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What is diabetes?

Diabetes is a complex group of diseases with a variety of causes. People with diabetes have high blood glucose, also called high blood sugar or hyperglycemia.

Diabetes is a disorder of metabolism—the way the body uses digested food for energy. The digestive tract breaks down carbohydrates—sugars and starches found in many foods—into glucose, a form of sugar that enters the bloodstream. With the help of the hormone insulin, cells throughout the body absorb glucose and use it for energy. Insulin is made in the pancreas, an organ located behind the stomach. As the blood glucose level rises after a meal, the pancreas is triggered to release insulin. Within the pancreas, clusters of cells called islets contain beta cells, which make the insulin and release it into the blood.

Diabetes develops when the body doesn’t make enough insulin or is not able to use insulin effectively, or both. As a result, glucose builds up in the blood instead of being absorbed by cells in the body. The body’s cells are then starved of energy despite high blood glucose levels.

Over time, high blood glucose damages nerves and blood vessels, leading to complications such as heart disease, stroke, kidney disease, blindness, dental disease, and amputations. Other complications of diabetes may include increased susceptibility to other diseases, loss of mobility with aging, depression, and pregnancy problems.

Main Types of Diabetes

The three main types of diabetes are type 1, type 2, and gestational diabetes:

Type 1 diabetes, formerly called juvenile diabetes, is usually first diagnosed in children, teenagers, and young adults. In this type of diabetes, the beta cells of the pancreas no longer make insulin because the body’s immune system has attacked and destroyed them.

Type 2 diabetes, formerly called adult-onset diabetes, is the most common type of diabetes. About 90 to 95 percent of people with diabetes have type 2.1 People can develop type 2 diabetes at any age, even during childhood, but this type of diabetes is most often associated with older age. Type 2 diabetes is also associated with excess weight, physical inactivity, family history of diabetes, previous history of gestational diabetes, and certain ethnicities.

Type 2 diabetes usually begins with insulin resistance, a condition linked to excess weight 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. But in time, the pancreas loses its ability to produce enough insulin in response to meals, and blood glucose levels rise.

Gestational diabetes is a type of diabetes that develops only during pregnancy.

The hormones produced during pregnancy increase the amount of insulin needed to control blood glucose levels. If the body can’t meet this increased need for insulin, women can develop gestational diabetes during the late stages of pregnancy.

Gestational diabetes usually goes away after the baby is born. Shortly after pregnancy, 5 to 10 percent of women with gestational diabetes continue to have high blood glucose levels and are diagnosed as having diabetes, usually type 2.1 Research has shown that lifestyle changes and the diabetes medication, metformin, can reduce or delay the risk of type 2 diabetes in these women. Babies born to mothers who had gestational diabetes are also more likely to develop obesity and type 2 diabetes as they grow up.

Other Types of Diabetes

Many other types of diabetes exist, and a person can exhibit characteristics of more than one type. For example, in latent autoimmune diabetes in adults, people show signs of both type 1 and type 2 diabetes. Other types of diabetes include those caused by genetic defects, diseases of the pancreas, excess amounts of certain hormones resulting from some medical conditions, medications that reduce insulin action, chemicals that destroy beta cells, infections, rare autoimmune disorders, and genetic syndromes associated with diabetes.

What is prediabetes?

Prediabetes is when blood glucose levels are higher than normal but not high enough for a diagnosis of diabetes. Prediabetes means a person is at increased risk for developing type 2 diabetes, as well as for heart disease and stroke. Many people with prediabetes develop type 2 diabetes within 10 years.

However, modest weight loss and moderate physical activity can help people with prediabetes delay or prevent type 2 diabetes.

How are diabetes and prediabetes diagnosed?

Blood tests are used to diagnosis diabetes and prediabetes because early in the disease type 2 diabetes may have no symptoms. All diabetes blood tests involve drawing blood at a health care provider’s office or commercial facility and sending the sample to a lab for analysis. Lab analysis of blood is needed to ensure test results are accurate. Glucose measuring devices used in a health care provider’s office, such as finger-stick devices, are not accurate enough for diagnosis but may be used as a quick indicator of high blood glucose.

