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Hypoglycemia is also a term of contemporary American folk medicine which refers to a recurrent state of symptoms of altered mood and cognitive efficiency, sometimes accompanied by adrenalin symptoms, but not by measurably low blood glucose. The symptoms are primarily those of altered mood, behavior, and mental efficiency. This condition is usually treated by dietary changes which range from simple to elaborate. Because glucose levels are not actually low, this type of hypoglycemia does not carry the same risks of coma or brain damage as measurable hypoglycemia. In the medical literature, this condition is sometimes referred to as idiopathic postprandial syndrome , and resembles reactive hypoglycemia but is distinguished by the inability to demonstrate low blood glucose. Many "hypoglycemia" websites are aimed at people with this condition or describe a conflated mixture of reactive hypoglycemia and this condition.
Varying values of blood glucose are defined as low depending on whether the definition is based on metabolic responses, population norms, or likelihood of clinical consequences. This article expresses glucose in mg/dl as is customary in North America, while mmoles/l are often the units used in Europe. Values in mg/dl can be converted to mM by dividing by 18 (e.g., 90 mg/dl=5 mM).
Research in healthy adults shows that mental efficiency declines measurably as blood glucose falls below 65 mg/dl. Hormonal defense mechanisms ( adrenaline and glucagon) are activated as it drops below 55. Furthermore, surveys of children and young adults show that fasting blood glucoses below 60 or above 100 mg/dl are uncommon in the healthy population. On the other hand, individuals vary and not everyone with a blood sugar below 60 will have symptoms, let alone a disease.
Fasting blood sugars in infancy and early childhood are lower, though still above 60 mg/dl after the newborn period except in illness or other unusual circumstances.
The normal range of newborn blood sugars is more problematic. Surveys and experience have revealed blood sugars often below 40 mg/dl and occasionally below 30 mg in apparently healthy full-term infants on day one. Newborn brains seem to be able to use alternate fuels when glucose levels are low. Experts continue to debate the significance and risk of such levels, though the trend has been to recommend maintenance of glucose levels above 60. In ill, undersized, or premature newborns, low blood sugars are even more common, but there is a consensus that sugars should be maintained at least above 40 mg/dl in such circumstances. Some experts advocate 70 as a therapeutic target, especially in circumstances such as hyperinsulinism where alternate fuels may be less available.
Glucose levels discussed above are venous serum levels. For clinical purposes, plasma and capillary serum levels are similar, and arterial levels slightly higher. On the other hand, whole blood glucose levels (e.g., by fingerprick meters) are about 15% lower than venous serum levels. Furthermore, available fingerstick glucose meters are warranted to be accurate to within 15% of a simultaneous laboratory value. In other words, a meter glucose of 39 could be properly obtained from a person whose serum glucose was 55 mg/dl.
Two other factors affect reported glucose values. 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 from the cells, the measurable glucose will be gradually lowered by in vitro consumption.