Diagnosis of Diabetes

The following tests are used for diagnosis of diabetes and pre-diabetes:

• A fasting plasma glucose (FPG) test measures blood glucose in a person who has not eaten anything for at least 8 hours. This test is used to detect diabetes and pre-diabetes.
• An oral glucose tolerance test (OGTT) measures blood glucose after a person fasts at least 8 hours and 2 hours after the person drinks a glucose-containing beverage. This test can be used to diagnose diabetes and pre-diabetes.
• A random plasma glucose test, also called a casual plasma glucose test, measures blood glucose without regard to when the person being tested last ate. This test, along with an assessment of symptoms, is used to diagnose diabetes but not pre-diabetes.

Test results indicating that a person has diabetes should be confirmed with a second test on a different day.

Fasting Plasma Glucose (FPG) Test

The FPG test is the preferred test for diagnosing diabetes because of its convenience and low cost. However, it will miss some diabetes or pre-diabetes that can be found with the OGTT. The FPG test is most reliable when done in the morning. Results and their meaning are shown in Table 1. People with a fasting glucose level of 100 to 125 milligrams per deciliter (mg/dL) have a form of pre-diabetes called impaired fasting glucose (IFG). Having IFG means a person has an increased risk of developing type 2 diabetes but does not have it yet. A level of 126 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.

Table 1. FPG test

Oral Glucose Tolerance Test (OGTT)

Research has shown that the OGTT is more sensitive than the FPG test for diagnosing pre-diabetes, but it is less convenient to administer. The OGTT requires fasting for at least 8 hours before the test. The plasma glucose level is measured immediately before and 2 hours after a person drinks a liquid containing 75 grams of glucose dissolved in water. Results and their meaning are shown in Table 2. If the blood glucose level is between 140 and 199 mg/dL 2 hours after drinking the liquid, the person has a form of pre-diabetes called impaired glucose tolerance (IGT). Having IGT, like having IFG, means a person has an increased risk of developing type 2 diabetes but does not have it yet. A 2-hour glucose level of 200 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.

Table 2. OGTT

Confirmed by repeating the test on a different day.

Gestational diabetes is also diagnosed based on plasma glucose values measured during the OGTT, preferably by using 100 grams of glucose in liquid for the test. Blood glucose levels are checked four times during the test. If blood glucose levels are above normal at least twice during the test, the woman has gestational diabetes. Table 3 shows the above-normal results for the OGTT for gestational diabetes.

Table 3. Gestational diabetes: Above-normal results for the OGTT*

Note: Some laboratories use other numbers for this test.

• These numbers are for a test using a drink with 100 grams of glucose.

Random Plasma Glucose Test (or "Casual Plasma Glucose Test")

A random, or casual, blood glucose level of 200 mg/dL or higher, plus the presence of the following symptoms, can mean a person has diabetes:

• increased urination
• increased thirst
• unexplained weight loss

Other symptoms can include fatigue, blurred vision, increased hunger, and sores that do not heal. The doctor will check the person’s blood glucose level on another day using the FPG test or the OGTT to confirm the diagnosis.

Who should be tested for diabetes and pre-diabetes?

The American Diabetes Association recommends that testing to detect pre-diabetes and type 2 diabetes be considered in adults without symptoms who are overweight or obese and have one or more additional risk factors for diabetes. In those without these risk factors, testing should begin at age 45. The Body Mass Index Table can be used to find out whether someone is normal weight, overweight, obese, or extremely obese.