Testing enables health care providers to find and treat diabetes before complications occur and to find and treat prediabetes, which can delay or prevent type 2 diabetes from developing.

Any one of the following tests can be used for diagnosis:*

  • an A1C test, also called the hemoglobin A1c, HbA1c, or glycohemoglobin test
  • a fasting plasma glucose (FPG) test
  • an oral glucose tolerance test (OGTT)

*Not all tests are recommended for diagnosing all types of diabetes. See the individual test descriptions for details.

Another blood test, the random plasma glucose (RPG) test, is sometimes used to diagnose diabetes during a regular health checkup. If the RPG measures 200 micrograms per deciliter or above, and the individual also shows symptoms of diabetes, then a health care provider may diagnose diabetes.

Symptoms of diabetes include

  • increased urination
  • increased thirst
  • unexplained weight loss

Other symptoms can include fatigue, blurred vision, increased hunger, and sores that do not heal.

Any test used to diagnose diabetes requires confirmation with a second measurement unless clear symptoms of diabetes exist.

The following table provides the blood test levels for diagnosis of diabetes for nonpregnant adults and diagnosis of prediabetes.

Table of blood test levels for diagnosis of diabetes and prediabetes. Shows A1C, Fasting Plasma Glucose, and Oral Glucose tests

blood test levels

A1C Test

The A1C test is used to detect type 2 diabetes and prediabetes but is not recommended for diagnosis of type 1 diabetes or gestational diabetes. The A1C test is a blood test that reflects the average of a person’s blood glucose levels over the past 3 months and does not show daily fluctuations. The A1C test is more convenient for patients than the traditional glucose tests because it does not require fasting and can be performed at any time of the day.

The A1C test result is reported as a percentage. The higher the percentage, the higher a person’s blood glucose levels have been. A normal A1C level is below 5.7 percent.

An A1C of 5.7 to 6.4 percent indicates prediabetes. People diagnosed with prediabetes may be retested in 1 year. People with an A1C below 5.7 percent maystill be at risk for diabetes, depending on the presence of other characteristics that put them at risk, also known as risk factors. People with an A1C above 6.0 percent should be considered at very high risk of developing diabetes. A level of 6.5 percent or above means a person has diabetes.

Laboratory analysis. When the A1C test is used for diagnosis, the blood sample must be sent to a laboratory using a method that is certified by the NGSP to ensure the results are standardized. Blood samples analyzed in a health care provider’s office, known as point-of-care tests, are not standardized for diagnosing diabetes.

Abnormal results. The A1C test can be unreliable for diagnosing or monitoring diabetes in people with certain conditions known to interfere with the results. Interference should be suspected when A1C results seem very different from the results of a blood glucose test. People of African, Mediterranean, or Southeast Asian descent or people with family members with sickle cell anemia or a thalassemia are particularly at risk of interference.

However, not all of the A1C tests are unreliable for people with these diseases. The NGSP provides information about which A1C tests are appropriate to use for specific types of interference and details on any problems with the A1C test at www.ngsp.orgExternal Link Disclaimer.

False A1C test results may also occur in people with other problems that affect their blood or hemoglobin such as chronic kidney disease, liver disease, or anemia.

Changes in Diagnostic Testing

In the past, the A1C test was used to monitor blood glucose levels but not for diagnosis. The A1C test has now been standardized, and in 2009, an international expert committee recommended it be used for diagnosis of type 2 diabetes and prediabetes.2

Fasting Plasma Glucose Test

The FPG test is used to detect diabetes and prediabetes. The FPG test has been the most common test used for diagnosing diabetes because it is more convenient than the OGTT and less expensive. The FPG test measures blood glucose in a person who has fasted for at least 8 hours and is most reliable when given in the morning.

People with a fasting glucose level of 100 to 125 mg/dL have impaired fasting glucose (IFG), or prediabetes. A level of 126 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.

