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Blood Sugar Tests

Fasting Blood Sugar (FBS or Fasting Glucose)

A test that measures blood sugar levels. Elevated levels are associated with diabetes and insulin resistance, in which the body cannot properly handle sugar (e.g. obesity).

Goal values:
  • Less than 100 mg/dL = normal
  • Between 110–125 mg/dL = impaired fasting glucose (i.e., prediabetes)
  • Greater than 126 mg/dL on two or more samples = diabetes
Preparation

This test requires a 12-hour fast. You should wait to eat and/or take a hypoglycemic agent (insulin or oral medication) until after test has been drawn, unless told otherwise.

Eating and digesting foods called carbohydrates forms glucose (blood sugar). Glucose is needed by your body to provide energy to carry out your normal activities. Insulin is needed by the body to allow glucose to go into the cells and be used as energy. Without insulin, the levels of glucose in the blood will rise. Diabetes is a disease that occurs when either the pancreas (an organ in your body) is not able to produce insulin or the pancreas makes insulin, but it does not work as it should. Fasting blood sugar is a part of diabetic evaluation and management. An FBS greater than 126 mg/dL on more than one occasion usually indicates diabetes.

Glycosylated Hemoglobin or Hemoglobin A1C (HbA1C)

Reflects average blood sugar levels over the preceding 90-day period. Elevated levels are associated with prediabetes and diabetes. Individuals with diabetes have an increased risk of a cardiac event. A diabetic person's risk for heart attack is the same as a non-diabetic person, who has experienced one heart attack, having a second heart attack. Aggressive global preventive risk reduction efforts, such as lower LDL targets, diet, exercise and blood pressure control, are recommended.

Goal values (per American Diabetes Association guidelines):
  • A range of 5.7-6.4 percent indicates an increased risk for development of diabetes (i.e., prediabetes), and lifestyle interventions may be beneficial.
  • A value equal or greater than 6.5 percent is considered diabetic.
Preparation

This test may be measured any time of the day without fasting.

Glycosylated hemoglobin is blood glucose attached to hemoglobin (a component of blood). This test is often called the "diabetic report card." It reflects the average blood sugar for the two to three month period before the test.

To calculate the average blood glucose level from the HbA1C:

HbA1C level x (multiplied by) 33.3 – 86 = average blood glucose level for the past 90 days. HbA1C can be helpful to track diabetic control over time.

Reviewed: 09/13

This information is about testing and procedures and may include instructions specific to Cleveland Clinic. Please consult your physician for information pertaining to your testing.

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This information is provided by Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition.

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