People aged 45 or older should consider getting tested for pre-diabetes or diabetes. People younger than 45 should consider testing if they are overweight, obese, or extremely obese and have one or more of the following risk factors:

• being physically inactive
• having a parent, brother, or sister with diabetes
• having a family background that is African American, Alaska Native, American Indian, Asian American, Hispanic/Latino, or Pacific Islander
• giving birth to a baby weighing more than 9 pounds or being diagnosed with gestational diabetes
• having high blood pressure—140/90 mmHg or above—or being treated for high blood pressure
• having an HDL, or “good,” cholesterol level below 35 mg/dL or a triglyceride level above 250 mg/dL
• having polycystic ovary syndrome, also called PCOS
• having IFG or IGT on previous testing
• having a condition called acanthosis nigricans, characterized by a dark, velvety rash around the neck or armpits
• having a history of cardiovascular disease—disease affecting the heart and blood vessels

If results of testing are normal, testing should be repeated at least every 3 years. Doctors may recommend more frequent testing depending on initial results and risk status. People whose test results indicate they have pre-diabetes should have their blood glucose checked again in 1 to 2 years and take steps to prevent type 2 diabetes.

When a woman is pregnant, the doctor will assess her risk for developing gestational diabetes at her first prenatal visit and order testing as needed during the pregnancy. Women who develop gestational diabetes should also have follow-up testing 6 to 12 weeks after the baby is born.

Type 2 diabetes has become more common in children and teens than in the past, and those at high risk for developing diabetes should be tested every 2 years. Testing should begin at age 10 or at puberty, whichever occurs first. Children and teens who are overweight or obese and have other risk factors, such as a family history of diabetes, are at high risk for developing diabetes.

Body Mass Index (BMI)

BMI is a measurement of body weight relative to height. Adults aged 20 or older can use the BMI Table to find out whether they are normal weight, overweight, obese, or extremely obese. To use the table,

• find the adult’s height in the left-hand column
• move across the row to the number closest to that person’s weight
• find the number at the top of that column

The number at the top of the column is the person’s BMI. The words above the BMI number indicate whether the person is normal weight, overweight, obese, or extremely obese. People who are overweight, obese, or extremely obese should consider talking with a doctor about ways to lose weight to reduce the risk of diabetes.

The BMI has certain limitations. It may overestimate body fat in athletes and others who have a muscular build and underestimate body fat in older adults and others who have lost muscle.

BMI for children and teens must be determined based on age, height, weight, and sex. See the following for BMI charts:

• Body Mass Index (BMI)
• BMI for Adults
• BMI for Children and Teens

In addition to weight, the location of excess fat on the body can be important. A waist measurement of 40 inches or more for men or 35 inches or more for women is linked to insulin resistance and increases a person’s risk for type 2 diabetes.

What steps can delay or prevent type 2 diabetes?

A major research study, the Diabetes Prevention Program (DPP), confirmed that people with IGT—pre-diabetes—were able to sharply reduce their risk of developing diabetes during the study by losing 5 to 7 percent of their body weight through dietary changes and increased physical activity. Study participants followed a low-fat, low-calorie diet and engaged in regular physical activity, such as walking briskly for 30 minutes, five times a week. These strategies worked well for both men and women and were especially effective for participants aged 60 and older.

More information about insulin resistance, the DPP, or how to lower risk for type 2 diabetes is available in the following publications:

• Insulin Resistance and Prediabetes
• Diabetes Prevention Program (DPP)
• Risk Factors for Type 2 Diabetes
• Diabetes Risk Self-Assessment

Points to Remember

• Diabetes, pre-diabetes, and gestational diabetes are diagnosed by checking blood glucose levels.
• Tests used for diagnosing diabetes and pre-diabetes include the fasting plasma glucose (FPG) test and the oral glucose tolerance test (OGTT).
• People aged 45 or older should consider getting tested for pre-diabetes or diabetes. People younger than 45 who are overweight, obese, or extremely obese and have one or more additional risk factors for diabetes should consider testing.
• Many people with pre-diabetes develop type 2 diabetes within 10 years.
• People with pre-diabetes can delay or prevent type 2 diabetes by losing a modest amount of weight through regular physical activity and a diet low in fat and calories.

For More Information

American Association of Diabetes Educators

Internet: www.diabeteseducator.org

American Diabetes Association

Internet: www.diabetes.org

Juvenile Diabetes Research Foundation International

Internet: www.jdrf.org

National Diabetes Education Program

Internet: www.ndep.nih.gov

National Diabetes Information Clearinghouse

Internet: www.diabetes.niddk.nih.gov