Serum Glucose Level

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Serum Glucose Level



The serum glucose or blood sugar level is a measurement of the amount of a particular form of simple sugar in the blood. When carbohydrates are ingested, they are broken down in the intestines into component parts, including sugars such as glucose. Glucose is absorbed from the small intestine into the bloodstream. It circulates throughout the body and is used by all of the body’s tissues and organs to generate the energy necessary for their normal functioning. In order for glucose to enter the body’s cells, insulin must be present. Insulin is a hormone produced in and excreted by the pancreas. Insulin functions to allow the transport of glucose into the cells of the body, as well as being involved in the body’s storage of excess glucose in the form of glycogen or triglycerides.

The blood levels of glucose and insulin are intimately related. When carbohydrates are metabolized after a meal, the blood glucose begins to rise. Under normal circumstances, the pancreas then secretes insulin, in an amount relative to the blood glucose elevation. Between meals, or after heavy exertion, glucose levels may begin to drop below a safe threshold for the body’s cells (particular cells of the brain and nervous system). In response to this lowering of blood glucose, the pancreas secretes a different hormone, called glucagon. Glucagon prompts the liver to convert glycogen into glucose, thereby elevating the blood glucose back into a safe range.

Abnormal levels of blood glucose can be life-threatening. High blood glucose is termed hyperglycemia; low blood glucose is termed hypoglycemia. Either of these conditions can result in organ failure, severe brain damage, coma, or death. Diabetes occurs when the pancreas fails to produce normal amounts of insulin, or when it completely stops producing any insulin at all (this is often referred to as insulin-dependent or type I diabetes). Diabetes can also occur when cells of the body become less responsive to the effects of insulin (this is often referred to as insulin-resistance, or type II diabetes). Diabetes causes abnormal perturbations of the serum glucose level. Chronic high levels of serum glucose (which may occur in poorly controlled diabetes) can result in severe damage over time to the heart, the eyes, the kidneys, the circulatory system, and the nervous system. In diabetics, sudden, acute increases in the serum glucose level can result in the condition called diabetic ketoacidosis, in which the extremely high levels of blood glucose lead a life-threatening illness. Diabetics can also suffer from sudden drops in serum glucose levels; if untreated, glucose deprivation can affect the organs and tissues of the body and may also be life-threatening.


A serum glucose level is usually drawn as part of a larger metabolic panel or screen. Other tests performed in this panel include electrolytes (sodium, potassium, chloride, and carbon dioxide), as well as calcium, creatnine, and BUN (blood urea nitrogen). A serum glucose level is usually checked during a routine physical examination or may be performed specifically to screen for diabetes, especially when there is a strong family history of diabetes, or when an individual has other specific risk factors, such as being overweight.

Serum glucose levels are also an important part of monitoring the health of pregnant women since some women develop gestational diabetes during pregnancy. Untreated, this can result in problems with the baby as well as the mother. Gestational diabetes in early pregnancy can cause birth defects (particularly of the brain and/or heart) and increase the chance of miscarriage. Gestational diabetes in the second and third trimesters can cause the baby to grow very large. The baby’s size can result in problems for the mother during labor and delivery. Additionally, once the baby is born, it can suffer sudden hypoglycemia. In utero, the baby will have acclimated to its mother’s high serum glucose levels by producing high levels of insulin. After birth, suddenly deprived of that glucose, the baby’s relatively high insulin levels can result in severe hypoglycemia.

A serum glucose level may be ordered when there are symptoms suspicious of diabetes, such as excessive thirst and/or hunger, urinary frequency, unintentional weight loss, severe fatigue and weakness, and poor healing. The diagnosis of diabetes requires that a random high serum glucose level be confirmed by a high fasting serum glucose level or by abnormal results of an oral glucose tolerance test. Patients who are diabetic may also be required to check their own blood glucose one or more times a day, to make sure that their condition is under good control.

A serum glucose level may be ordered when there are symptoms suspicious of low blood sugar (hypoglycemia), such as shakiness, sweating, anxiety, confusion, dizziness, or fainting.


The serum glucose level is highly affected by when an individual has last eaten, therefore, appropriate interpretation of the test results must take this into consideration. Serum glucose levels may be examined under random conditions, after an eight to ten hour fast (referred to as a fasting serum glucose level); two hours after a meal has been completed (referred to as a two-hour post-prandial serum glucose level); or after an individual has been given a standardized amount of a glucose-containing beverage (referred to as an oral glucose tolerance test or OGTT).

Serum glucose levels can be affected by a number of medications. Patients who are on these medications should inform their doctor, so that test results can be interpreted appropriately. Medications that may affect serum glucose levels include birth control pills, high blood pressure medications, phenytoin, furosemide, triamterene, hydrochlorothiazide, niacin, propranolol, and steroid medications. Additionally, the use of alcohol, the use of caffeine, recent illness, infection, or emotional distress may affect test results.

Patients who are taking anticoagulant medications should inform their healthcare practitioner, since this may increase their chance of bleeding or bruising after a blood test.


This test requires serum to be drawn from a vein (usually one in the forearm), generally by a nurse or phlebotomist (an individual who has been trained to draw serum). A tourniquet is applied to the arm above the area where the needle stick will be performed. The site of the needle stick is cleaned with antiseptic, and the needle is inserted. The serum is collected in vacuum tubes. After collection, the needle is withdrawn, and pressure is kept on the serum draw site to stop any bleeding and decrease bruising. A bandage is then applied.

