Diagnosing Diabetes or Prediabetes

By American Diabetes Association, Sheri R. Colberg

Some people have symptoms of diabetes before they’re diagnosed, but many more never have any or realize that any symptoms they’re having are related to diabetes. That makes it even more important to get annual checkups — particularly when you’re getting older — that measure your fasting blood glucose. Of course, that’s only one way to diagnose it, and testing only fasting levels misses some people who experience spikes in their blood glucose after eating although their morning levels are just fine.

Recognizing the symptoms

The more classic symptoms of hyperglycemia, or elevated blood glucose levels, include increased thirst, excessive urination, unusual fatigue, blurred vision, unexplained hunger, rapid weight loss, and slow-healing cuts and infections. These symptoms are common in youth who develop type 1 diabetes rapidly. However, diabetes can have subtle symptoms and may go undetected for some time, particularly in adults who develop it slowly.

If you or a loved one has complained recently about excessive thirst, frequent urination, or excessive hunger, schedule an appointment with your doctor or health care provider to check for diabetes.

These symptoms aren’t always indicative of diabetes. Sometimes elevated glucose levels can occur temporarily due to illness or medication use.

Testing for diabetes and prediabetes

Three main clinical methods are currently approved for diagnosing diabetes or prediabetes: fasting plasma glucose, oral glucose tolerance, and the A1C test (glycated hemoglobin). Any test that appears to indicate that you have either of these conditions should be repeated a second time (on another day) before your diagnosis is officially confirmed.

Though this test isn’t a usual official method, diabetes can sometimes be diagnosed when someone experiences the classic symptoms of hyperglycemia and has a random plasma glucose value of 200 mg/dL or higher.

Fasting plasma glucose

This simple blood test measures your blood glucose levels after an overnight fast of at least eight hours. It determines the amount of glucose in plasma, which is the clear part of the blood with all the red blood cells removed.

The fasting value is reported (in the United States) in mg/dL, which is simply a measure of the amount of glucose (in milligrams, or mg) in a set amount of plasma (100 milliliters, which equals 1 deciliter, or dL). Outside of the United States or in research papers, it’s reported as mmol/L (millimoles per liter, or sometimes mM). Note: To convert from mg/dL to mmol/L, divide the value in mg/dL by 18.

The fasting plasma levels used for diagnosis are

  • Normal: 70 to 99 mg/dL (3.9 to 5.5 mmol/L)
  • Prediabetes: 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
  • Diabetes: 126 mg/dL (7.0 mmol/L) or above

Prediabetes is diagnosed when your fasting glucose levels are elevated above normal (also known as impaired fasting glucose, or IFG). Diabetes is diagnosed when your fasting plasma glucose exceeds the prediabetes range.

Being on the lower end of the normal range in the morning is always better, and you should take steps to lower it if it rises over time toward the high end of normal.

Oral glucose tolerance

An alternate testing method is the oral glucose tolerance test (OGTT), which involves drinking 75 grams of glucose and having your blood glucose monitored for two to three hours afterward. This approach tests your body’s ability to respond to a large influx of sugar. If your blood glucose goes up or stays up too high for long from this oral sugar load, you’re said to have impaired glucose tolerance (IGT). This test is used to diagnose diabetes, prediabetes, and gestational diabetes.

Make sure to have your fasting blood glucose levels tested annually, and an A1C test (discussed in the following section) as well if you can swing it.

A1C test (glycated hemoglobin)

A third approved method to diagnose diabetes is to test your A1C (previously called glycated hemoglobin or hemoglobin A1C). The A1C indicates your average blood glucose over the past two or three months. Basically, the higher your blood glucose has been, the more glucose will be “stuck” to the hemoglobin part of red blood cells, and those blood cells live about 120 days.

This simple blood test can also be used to diagnose prediabetes because it averages in post-meal spikes in your blood glucose that a fasting value may not detect. Finding out your risk for developing diabetes is important because complications can occur when your A1C test is still in the normal range (at the high end).

Interpreting your test results

The following table illustrates how the results of these tests are used to diagnose diabetes and prediabetes.

Diagnosis of Diabetes and Prediabetes

Diabetes Diagnosis Fasting Plasma Glucose Oral Glucose Tolerance Test (OGTT) A1C Symptoms
Type 1 ≥ 126 mg/dL (7.0 mM) 2-hour value: ≥ 200 mg/dL (11.1 mM) 6.5% or higher Classic symptoms of hyperglycemia or a random plasma glucose ≥ 200 mg/dL (11.1 mM)
Type 2 ≥ 126 mg/dL (7.0 mM) 2-hour value: ≥ 200 mg/dL (11.1 mM) 6.5% or higher Classic symptoms of hyperglycemia or a random plasma glucose ≥ 200 mg/dL (11.1 mM)
Gestational ≥ 92 mg/dL (5.1 mM) 1-hour value: ≥ 180 mg/dL (10.0 mM) or 2-hour value: ≥ 153 mg/dL (8.5 mM)
Prediabetes 100–125 mg/dL (5.6–6.9 mM) 2-hour value: 140–199 mg/dL (7.8–11.0 mM) 5.7–6.4%

Your test results can be confusing because you may not get diagnosed with diabetes with one test, but meet the criteria for another. To be considered as having diabetes, you only have to meet the criteria for one test. But then how your diabetes is managed may vary based on which category you met. For instance, if you just have elevated fasting levels but your A1C is okay, your doctor may put you on a medication that will lower your morning blood glucose. If your blood glucose shoots up after meals (as indicated by an oral glucose test), you may need a medication that makes your pancreas release more insulin when you eat but not at other times of day.

Talk to your doctor or health care provider about your diagnosis and the best course of action to follow based on your test results.

Getting tested for gestational diabetes

Gestational diabetes is typically tested for and diagnosed between 24 and 28 weeks of pregnancy with an oral glucose tolerance test. Managing it may involve using insulin or other medications, along with diet changes and regular exercise. All pregnant women should be screened for this condition no later than 28 weeks and possibly even earlier if it was diagnosed during previous pregnancies.

Being misdiagnosed with type 2 diabetes

Given the current diagnosis methods for diabetes, determining which kind of diabetes a person has can sometimes be difficult. In addition, diabetes can sometimes have other causes, such as pancreatic cancer or other rare conditions. Having excess body fat used to lead to an almost guaranteed diagnosis of type 2 diabetes, but that is no longer the case because people who develop type 1 are often overweight and can develop an insulin-resistant state related to weight gain, dietary choices, and physical inactivity.

As many as 20 percent of adults who develop type 1 later in life may initially be misdiagnosed with type 2 due to their older age and slower onset. Being misdiagnosed because of your adult age is common, and you may initially respond well to oral diabetes medications (which further confuses the diagnosis). But you’re not likely to be as insulin resistant as someone who has type 2 diabetes.

If you’re an extremely athletic adult (age 25 or older) and you were diagnosed with type 2 over the age of 18 while regularly active and at normal or near-normal body weight, you likely have a slow-onset form of type 1 diabetes instead.

You can get antibody tests done to help make the diagnosis between type 1 and type 2. Knowing which you have can help because starting insulin therapy (rather than diabetes pills) early may help preserve your remaining beta cells for a little longer.