Oral Glucose Tolerance Test

The OGTT can be used to diagnose diabetes, prediabetes, and gestational diabetes. Research has shown that the OGTT is more sensitive than the FPG test, but it is less convenient to administer. When used to test for diabetes or prediabetes, the OGTT measures blood glucose after a person fasts for at least 8 hours and 2 hours after the person drinks a liquid containing 75 grams of glucose dissolved in water.

If the 2-hour blood glucose level is between 140 and 199 mg/dL, the person has a type of prediabetes called impaired glucose tolerance (IGT). If confirmed by a second test, a 2-hour glucose level of 200 mg/dL or above means a person has diabetes.

Are diabetes blood test results always accurate?

All laboratory test results can vary from day to day and from test to test. Results can vary

  • within the person being tested. A person’s blood glucose levels normally move up and down depending on meals, exercise, sickness, and stress.
  • between different tests. Each test measures blood glucose levels in a different way.
  • within the same test. Even when the same blood sample is repeatedly measured in the same laboratory, the results may vary due to small changes in temperature, equipment, or sample handling.

Although all these tests can be used to indicate diabetes, in some people one test will indicate a diagnosis of diabetes when another test does not. People with differing test results may be in an early stage of the disease, where blood glucose levels have not risen high enough to show on every test.

Health care providers take all these variations into account when considering test results and repeat laboratory tests for confirmation. Diabetes develops over time, so even with variations in test results, health care providers can tell when overall blood glucose levels are becoming too high.

Diagnosis of Gestational Diabetes

Health care providers test for gestational diabetes using the OGTT. Women may be tested during their first visit to the health care provider after becoming pregnant or between 24 to 28 weeks of pregnancy depending on their risk factors and symptoms. Women found to have diabetes at the first visit to the health care provider after becoming pregnant may be diagnosed with type 2 diabetes.

Defining Safe Blood Glucose Levels for Pregnancy

Many studies have shown that gestational diabetes can cause complications for the mother and baby. An international, multicenter study, the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study, showed that the higher a pregnant woman’s blood glucose is, the higher her risk of pregnancy complications. The HAPO researchers found that pregnancy complications can occur at blood glucose levels that were once considered to be normal.

Based on the results of the HAPO study, new guidelines for diagnosis of gestational diabetes were recommended by the International Association of the Diabetes and Pregnancy Study Groups in 2011. So far, the new guidelines have been adopted by the American Diabetes Association (ADA)3 but not by the American College of Obstetricians and Gynecologists (ACOG)4 or other medical organizations. Researchers estimate these new guidelines, if widely adopted, will increase the proportion of pregnant women diagnosed with gestational diabetes to nearly 18 percent.5

Both ADA and ACOG guidelines for using the OGTT in diagnosing gestational diabetes are shown in the following tables.

Recommendations for Testing Pregnant Women for Diabetes

pregnant women


gestational diabetes

Who should be tested for diabetes and prediabetes?

Adults, pregnant women, children, and teens should be tested for diabetes and prediabetes according to their risk factors. Adults

Anyone age 45 or older should consider getting tested for diabetes or prediabetes. Testing is strongly recommended for people older than age 45 who are overweight or obese. People younger than 45 should consider testing if they are overweight or obese*** and have one or more of the following risk factors:

  • physical inactivity
  • parent, brother, or sister with diabetes
  • family background that is African American, Alaska Native, American Indian, Asian American, Hispanic/Latino, or Pacific Islander American
  • history of giving birth to at least one baby weighing more than 9 pounds
  • history of gestational diabetes
  • high blood pressure—140/90 mmHg or higher—or being diagnosed with high blood pressure
  • high-density lipoprotein, or HDL, cholesterol—“good” cholesterol—level below 35 mg/dL or a triglyceride level above 250 mg/dL
  • polycystic ovary syndrome, also called PCOS
  • prediabetes—an A1C level of 5.7 to 6.4 percent; an FPG test result of 100–125 mg/dL, indicating IFG; or a 2-hour OGTT result of 140–199 mg/dL, indicating IGT
  • acanthosis nigricans, a condition associated with insulin resistance and characterized by a dark, velvety rash around the neck or armpits
  • history of cardiovascular disease—disease affecting the heart and blood vessels

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 and 35 inches or more for women is linked to insulin resistance and increases a person’s risk for type 2 diabetes. This is true even if a person’s body mass index (BMI) falls within the normal range.

