waiting for you to fast. A random blood sugar test can be done at any time, regardless of when you last ate.^[15] This test is most useful when you’re having clear symptoms of diabetes, such as extreme thirst and frequent urination.

A random blood sugar level of 200 mg/dL or higher strongly suggests diabetes, especially if you also have symptoms.^[15] This test gives doctors a quick snapshot of what’s happening with your blood sugar at that moment. While it’s helpful in urgent situations, doctors typically confirm the results with other tests before finalizing a diagnosis.

Oral Glucose Tolerance Test

The oral glucose tolerance test is another diagnostic option, though it’s used less frequently than the A1C or fasting blood sugar tests. This test is more commonly used for pregnant women to check for gestational diabetes, or for people with certain conditions like cystic fibrosis.^[15]

During this test, you first fast for a certain period, usually overnight. Then you drink a sweet liquid containing a specific amount of sugar. Your blood sugar is measured before you drink the liquid and then again at regular intervals over the next two hours to see how your body processes the sugar.^[15] A blood sugar level less than 140 mg/dL after two hours is healthy. A level between 140 and 199 mg/dL indicates prediabetes, while a level of 200 mg/dL or higher after two hours suggests diabetes.^[15]

Additional Tests to Identify Diabetes Type

Once diabetes is diagnosed, your doctor may need additional tests to determine which type you have. This is important because type 1 and type 2 diabetes require different treatment approaches. In type 1 diabetes, the body’s immune system mistakenly attacks the cells in the pancreas that make insulin.^[1] To check for this, doctors can test your blood for specific autoantibodies—proteins that indicate your immune system is attacking your own body.

Your doctor might also check for ketones in your urine. Ketones are byproducts created when your body breaks down fat for energy instead of using glucose. Their presence, especially along with high blood sugar, is more common in type 1 diabetes.^[6] Finding ketones can help your doctor understand what’s happening in your body and guide treatment decisions.

⚠️ Important

Distinguishing between type 1 and type 2 diabetes is crucial because the treatments are quite different. People with type 1 diabetes must take insulin every day to survive, while many people with type 2 diabetes can initially manage their condition with lifestyle changes and oral medications.^[8] Getting the right diagnosis ensures you receive the most appropriate care from the start.

Diagnostic Testing for Clinical Trial Qualification

When researchers test new treatments for diabetes in clinical trials, they need to make sure participants truly have diabetes and meet specific health criteria. The diagnostic tests used in clinical trials are similar to those used in regular medical care, but they follow strict protocols to ensure consistency and accuracy across all participants.

Clinical trials typically use the same standard blood tests—A1C, fasting blood sugar, or oral glucose tolerance tests—to confirm that potential participants have diabetes.^[10] However, trials often have precise cutoff values for blood sugar levels that determine who can join. For example, a study might only accept people whose A1C falls within a certain range, such as between 7 and 10 percent, to ensure the group being studied is relatively similar.

Beyond confirming diabetes, clinical trials often require additional screening tests to assess overall health and identify any other medical conditions that might affect the study results or put participants at risk. These might include tests of kidney function, liver function, cholesterol levels, and blood pressure measurements.^[10] Researchers need this comprehensive health information to make sure the treatment being tested is safe for each participant and to understand how it works in different situations.

Some trials specifically focus on people with type 1 or type 2 diabetes, while others might include both. To determine type, researchers use the same autoantibody tests and ketone checks that doctors use in clinical practice. They may also measure something called C-peptide, a substance your pancreas releases along with insulin. C-peptide levels can help determine how much insulin your body is still producing, which is useful for distinguishing between diabetes types.

For trials testing new ways to monitor diabetes, such as continuous glucose monitoring devices, participants typically need to demonstrate that they can successfully use a blood glucose meter and follow testing schedules.^[10] This ensures they can compare the new technology against standard methods. Continuous glucose monitoring involves wearing a small sensor under your skin that automatically checks blood sugar throughout the day and night, giving a much more detailed picture of glucose patterns than finger-prick tests alone.

