Several kinds of glucose blood tests are used to diagnose or monitor diabetes. The major tests include several screening tests for diabetes:

• Fasting plasma glucose test (FPG). Measures the level of glucose (blood sugar) after a patient has refrained from eating for 10 to 16 hours. This is usually done in the morning following the patient’s nightly sleep cycle. This test is often offered when a nondiabetic patient has a physical examination, but recent recommendations from endocrinologists may make other glucose tests more popular.
  
  
• Glucose challenge test. Evaluates the body’s ability to metabolize glucose. It is used to screen pregnant women for gestational diabetes. The patient drinks 50 grams of a glucose solution. A blood sample is drawn an hour later. If results are abnormal, an oral glucose tolerance test is used to confirm diagnosis.
  
  
• Oral glucose tolerance test (OGTT). Measures a person’s ability to use glucose. This test can be used to diagnose gestational diabetes, usually when a woman is 24 to 28 weeks pregnant. It can also be used in diagnosing other forms of diabetes, in screening for prediabetes (impaired glucose tolerance) or in calculating insulin sensitivity and insulin resistance for conditions such as polycystic ovarian syndrome. The OGTT differs from the glucose challenge test, but the two terms are sometimes used interchangeably.
  
  According to the American Diabetes Association (ADA), either the fasting plasma glucose test or the OGTT can be used in diagnosing diabetes or prediabetes, but it recommends the FPG as easier, quicker and less expensive. However, in 2005 the American College of Endocrinology and the American Association of Clinical Endocrinologists issued new guidelines urging that physicians who suspect diabetes in a patient perform a two-hour OGTT rather than a fasting glucose test.
  
  Patients preparing for an OGTT are asked to consume a diet rich in carbohydrates for two or three days, then to fast for 10 to 16 hours before the test. After a blood sample is drawn, the patient is asked to consume a sweet-tasting concentrated glucose liquid. During the next few hours, blood samples will be taken several times. In patients without diabetes, blood glucose levels will rise, then fall quickly after the glucose is consumed. Those with diabetes will see their blood glucose levels remain elevated.
  
  
• Two-hour postprandial blood sugar test (2-hour PC). A measurement of blood glucose taken two hours after a meal. It reveals the body’s ability to metabolize carbohydrates and produce insulin. It is sometimes used to confirm results of a fasting glucose test.
  
  
• Random glucose test or random blood sugar test (RBS). Also known as a casual blood glucose test, it is a measurement of blood glucose taken regardless of when the patient last ate. Several random measurements may be obtained during the day. Most healthy people have blood glucose levels that do not vary widely during the day, and random blood sugar tests that vary widely may indicate a problem.

There are also tests used to monitor diabetes. Though they do not directly assess glucose levels, they are often thought of as glucose tests because they do indicate glycemic control:

• Glycohemoglobin test (also called a GHb, HbA1C or A1C test). Measures the amount of glycosylated hemoglobin (glucose that has attached to the oxygen-carrying portion of red blood cells). This test helps determine glucose levels over the previous two to three months. A high GHb measurement indicates hyperglycemia. The glycohemoglobin test is primarily used to assess how well diabetic individuals are managing their disease.
  
  Target glucose levels vary among patients and physicians. A common goal, and one recommended by the U.S. National Institutes of Health, is a maximum glycohemoglobin of 7 percent. In 2005 the International Diabetes Federation (IDF) issued guidelines calling for a glycohemoglobin level of 6.5 percent or less to reduce the rate of diabetic complications such as cardiovascular disorders and the eye disease diabetic retinopathy. However, the IDF, which represents 190 diabetes associations in 150 nations, acknowledged that most people with diabetes fell short of this goal. Some patients, such as those prone to hypoglycemia, have a higher GHb target set by their physician.

• Fructosamine test. Measures a person’s average glucose level over the previous several weeks. The glycohemoglobin test is more commonly performed to evaluate management of a person’s diabetes, but the fructosamine test may be used instead when information is sought about the past few weeks rather than months. These circumstances may include pregnancy, a recent change in the treatment plan, or recent blood loss or illness.

Glucose levels can also be measured in glucose urine tests. However, blood glucose can rise significantly before it is detectable in urine, and these tests are not especially accurate. For these reasons, glucose urine tests are not commonly used to diagnose or monitor diabetes.

Scientists are developing noninvasive, painless methods of assessing glucose, such as using fluorescent light to scan the forearm for biological markers in the skin, and testing teardrops with a solution of gold salts.

Diabetes patients can use glucose monitoring at home, work, school or elsewhere to measure their glucose themselves. The frequency of this test depends on their physician's recommendation and their form of diabetes. A small, portable device called a glucose meter is needed. The patient pricks a finger with a small needle and collects a drop of blood. This is placed on a test strip, which is inserted into the meter for a reading of the blood glucose level at that moment. A blood glucose level of between 90 mg/dL and 130 mg/dL before a meal (preprandial) and less than 180 after a meal (postprandial) is considered normal, according to the ADA. Some patients, such as those prescribed insulin, must perform glucose monitoring several times a day.