How to Measure the Waist

To measure the waist, a person should

  • place a tape measure around the bare abdomen just above the hip bone
  • make sure the tape is snug but isn’t digging into the skin and is parallel to the floor
  • relax, exhale, and measure

If results of testing are normal, testing should be repeated at least every 3 years. Health care providers may recommend more frequent testing depending on initial results and risk status. People whose test results indicate they have prediabetes may be tested again in 1 year and should take steps to prevent or delay type 2 diabetes.

Pregnant Women

All pregnant women with risk factors for type 2 diabetes should be tested using standard diabetes blood tests during their first visit to the health care provider during pregnancy to see if they had undiagnosed diabetes before becoming pregnant. After that, pregnant women should be tested for gestational diabetes between 24 and 28 weeks of their pregnancy using the OGTT.

Women who develop gestational diabetes should also have follow-up testing 6 to 12 weeks after the baby is born to find out if they have type 2 diabetes or prediabetes. If results of testing are normal, testing should be repeated at least every 3 years. Blood glucose tests, rather than the A1C test, should be used for testing within 12 weeks of delivery.

Children and Teens

Type 2 diabetes has become increasingly common in children and teens. Children are at high risk for developing type 2 diabetes and should be tested if they are

  • overweight or obese and have other risk factors, such as a family history of diabetes
  • older than age 10 or have already gone through puberty

Body Mass Index (BMI)

Body mass index is a measurement of body weight relative to height for adults age 20 or older.

People who are overweight or obese should consider talking with a health care provider about ways to lose weight and reduce the risk of diabetes.

The BMI has certain limitations. The BMI 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.

The BMI for children and teens must be determined based on age, height, weight, and sex. The Centers for Disease Control and Prevention (CDC) has information about BMI in children and teens, including a BMI calculator, at Link Disclaimer. The CDC website also has a BMI calculator for adults.

What steps can delay or prevent type 2 diabetes?

A major research study, the Diabetes Prevention Program (DPP), proved that people with prediabetes 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 five times a week for 30 minutes. These strategies worked well for both men and women in all racial and ethnic groups, but were especially effective for participants age 60 and older. A follow-up study, the Diabetes Prevention Program Outcomes Study (DPPOS), showed losing weight and being physically active provide lasting results. Ten years after the DPP, modest weight loss delayed onset of type 2 diabetes by an average of 4 years.

The diabetes medication metformin also lowers the risk of type 2 diabetes in people with prediabetes, especially those who are younger and heavier and women who have had gestational diabetes. The DPPOS showed that metformin delayed type 2 diabetes by 2 years. People at high risk should ask their health care provider if they should take metformin to prevent type 2 diabetes. Metformin is a medication that makes insulin work better and can reduce the risk of type 2 diabetes.

Points to Remember

  • Tests used for diagnosing diabetes and prediabetes include the A1C test—for type 2 diabetes and prediabetes—the fasting plasma glucose (FPG) test, and the oral glucose tolerance test (OGTT). Another blood test, the random plasma glucose (RPG) test, is sometimes used to diagnose diabetes when symptoms are present during a regular health checkup.
  • Anyone age 45 or older should consider getting tested for diabetes or prediabetes. People younger than 45 should consider testing if they are overweight or obese and have one or more additional risk factors for diabetes.
  • If results of testing are normal, testing should be repeated at least every 3 years. Health care providers may recommend more frequent testing depending on initial results and risk status.
  • People whose test results indicate they have prediabetes may be tested again in 1 year and should take steps to prevent or delay type 2 diabetes.
  • Many people with prediabetes develop type 2 diabetes within 10 years.
  • Modest weight loss and moderate physical activity can help people with prediabetes delay or prevent type 2 diabetes.

National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
June 2014



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