| Subject | Contents |
| Definition | A test that measures the amount of glucose in the blood. |
| Alternative Names | FBS; Blood sugar levels; Fasting blood sugar |
| How the test is performed | Adult or child: Blood is drawn from a vein ( venipuncture ), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding . Infant or young child: The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding. |
| How to prepare for the test | Fast for 6 hours. Infants and children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:Infant test or procedure preparation (birth to 1 year) Toddler test or procedure preparation (1 to 3 years) Preschooler test or procedure preparation (3 to 6 years) Schoolage test or procedure preparation (6 to 12 years) Adolescent test or procedure preparation (12 to 18 years) A random serum glucose can be performed at any time, but will reflect such things as dietary intake and activity (see also glucose - random ). |
| How the test will feel | When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing. |
| Why the test is performed | This test is used to evaluate glucose (blood sugar) levels. It may be used to diagnose diabetes , monitor diabetic control, or as a screening test. Most dietary carbohydrate eventually ends up as glucose in the blood. Excess glucose is converted to glycogen for storage by the liver and skeletal muscles after meals. Glycogen is gradually broken down to glucose and released into the blood by the liver between meals. Excess glucose is converted to triglyceride for energy storage. Glucose is a major source of energy for most cells of the body. Some cells (for example, brain and red blood cells), are almost totally dependent on blood glucose as a source of energy. The brain, in fact, requires that glucose concentrations in the blood remain within a certain range in order to function normally. Concentrations less than about 30 milligrams per deciliter (mg/dl) or greater than about 300 mg/dl can produce confusion or unconsciousness . The major hormone regulating glucose concentration in the body is insulin (although other hormones such as glucagon , epinephrine, and cortisol also affect it). Glucose levels are measured most commonly to diagnose diabetes or to monitor adequacy of diabetic control. Diabetes is a very common disease; affecting about 2% of the general population. Diabetes results from deficient insulin or insensitivity to insulin. Type I diabetics require daily injections of insulin. Injection of too much or too little insulin can be dangerous. |
| Normal Values | 64 to 126 mg/dl Note: mg/dl = milligrams per deciliter. |
| What abnormal results mean | Greater-than-normal levels (hyperglycemia) may indicate:Acromegaly (very rare) Cushing's syndrome (rare) Diabetes mellitusHyperthyroidismPancreatic cancerPancreatitisPheochromocytoma (very rare) Insufficient amount of insulin Excessive food intake Lower-than-normal levels ( hypoglycemia ) may indicate:HypopituitarismHypothyroidismInsulinoma (very rare) Injection of too much insulin Insufficient dietary intake Additional conditions under which the test may be performed:Acute adrenal crisisCushing's syndrome caused by adrenal tumorCushing's syndrome - exogenousDeliriumDementiaDementia due to metabolic causesDiabetic hyperglycemic hyperosmolar comaDiabetic ketoacidosisDiabetic nephropathy/sclerosisEctopic Cushing's syndromeEpilepsyGeneralized tonic-clonic seizureGlucagonomaInsulin-dependent diabetes mellitus (IDDM)Islet of Langerhans' tumorMultiple endocrine neoplasia (MEN) INoninsulin-dependent diabetes mellitus (NIDDM)Pituitary Cushing's (Cushing's disease)SyphilisSyphilis - primarySyphilis - secondarySyphilis - tertiaryTransient ischemic attack (TIA) |
| What the risks are | Excessive bleedingFainting or feeling lightheaded Hematoma (blood accumulating under the skin) Infection (a slight risk any time the skin is broken) Multiple punctures to locate veins |
| Special considerations | Many forms of severe stress (for example, trauma, stroke , heart attack , and surgery) can temporarily increase glucose levels. Drugs that can increase glucose measurements include tricyclic antidepressants, corticosteroids, diazoxide, IV dextrose, diuretics, epinephrine, estrogens, glucagon , isoniazid, lithium, phenothiazines, phenytoin, salicylates ( acute toxicity--see aspirin overdose ), and triamterene. Drugs that can decrease glucose measurements include acetaminophen - oral, alcohol, anabolic steroids, clofibrate, disopyramide, gemfibrozil, monoamine oxidase inhibitors, pentamidine, tolazamide, and tolbutamide. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others. |
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