According to the National Institutes of Health (NIH) site Medline Plus, a fasting glucose test is conducted to determine the amount of sugar, or glucose, that is in your blood after refraining from eating or drinking for a set time prior to the test. Also known as a blood sugar or plasma glucose test, this laboratory test is usually the first one conducted when diagnosing diabetes and may also be used periodically to monitor the progress of diabetic individuals. The test is simple, relatively painless and takes little time to perform, although it does require advance preparation in order not to disrupt the results.
Significance
A fasting blood glucose test is performed for one reason: to determine if your circulating glucose levels are higher than normal, which could indicate that you are not producing sufficient amounts of the glucose-regulating hormone insulin to deal with the sugar, or glucose, you take into your body through food. According to the University of Michigan Health System, an increase in the level of glucose in your blood normally triggers your pancreas to produce insulin to decrease circulating glucose amounts. If there is insufficient insulin and the glucose levels remain high over an extended period of time, your kidneys, nerves, eyes and cardiovascular system may become damaged.
Preparation
For a fasting blood glucose test, you will be asked to not eat or drink anything except water for at least eight hours prior to having your blood drawn. There are a number of medications that can either increase or decrease your glucose measurement, and you should be certain that your physician is aware if you are currently taking one of these drugs. Medications that can decrease your glucose level include acetaminophen, monoamine oxidase inhibitors, anabolic steroids and sulfonylurea medications like glyburide. Conversely, your glucose levels can be increased by the following drugs: tricyclic antidepressants, estrogens, epinephrine, diuretics, certain antipsychotic drugs, lithium and salicylates. Stress, smoking and drinking excessive alcohol or caffeinated beverages can also skew the results of the test.
Procedure
The site where the blood sample will be taken from--usually the back of a hand or the inside of an elbow--will be thoroughly cleaned with an antiseptic fluid and an elastic tourniquet will be tied firmly around your upper arm to cause your lower-arm veins to swell slightly with blood. After locating a vein, the technician performing the test will insert a needle and collect blood in an attached vial. Once the blood has been collected, the tourniquet and the needle are removed and the site is covered with a bandage. You may be asked to apply gentle pressure to the site for a few minutes to prevent excessive bruising or additional bleeding.
Results
Your results after a fasting blood glucose test will fall into one of three ranges. According to the NIH, anything under 100 mg/dL is considered normal, while a measurement of 100 to 125 mg/dL is considered to be indicative of a prediabetic condition. According to the University of Michigan Health System, you are at an increased risk of developing diabetes if your glucose measurements are within this range. Measurements above 126 mg/dL can be used as a positive diagnosis for diabetes. If, however, your fasting blood glucose levels are abnormally low--under 40 mg/dL for women, 50 mg/dL for men--you may have another serious medical condition such as hypothyroidism, Addison's disease or kidney or liver disease.
Risks
While the risks associated with a fasting blood glucose test are small, any medical procedure can result in unwanted developments or side effects. According to Medline Plus, risks with a fasting glucose test include too much bleeding or excessive blood pooling just beneath the skin, an infection later developing at the site where the needle punctured the skin, and dizziness or lightheadedness during or after the test is performed.
