Hypoglycemia is a medical term referring to a pathologic state produced by a lower than normal amount of sugar (glucose) in the blood. The term hypoglycemia literally means "low blood sugar". Hypoglycemia can produce a variety of symptoms and effects but the principal problems arise from an inadequate supply of glucose as fuel to the brain, resulting in impairment of function (neuroglycopenia). Derangements of function can range from vaguely "feeling bad" to coma and (rarely) death. Hypoglycemia can arise from many causes, and can occur at any age. The most common forms of moderate and severe hypoglycemia occur as a complication of treatment of diabetes mellitus with insulin or oral medications. more...
Endocrinologists (specialists in disorders of blood glucose metabolism) typically consider the following criteria (referred to as Whipple's triad) as proving that an individual's symptoms can be attributed to hypoglycemia:
- Symptoms known to be caused by hypoglycemia
- Low glucose at the time the symptoms occur
- Reversal or improvement of symptoms or problems when the glucose is restored to normal
However, not everyone has accepted these suggested diagnostic criteria, and even the level of glucose low enough to define hypoglycemia has been a source of controversy in several contexts. For many purposes, plasma glucose levels below 70 mg/dl or 3.9 mmol/L are considered hypoglycemic, but these issues are elaborated in more detail below.
Defining hypoglycemia: what's normal and what's low?
Although 70 mg/dl (3.9 mmol/l) is commonly cited as the lower limit of normal glucose, different values may be defined as low for different populations, purposes, or circumstances. The precise level of glucose considered low enough to define hypoglycemia is dependent on (1) the measurement method, (2) the age of the person, (3) presence or absence of effects, and (4) the purpose of the definition. This article expresses glucose in milligrams per deciliter (mg/dl or mg/100 ml) as is customary in the United States, while millimoles per liter (mmol/l or mM) are the SI (International System) units used in most of the rest of the world. Values in mg/dl can be converted to mmol/l by dividing by 18 (e.g., 90 mg/dl = 5 mmol/l or 5 mM).
Measurement method: different methods can yield different values
Glucose levels discussed in this article are venous plasma or serum levels measured by standard glucose oxidase methods used in medical laboratories. For clinical purposes, plasma and serum levels are similar enough to be interchangeable. Arterial plasma or serum levels are slightly higher than venous levels, and capillary levels typically in between. This difference between arterial and venous levels is small in the fasting state but is amplified and can be greater than 10% in the postprandial state. On the other hand, whole blood glucose levels (e.g., by fingerprick meters) are about 10-15% lower than venous plasma levels. Furthermore, available fingerstick glucose meters are only warranted to be accurate to within 15% of a simultaneous laboratory value. In other words, a meter glucose reading of 39 mg/dl could be properly obtained from a person whose serum glucose was 55 mg/dl.
Two other factors significantly affect glucose measurement. The disparity between venous and whole blood concentrations is greater when the hematocrit is high, as in newborns. High neonatal hematocrits are particularly likely to confound meter glucose measurement. Second, unless the specimen is drawn into a fluoride tube or processed immediately to separate the serum or plasma from the cells, the measurable glucose will be gradually lowered by in vitro metabolism of the glucose.
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