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What is a Fasting Blood Sugar Level?

By Dulce Corazon
Updated Feb 17, 2024
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A fasting blood sugar level is the measure of sugar in the blood of individuals who didn't eat or drink anything for at least eight hours before undergoing a fasting blood sugar test. This test is usually requested by physicians to evaluate patients who have symptoms of diabetes. Diabetes is a medical condition mostly characterized by increased sugar in the blood. Its symptoms include increased frequency of urination, constant hunger, and excessive thirst. Fasting blood sugar test is also regularly performed to monitor diabetic patients.

The expected fasting blood sugar level of a healthy individual is usually between 70 milligrams per deciliter (mg/dL) and 99 mg/dL. The values may, however, sometimes vary according to the laboratory performing the test. A single test showing an elevated fasting blood sugar level is usually not enough to make a diagnosis of diabetes. Physicians often base their diagnosis on two or more abnormal fasting blood sugar level results.

Testing for blood sugar level is generally done by collecting a blood sample from the vein of the patient. Discomfort and slight pain from the needle puncture may be experienced. The sample is then processed in the laboratory for determination of the fasting blood sugar level of the patient.

Results from 100 mg/dL up to 125 mg/dL are frequently diagnosed as prediabetes, a state in which the fasting blood sugar level is increased, but not yet considered diabetes. Patients with these fasting blood sugar levels are often encouraged to review their lifestyle habits and make changes to prevent or delay the development of diabetes. These lifestyle changes include eating a healthy diet, getting enough exercise, and losing weight.

If a patient's blood tests show results ranging above 125 mg/dl on two or more occasions, it is often an indication of diabetes. Aside from diabetes, other diseases that can increase the fasting blood sugar level include long-term kidney disorder, inflammation and cancer of the pancreas, and hyperthyroidism. Hyperthyroidism is the excessive release of thyroid hormone due to the thyroid gland's abnormal activities. Drugs such as oral contraceptive pills, antidepressants, and corticosteroids may also affect an increase.

Decreased or low fasting blood sugar level measurements, under 70 mg/dL, can also be seen in patients with diabetes. This is commonly known as hypoglycemia. Common hypoglycemia symptoms are sweating, intense hunger, rapid heart beats, and weakness. When blood sugar level becomes very low, patients may become confused, lose consciousness, or lapse into a coma.

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Discussion Comments
By Wisedly33 — On Apr 27, 2014

@Scrbblechick -- Diabetics also have to be aware that their fasting readings may be affected by what is called "dawn phenomenon," which is when the liver starts dumping glucose in the early morning, making the blood sugar levels rise.

For some diabetics, a better reading is one taken about three hours after breakfast or lunch. Fasting blood sugar levels can be deceptive because of dawn phenomenon, but they do set a benchmark.

By Scrbblchick — On Apr 26, 2014

If a patient tests with an elevated blood sugar reading, any decent doctor will then run a Hemoglobin A1c. This test shows the average blood sugar range over the previous three or four months. A reading above 6.0 usually indicates diabetes. Most diabetics want their A1C to be below 6.0, and their morning blood sugar to be in the 80s to low 90s. This usually means their disease is well controlled.

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