Fasting Blood Sugar

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Fasting Blood Sugar

A fasting blood sugar (FBS) level is one of the tests used to diagnose diabetes mellitus (two others being the oral glucose tolerance test and the A1C level). In a person with symptoms of osmotic diuresis and an elevated fasting blood sugar level, the diagnosis of diabetes mellitus is usually made.

The fasting blood sugar is determined by taking a sample of venous blood after at least eight hours of fasting. The sugar level is then evaluated in the blood sample.

Interpreting your Fasting Blood Sugar (FBS)

According to the 2013 Standards of Medical Care in Diabetes made by the American Diabetes Association, you may interpret your fasting blood sugar as follows:

  • FBS < 100 mg/dl (5.6 mmol/l) = normal fasting blood sugar;
  • FBS 100-125 mg/dl (5.6-6.9 mmol/l) = IFG (impaired fasting glucose);
  • FBS ≥ 126 mg/dl (7.0 mmol/l) = provisional diagnosis of diabetes

Rationale for using fasting blood sugar

The blood glucose level is usually maintained in a range of 2.2-3.9 mmol/L (40-70 mg/dl). Between meals, and even during an overnight fast, that level is still maintained, even in the absence of food intake. The body does this by converting glycogen to glucose and, when necessary, fats to glucose (see carbohydrate metabolism). These two methods of producing glucose maintain a normal glucose level during fasting.

The ability to convert glycogen to glucose presupposes that the body converts glucose to glycogen when the glucose level is high, that is, after a meal. This process, which requires insulin, brings the glucose level back to normal a few hours after a meal.

In a diabetic, the lack of insulin keeps the glucose level elevated even after fasting overnight. When the blood sugar level reaches a critical level, symptoms of osmotic diuresis will be felt.

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