Glucose Tests

Glucose Tests

Also called: Random Glucose, Glucose Tolerance Test, Serum Glucose, Fasting Glucose, Oral Glucose Tolerance Test, Glucose Blood Test, Postprandial Blood Glucose, OGTT


Glucose tests measure the amount of glucose (blood sugar) in a person’s system. There are several types of glucose tests. They generally involve a blood sample taken from the patient and analyzed to measure the amount of glucose present in the bloodstream.

Some types of glucose tests indicate whether or not the patient has diabetes. In other situations, physicians may use glucose tests to monitor the progression of diabetes or the level of metabolic control maintained by a patient. These tests can also help uncover other illnesses, such as thyroid, adrenal or pituitary disorders. 

The various tests have particular strengths, such as detecting prediabetes, diagnosing gestational diabetes, evaluating the treatment plan or monitoring pregnancy.

Glucose tests cause patients little discomfort. Blood is usually drawn from a vein located along the inner elbow or on the back of the hand. In some forms of testing, patients are required to modify their diet before the test.

About glucose tests

Most glucose tests use a blood sample to measure the amount of glucose (blood sugar) in a person’s bloodstream. Urine tests can also indicate the level of a person’s glucose but are less accurate than blood tests and rarely used for this purpose. Several versions of glucose tests are used to diagnose diabetes, monitor diabetes management or assess the progression of the disease.

Glucose is the major source of energy in the body. Most of the carbohydrates people eat are broken down into glucose before being absorbed into the bloodstream. In healthy individuals, a hormone called insulin carries the glucose to the cells, where it is converted into energy.

However, diabetic individuals either do not manufacture insulin in their bodies (type 1 diabetes) or cannot properly use the insulin they do create (type 2 diabetes and gestational diabetes). As a result, glucose stays in the bloodstream and builds up, a condition known as hyperglycemia. Excess glucose can lead to heart conditions, nerve damage, kidney disease, eye diseases and other serious health problems. Diabetic individuals can also experience bouts of hypoglycemia (low glucose), which can cause seizures and a diabetic coma.

Glucose tests allow physicians to measure how much glucose is in a patient’s bloodstream. These tests can be used to diagnose diabetes and to monitor its progression. Glucose tests reveal:

  • How effectively the body is breaking down foods and converting glucose into energy
  • How effectively the liver is working
  • How effectively other organs are working, particularly those that help regulate glucose

In each test, a sample of blood is analyzed for its glucose content and compared to a range of blood glucose levels that indicate whether the patient’s results are within the normal range or indicate prediabetes or diabetes.

Different tests are available to check blood glucose. For example, some tests examine blood glucose after periods of fasting, whereas others check levels shortly after meals. The results describe how insulin and glucose are interacting in a patient.

Types and differences of glucose tests

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.

Before the glucose test

Preparation for glucose blood tests focuses on general preparation, food and medication.

  • General preparation
    • Patients should be in good health before glucose tests and free of all illnesses, including colds. 
  • Foods
    • Fasting plasma glucose test. Requires patients to fast overnight prior to testing. On the day of testing, patients should not smoke or consume coffee or other caffeinated products.
    • Oral glucose tolerance test (OGTT). Requires patients to eat a diet rich in carbohydrates for three days prior to the test. Such foods include bread, grains, starchy vegetables and fruit. On the evening before the test the patient fasts. On the testing day patients should not smoke or consume caffeine. Carbohydrate loading is not necessary for pregnant women taking an OGTT as a means of screening for gestational diabetes.
    • Two-hour postprandial blood sugar test. Requires patients to eat a high-carbohydrate meal exactly two hours before the test.
  • Medications and supplements. A variety of medications and dietary supplements may raise or lower glucose (blood sugar) levels. Patients should inform their physician about medications taken, including those for the following conditions:
    • Depression
    • High blood pressure
    • Abnormal cholesterol levels
    • Hormonal therapy
    • Seizures
    • Tuberculosis
    • Pain relief, including aspirin

During the glucose test

A glucose test follows the same general pattern as any other blood test. An elastic band is wrapped around the patient’s upper arm. This temporarily stops the flow of blood through the veins in the arm, keeping veins below the band larger and free from collapse. Veins in this state are more easily punctured by a needle.

The needle site is cleaned with alcohol before the needle is inserted. Occasionally, more than one needle stick may be necessary if the vein does not provide enough blood.

Once the needle is properly placed in the vein, a collection tube is attached and the blood flows into the tube. Once the blood has been captured, the band around the patient’s arm is removed and a gauze pad or cotton ball is placed over the puncture site. Pressure is applied for several minutes until the blood clots.