Self-glucose testing is often performed one or more times per day by diabetics themselves. This involves using a special sharp instrument, called a lancet, to prick a finger. Frequently, these lancets are placed in a spring-loaded mechanism to make it easier to accomplish the finger prick. A drop of blood from this finger prick is then put onto a special strip of paper and slipped into a machine called a blood glucose meter. The meter gives a digital readout of the serum glucose level. Alternatively, the drop of blood can be put onto a special strip of test paper which changes color based on the glucose level; this is less accurate than the blood glucose meter.


There are no special preparations necessary prior to a random serum glucose level. For a two-hour postprandial serum glucose level, the individual should be instructed to eat a meal exactly two hours before the blood draw. For a fasting serum glucose level, the individual should ingest nothing other than water for a minimum of eight hours prior to the blood draw. Diabetics may be asked to delay their morning dose of insulin or oral diabetes medication (oral hypoglycemic agents) prior to the blood draw.


As with any blood tests, discomfort, bruising, and/or a very small amount of bleeding is common at the puncture site. Immediately after the needle is withdrawn, it is helpful to put pressure on the puncture site until the bleeding has stopped. This decreases the chance of significant bruising. Warm packs may relieve minor discomfort. Some individuals may feel briefly woozy after a serum test, and they should be encouraged to lie down and rest until they feel better.


Gestational diabetes— A type of diabetes that occurs during pregnancy. Untreated, it can cause severe complications for the mother and the baby; however, it usually does not lead to long-term diabetes in either the mother or the child.

Glucose— A simple sugar that is the product of carbohydrate metabolism. It is the major source of energy for all of the organs and tissues of the body.

Glucagon— A hormone produced in the pancreas that is responsible for elevating blood glucose when it falls below a safe level for the body’s organs and tissues.

Glycogen— The form in which glucose is stored in the body.

Hyperglycemia— Elevated blood glucose levels.

Hypoglycemia— Low blood glucose levels.

Insulin— A hormone produced by the pancreas that is responsible for allowing the body’s cells to utilize glucose. The deficiency or absence of insulin is one of the causes of the disease diabetes.

Insulinoma— A tumor within the pancreas that produces insulin, potentially causing the serum glucose level to drop to dangerously low levels.

Ketoacidosis— A potentially life-threatening condition in which abnormally high blood glucose levels result in the blood become too acidic.

Pancreas— An organ located near the liver and stomach, responsible for various digestive functions. The pancreas produces insulin and glucagon, hormones that are responsible for maintaining safe blood levels of glucose.


Basic blood tests, such as serum glucose levels, do not carry any significant risks, other than slight bruising and the chance of brief dizziness.


Normal results of a random serum glucose test range from 70-125 milligrams per deciliter (mg/dL). Normal results of a two-hour post-prandial serum glucose level range from 70-145 mg/dL. Normal results of a fasting serum glucose level range from 70-99 mg/dL.

High levels

High serum glucose levels suggest the possibility of diabetes; however, a single high, random serum glucose level is not sufficient for definitively diagnosing diabetes. The American Diabetes Association has specific criteria that must be met in order to diagnose diabetes. They require that results are verified through testing on a minimum of two different days. Levels indicative of diabetes are as follows:

  • random serum glucose level of 200 mg/dL in the presence of actual symptoms of diabetes (such as increased thirst and/or hunger, urinary frequency, unintentional weight loss, weakness and fatigue, numbness/tingling in hands and feet, blurred vision, or erection problems;
  • fasting serum glucose level of at least 126 mg/dL;
  • two-hour oral glucose tolerance test of at least 200 mg/dL.

Individuals who don’t meet the criteria for an actual diagnosis of diabetes, but who have a higher-than-normal fasting serum glucose level, also known as an impaired fasting glucose (ranging from 100 mg/dL to 125 mg/dL), have an increased risk of eventually developing diabetes, and should be followed closely. These individuals are considered to have “prediabetes.”

Other causes of high serum glucose levels include:

Low levels

Low serum glucose levels may be due to:

  • the presence of an insulinoma (a tumor that secretes insulin);
  • Addison’s disease;
  • hypothyroidism (underactive thyroid);
  • pituitary gland tumor;
  • liver disease, including cirrhosis;
  • kidney disease;
  • malnutrition;
  • eating disorders, including anorexia nervosa; and
  • nappropriate doses of medicines used to treat diabetes, such as insulin or oral hypoglycemic agents.



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Kronenberg H. M., S. Melmed, K. S. Polonsy, P. R. Larsen. Williams Textbook of Endocrinology, 11th ed. Philadelphia: Saunders Elsevier, 2007.

McPherson R.A., and M. R. Pincus, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods, 21st ed. Philadelphia: Saunders, 2006.


American Diabetes Association. (February 10, 2008).

Medical Encyclopedia. Medline Plus. National Institutes of Health. (February 10, 2008).


American Association for Clinical Chemistry, 1850 K Street, NW, Suite 625, Washington, DC, 20006, (800) 892-1400,

Rosalyn Carson-DeWitt, M.D.