Glucose Tests Health Article

Definition

Glucose tests are used to determine the concentration of glucose in blood, urine, cerebrospinal fluid (CSF), and other body fluids. These tests are used to detect increased blood glucose (hyperglycemia), decreased blood glucose (hypoglycemia), increased glucose in the urine (glycosuria), and decreased glucose in CSF, serous, and synovial fluid glucose.

Purpose

The results of glucose tests are used in a variety of situations, including:

• Screening persons for diabetes mellitus. The American Diabetes Association (ADA) recommends that a fasting plasma glucose (fasting blood sugar) be used to diagnose diabetes. People without symptoms of diabetes should be tested when they reach the age of 45 years, and again every three years. People in high-risk groups should be tested before the age of 45, and then more frequently. If a person already has symptoms of diabetes, a blood glucose test without fasting (a casual plasma glucose test) may be performed. In difficult diagnostic cases, a glucose challenge test called a two-hour oral glucose tolerance test (OGTT) is recommended. If the result of any of these three tests is abnormal, it must be confirmed with a second test—performed on another day. The same test or a different test can be used. However, the result of the second test must be abnormal as well to establish a diagnosis of diabetes.
• Screening for gestational diabetes. Diabetes that occurs during pregnancy is called gestational diabetes. This condition is associated with hypertension, increased birth weight of the fetus, and a higher risk for preeclampsia. Women who are at risk are screened when they are 24–28 weeks pregnant. A woman is considered at risk if she is older than 25 years; is not at her normal body weight; has a parent or sibling with diabetes; or is in an ethnic group that has a high rate of diabetes (such as Hispanic, Native American, or African-American).
• Blood glucose monitoring. Daily measurement of whole blood glucose identifies persons with diabetes who require intervention to maintain their blood glucose within an acceptable range as determined by their doctors. The Diabetes Control and Complications Trial (DCCT) demonstrated that persons with diabetes who maintained blood glucose and glycated hemoglobin (hemoglobin with glucose bound to it) at or near normal decreased their risk of complications by 50–75%. Based on results of this study, the ADA recommends routine glycated hemoglobin testing to measure long-term control of blood sugar. The most common glycated hemoglobin test, is the HbA_1c, which provides the average, overall blood glucose levels over the prior two to three month period. A DCCT randomized study found that the knowledge alone that their glycated hemoglobin results were good improved blood glucose control in some patients.
• Diagnosis and differentiation of hypoglycemia. Low blood glucose may be associated with such symptoms
  as confusion, memory loss, and seizures. Demonstration that such symptoms are the result of hypoglycemia requires evidence of low blood glucose at the time of symptoms and reversal of the symptoms by glucose. In documented hypoglycemia, blood glucose tests are used along with measurements of insulin and C-peptide (a fragment of proinsulin) to differentiate between fasting and postprandial (after a meal) causes.
• Analysis of glucose in body fluids. High levels of glucose in body fluids reflect a hyperglycemic state and are not otherwise clinically significant. Low body fluid glucose levels, however, indicate increased glucose utilization, often caused by infection (meningitis causes a low CSF glucose); inflammatory disease (rheumatoid arthritis causes a low pleural fluid glucose); or malignancy (a leukemia or lymphoma, such as Hodgkin's disease infiltrating the CNS or serous cavity).

Precautions

Diabetes must be diagnosed as early as possible so that treatment can begin. If left untreated, it will result in progressive vascular disease that may damage the blood vessels, nerves, kidneys, heart, and other organs. Brain damage can occur from glucose levels below 40 mg/dL and coma from levels above 450 mg/dL. For this reason, plasma glucose levels below 40 mg/dL or above 450 mg/dL are commonly used as alert values. Point-of-care and home glucose monitors measure glucose in whole blood rather than plasma. They are accurate, for the most part, within a range of glucose concentration between 40 mg/dL and 450 mg/dL. In addition, whole blood glucose measurements are generally 10% lower than those of serum or plasma glucose.

Other endocrine disorders and a number of medications can cause both hyperglycemia and hypoglycemia. For this reason, abnormal glucose test results must be interpreted by a doctor.

Glucose is affected by heat; therefore, plasma or serum must be separated from the blood cells and refrigerated as soon as possible. Splenectomy, for example, can result in an increase in glycated hemoglobin, but hemolytic anemia can produce a decrease in it.

There are other factors that can also affect the OGTT, such as exercise, diet, anorexia, and smoking. Drugs that decrease tolerance to glucose and affect the test include steroids, oral contraceptives, estrogens, and thiazide diuretics.