Pregnancy affects many aspects of diabetes management and treatment, so clinical trials often exclude pregnant women or study them separately in trials specifically designed for gestational diabetes. Similarly, trials may have age restrictions, with separate studies for children, adults, and older people, since diabetes behaves differently across age groups and treatments may need to be adjusted accordingly.

Understanding these qualification criteria helps potential participants know what to expect if they’re considering joining a diabetes clinical trial. The screening process, while thorough, ensures that participants are well-matched to the study and that the treatments being tested can be evaluated safely and effectively. If you’re interested in participating in diabetes research, your healthcare provider can help you understand whether you might qualify for ongoing trials.

Prognosis and Survival Rate

Prognosis

The outlook for people with diabetes depends largely on how well the condition is managed. With proper care and consistent treatment, many people with diabetes live long, active lives. However, the condition is progressive, meaning it can change over time, and without good management it can lead to serious complications.^[7]

Over time, consistently high blood glucose can damage many parts of your body. It affects blood vessels throughout your body, increasing your risk for heart disease, stroke, and problems with circulation in your legs and feet. Diabetes can damage the tiny blood vessels in your eyes, potentially leading to vision loss or blindness. It can also harm your kidneys, sometimes progressing to kidney failure that requires dialysis or transplant. Nerve damage, especially in the feet and legs, is another common complication.^[1]^[7]

The good news is that keeping your blood sugar, blood pressure, and cholesterol levels within target ranges significantly reduces your risk of developing these complications.^[13] People who maintain good control of their diabetes through medication, healthy eating, regular exercise, and consistent medical care can prevent or delay many complications. Regular screening for eye problems, kidney disease, and nerve damage allows doctors to detect issues early when they’re easier to treat.

Type 1 and type 2 diabetes have different trajectories, but both require lifelong management. Type 1 diabetes appears suddenly and requires immediate insulin treatment that continues for life.^[1] Type 2 diabetes typically develops gradually over years. Many people with type 2 diabetes can initially manage their condition with lifestyle changes and oral medications, though some eventually need insulin as the disease progresses and the body’s ability to produce insulin declines.^[13]

Factors that affect your prognosis include how early diabetes is diagnosed, how well you follow your treatment plan, whether you have other health conditions, and your access to quality healthcare. Smoking dramatically worsens the outlook for people with diabetes by further damaging blood vessels, so quitting smoking is one of the most important steps you can take.^[7]

Survival Rate

In 2021, diabetes was the direct cause of 1.6 million deaths worldwide. Importantly, 47 percent of all deaths from diabetes occurred before the age of 70 years, highlighting that diabetes can significantly shorten life expectancy when not properly managed.^[7] Another 530,000 kidney disease deaths were caused by diabetes, and high blood glucose contributes to approximately 11 percent of cardiovascular deaths globally.^[7]

However, these statistics represent the broader picture and include many people who didn’t have access to good diabetes care or didn’t manage their condition effectively. With modern treatments and proper disease management, people with diabetes are living longer and healthier lives than ever before. The key difference lies in how well the condition is controlled.

Since 2000, mortality rates specifically from diabetes have been increasing, which reflects the growing number of people living with the condition worldwide.^[7] In 2022, 14 percent of adults aged 18 years and older were living with diabetes, up from 7 percent in 1990, and the number rose from 200 million people in 1990 to 830 million in 2022.^[7] This dramatic increase makes diabetes one of the fastest-growing health challenges globally, particularly in low- and middle-income countries.

More than half of people living with diabetes—59 percent of adults aged 30 and over—were not taking medication for their diabetes in 2022, and treatment coverage was lowest in low- and middle-income countries.^[7] This treatment gap means many people aren’t getting the care they need to prevent complications and extend their lives. Access to diagnosis, medication, and ongoing healthcare makes an enormous difference in survival and quality of life for people with diabetes.