In addition to blood testing, an oral glucose tolerance test (OGTT) involves the ingestion of glucose and water. Patients ingest glucose before their blood is sampled. In a standard OGTT, the patient will drink between 75 grams and 100 grams of glucose. Pregnant women consume 100 grams of glucose. Blood is drawn at fasting, one hour and two hours after ingesting glucose if patients are being tested for gestational diabetes. Samples are taken one, two and three hours after glucose ingestion for others.

A glucose challenge test also involves drinking a sugary solution, this one containing 50 grams of glucose. Blood is drawn an hour later.

The drinks used in these two tests are extremely sweet and may cause nausea or vomiting in some patients. If vomiting occurs, the test is usually rescheduled for another day.

After the glucose test

Once the blood sample has been taken, it will be analyzed for glucose (blood sugar) content. The physician can then make a diagnosis about whether or not the patient has diabetes or prediabetes. If so, a treatment plan will be developed to manage the patient’s glucose levels. This usually involves lifestyle changes (exercise, weight loss, diet modification) and may involve medications such as antidiabetic agents or insulin therapy.

The following are blood glucose levels and how they are interpreted by most physicians. All measures in the chart below are in milligrams per deciliter (mg/dL):

Fasting plasma 
glucose test
Less than 100Between
100 and 125
126 or more
Oral glucose
tolerance test
Less than 140 (at two hours)Between
140 and 199
200 or more
Two-hour postprandial blood sugar testLess than 140 N/A200 or more
Random blood
sugar test
N/AN/A200 or more

With the glucose challenge test, which screens pregnant women for gestational diabetes, results below 140 mg/dL are considered normal. Above-normal results do not always indicate gestational diabetes and will prompt the physician to order an oral glucose tolerance test, which is more complicated but more specific.

In some cases, blood glucose abnormalities may indicate a condition other than diabetes or prediabetes. Other conditions that can cause high blood glucose (hyperglycemia) include:

  • Overeating
  • Severe stress
  • Heart attack
  • Stroke
  • Cushing’s syndrome (excess production of cortisol hormone)
  • Medications including corticosteroids, aspirin, diuretics, beta blockers (a class of antihypertensives), antidepressants, lithium, epinephrine and estrogens
  • Hyperthyroidism (overproduction of thyroid hormone)
  • Acromegaly (bone and muscle disease involving excess production of growth hormone)
  • Pancreatitis (inflammation of the pancreas)
  • Some cancers of the pancreas
  • Pheochromocytoma (noncancerous tumor that can exacerbate high blood pressure)

Low blood glucose (hypoglycemia) can also indicate health problems. Lower-than-normal glucose levels generally considered to be a health threat are:

  • Below 40 mg/dL for women
  • Below 50 mg/dL for men

Conditions that can cause lower-than-normal blood glucose levels include:

  • Addison’s disease (decreased production of adrenal hormone)
  • Hypothyroidism (decreased production of thyroid hormone)
  • Tumor in the pituitary gland or hypopituitarism (decreased production of pituitary hormone)
  • Liver diseases such as cirrhosis
  • Kidney failure
  • Malnutrition
  • Medications including acetaminophen
  • Insulinoma (tumor in the pancreas that causes overproduction of insulin)

Potential risks with glucose tests

There are very few risks associated with blood tests. Occasionally, a bruise may form at the puncture site. Bruising may be prevented by keeping pressure on this area for several minutes after the needle is withdrawn.

In rare cases, the vein used to obtain the blood sample may become inflamed. This condition is known as phlebitis. It can be alleviated by applying a warm compress several times throughout the day until the swelling disappears.

People with bleeding disorders may experience continued bleeding. Patients who are taking anticoagulants may also experience bleeding.

Some patients vomit after consuming glucose before an oral glucose tolerance test or glucose challenge test because the solution is so sweet. If this happens, the test may need to be canceled for that day and rescheduled. Other patients may find that their level of blood glucose plunges. Such patients are likely to receive juice or another drink to restore their glucose level.

Questions for your doctor about glucose tests

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about glucose tests:

  1. How often should I have a glucose test?

  2. Which type or types of glucose testing should I have?

  3. What are the advantages and disadvantages of this kind of glucose test compared to the others?

  4. Under what circumstances would I have a different glucose test?

  5. Is fasting or any other preparation needed for my test?

  6. Could any of my medications or supplements affect my results?

  7. When and how will I find out the test results?

  8. What do my test results indicate?

  9. How do these results compare to the results of my home glucose monitoring?

  10. Does my glucose test show the need for other tests or treatments?

  11. May I have a copy of my test results for my records?
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