Diagnosis of Diabetes

What is diabetes?

Diabetes is a disease in which levels of blood glucose, also called blood sugar, are above normal. People with diabetes have problems converting food to energy. Normally, after a meal, the body breaks food down into glucose, which the blood carries to cells throughout the body. Cells use insulin, a hormone made in the pancreas, to help them convert blood glucose into energy.

People develop diabetes because the pancreas does not make enough insulin or because the cells in the muscles, liver, and fat do not use insulin properly, or both. As a result, the amount of glucose in the blood increases while the cells are starved of energy. Over the years, high blood glucose, also called hyperglycemia, damages nerves and blood vessels, which can lead to complications such as heart disease, stroke, kidney disease, blindness, nerve problems, gum infections, and amputation.

Main Types of Diabetes

The two main types of diabetes are called type 1 and type 2. A third form of diabetes is called gestational diabetes.

  • Type 1 diabetes, formerly called juvenile diabetes, is usually first diagnosed in children, teenagers, and young adults. In this form of diabetes, the pancreas no longer makes insulin because the body’s immune system has attacked and destroyed the pancreatic cells specialized to make insulin. These insulin-producing cells are called beta cells.
  • Type 2 diabetes, formerly called adult-onset diabetes, is the most common form. People can develop type 2 diabetes at any age, even during childhood. This form of diabetes usually begins with insulin resistance, a condition in which muscle, liver, and fat cells do not use insulin properly. As a result, the body needs more insulin to help glucose enter cells to be used for energy. At first, the pancreas keeps up with the added demand by producing more insulin. In time, however, the pancreas loses its ability to secrete enough insulin in response to meals.
  • Gestational diabetes is diabetes that first occurs during pregnancy. When women are pregnant, their need for insulin appears to increase, and many can develop gestational diabetes during the late stages of pregnancy. Although this form of diabetes usually goes away after the baby is born, a woman who has had it is more likely to develop type 2 diabetes later in life.

Other Types of Diabetes

A number of other types of diabetes exist. A person may exhibit characteristics of more than one type. For example, in latent autoimmune diabetes in adults (LADA), also called type 1.5 diabetes or double diabetes, people show signs of both type 1 and type 2 diabetes. Diagnosis usually occurs after age 30.

Most people with LADA still produce their own insulin when first diagnosed, like those with type 2 diabetes, but within a few years, they must take insulin to control blood glucose levels. In LADA, as in type 1 diabetes, the beta cells of the pancreas stop making insulin because the body’s immune system attacks and destroys them. Some experts believe that LADA is a slowly developing kind of type 1 diabetes.

Other types of diabetes include those caused by

  • genetic defects of the beta cell, such as maturity-onset diabetes of the young (MODY) and neonatal diabetes mellitus
  • genetic defects in insulin action, resulting in the body’s inability to control blood glucose levels, as seen in leprechaunism and the Rabson-Mendenhall syndrome
  • diseases of the pancreas or conditions that damage the pancreas, such as pancreatitis and cystic fibrosis
  • excess amounts of certain hormones resulting from some medical conditions—such as cortisol in Cushing’s syndrome—that work against the action of insulin
  • medications that reduce insulin action, such as glucocorticoids, or chemicals that destroy beta cells
  • infections, such as congenital rubella and cytomegalovirus
  • rare autoimmune disorders, such as stiff-man syndrome, an autoimmune disease of the central nervous system
  • genetic syndromes associated with diabetes, such as Down syndrome and Prader-Willi syndrome

More information about MODY and neonatal diabetes mellitus is in the National Diabetes Information Clearinghouse’s fact sheet Monogenic Forms of Diabetes: Neonatal Diabetes Mellitus and Maturity-onset Diabetes of the Young. The fact sheet is available at diabetes.niddk.nih.gov/dm/pubs/mody/index.htm or by calling 1–800–860–8747.

Type 1 and type 2 diabetes

In 1997, to move away from naming the two main types of diabetes based on treatment or the age at onset, an American Diabetes Association expert committee recommended universal adoption of simplified terminology. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) agrees.

What is pre-diabetes?

In pre-diabetes, blood glucose levels are higher than normal but not high enough for a diagnosis of diabetes. However, many people with pre-diabetes develop type 2 diabetes within 10 years. Experts disagree about the specific blood glucose level they should use to diagnose diabetes, and through the years, that number has changed. Individuals with pre-diabetes have an increased risk of heart disease and stroke. With modest weight loss and moderate physical activity, people with pre-diabetes can delay or prevent type 2 diabetes.

Former Name Preferred Names
Type I

juvenile diabetes

diabetes mellitus


type 1 diabetes
Type II

adult-onset diabetes

diabetes mellitus


type 2 diabetes


How are diabetes and pre-diabetes diagnosed?

The following tests are used for diagnosis:

  • 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.

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
Plasma Glucose
Result (mg/dL)
99 or below Normal
100 to 125 Pre-diabetes

(impaired fasting

126 or above Diabetes*

*Confirmed by repeating the test on a different day.


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
2-Hour Plasma
Glucose Result
139 and below Normal
140 to 199 Pre-diabetes

(impaired glucose

200 and above Diabetes*

*Confirmed by repeating the test on a different day.

Table 3.
Gestational diabetes:
Above-normal results for
the OGTT*
When Plasma Glucose
Result (mg/dL)
Fasting 95 or higher
At 1 hour 180 or higher
At 2 hours 155 or higher
At 3 hours 140 or higher

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.

Note: Some laboratories use other numbers for this test.
*These numbers are for a test using a drink with 100 grams of glucose.

Additional information about the diagnosis and treatment of gestational diabetes is in the publication What I need to know about Gestational Diabetes. This publication is available at diabetes.niddk.nih.gov/dm/pubs/gestational/index.htm or by calling 1–800–860–8747.