The blood glucose test may be used to:

  • Detect high blood glucose (hyperglycemia) and low blood glucose (hypoglycemia)
  • Screen for diabetesin people who are at risk before signs and symptoms are apparent; in some cases, there may be no early signs or symptoms of diabetes. Screening can therefore be useful in helping to identify it and allowing for treatment before the condition worsens or complications arise.
  • Help diagnose diabetes, prediabetesand gestational diabetes
  • Monitor glucose levels in people diagnosed with diabetes

A few different testing protocols may be used to evaluate blood glucose levels, depending on the purpose.

Screening and Diagnosis
The following tests may be used for screening and diagnosis of type 1, type 2 or prediabetes. (Gestational diabetes testing is different—see below.) If the initial screening result from one of the tests is abnormal, the test is repeated on another day. The repeat result must also be abnormal to confirm a diagnosis of diabetes.

  • Fasting glucose (fasting blood glucose, FBG) – this test measures the level of glucose in the blood after fasting for at least 8 hours.
  • 2-hour glucose tolerance test (GTT) – for this test, the person has a fasting glucose test done (see above), then drinks a 75-gram glucose drink. Another blood sample is drawn 2 hours after the glucose drink. This protocol “challenges” the person’s body to process the glucose. Normally, the blood glucose level rises after the drink and stimulates the pancreas to release insulin into the bloodstream. Insulin allows the glucose to be taken up by cells. As time passes, the blood glucose level is expected to decrease again. When a person is unable to produce enough insulin, or if the body’s cells are resistant to its effects (insulin resistance), then less glucose is transported from the blood into cells and the blood glucose level remains high.
  • A different test called hemoglobin A1c may be used as an alternative to glucose testing for screening and diagnosis.

Sometimes a blood sample may be drawn and glucose measured when a person has not been fasting, for example, when a comprehensive metabolic panel (CMP) is performed. If the result is abnormal, it is typically followed up with a fasting blood glucose test or a GTT.

Glucose blood tests are also used to screen pregnant women for gestational diabetes between their 24th and 28th week of pregnancy. The American Diabetes Association and the U.S. Preventive Services Task Force recommend that pregnant women not previously known to have diabetes be screened and diagnosed, using either a one-step or two-step approach. The American College of Obstetricians and Gynecologists (ACOG) recommends the two-step approach.

  • One-step 2-hour oral glucose tolerance test (OGTT). After a fasting glucose level is measured, a woman is given a 75-gram dose of glucose to drink and her glucose levels are measured at 1 hour and 2 hours after the dose. Only one of the values needs to be above a cutoff value for diagnosis.
  • Two-step
    • Perform a glucose challenge test as a screen: a woman is given a 50-gram glucose dose to drink and her blood glucose level is measured after 1 hour.
    • If the challenge test is abnormal, perform a 3-hour oral glucose tolerance test. After a woman’s fasting glucose level is measured, she is given a 100-gram glucose dose and her glucose is measured at timed intervals. If at least two of the glucose levels at fasting, 1 hour, 2 hour, or 3 hour are above a certain level, then a diagnosis of gestational diabetes is made.
      Glucose testing is also used to test women who were diagnosed with gestational diabetes 6-12 weeks after they have delivered their baby to detect persistent diabetes.

Monitoring
Diabetics must monitor their own blood glucose levels, often several times a day, to determine how far above or below normal their glucose is and to determine what oral medications or insulin(s) they may need. This is usually done by placing a drop of blood from a skin prick onto a glucose strip and then inserting the strip into a glucose meter, a small machine that provides a digital readout of the blood glucose level.

Urine
Urine glucose is one of the substances tested when a urinalysis is performed. A urinalysis may be done routinely as part of a physical or prenatal checkup. The health practitioner may follow up an elevated urine glucose test with blood glucose testing. Urine glucose testing is a screening tool, but it is not sensitive enough for diagnosis or monitoring.

Other tests, such as diabetes autoantibodies, insulin, and C-peptide, may sometimes be performed along with these tests to help determine the cause of abnormal glucose levels, to distinguish between type 1 and type 2 diabetes, and to evaluate insulin production.

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