No single glucose value alone serves to define the medical condition termed hypoglycemia for all people and purposes. Throughout the 24 hour cycles of eating, digestion, and fasting, blood plasma glucose levels of healthy people past infancy are generally maintained between 72 and 144 mg/dL (4-8 mmol/L) throughout a 24 hour period. Although 60 or 70 mg/dL (3.3 or 3.9 mmol/L) is commonly cited as the lower limit of normal glucose, different values (typically below 40, 50, 60, or 70 mg/dL) have been defined as low for different populations, clinical purposes, or circumstances. In other words, many healthy people can occasionally have glucose levels in the hypoglycemic range without symptoms or disease.
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. While there is no disagreement as to the normal range of blood sugar, debate continues as to what degree of hypoglycemia warrants medical evaluation or treatment, or can cause harm.
Glucose concentrations are expressed as milligrams per deciliter (mg/dL or mg/100 mL) in the United States, Japan,Spain, France, Egypt, and Columbia, while millimoles per liter (mmol/L or mM) are the units used in most of the rest of the world. Glucose concentrations expressed as mg/dL can be converted to mmol/L by dividing by 18.0 g/dmol (the molar mass of glucose). For example, a glucose concentration of 90 mg/dL is 5.0 mmol/L or 5.0 mM.
Method of measurement
Blood glucose levels discussed in this article are venous plasma or serum levels measured by standard, automated 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 are typically in between. 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 under optimal conditions, and home use in the investigation of hypoglycemia is fraught with misleading low numbers. In other words, a meter glucose reading of 39 mg/dL could be properly obtained from a person whose laboratory serum glucose was 53 mg/dL; even wider variations can occur with "real world" home use.
Two other factors significantly affect glucose measurement: hematocrit and delay after blood drawing. The disparity between venous and whole blood concentrations is greater when the hematocrit is high, as in newborn infants, or adults with polycythemia. The delay that occurs when blood is drawn at a satellite site and transported to a central laboratory hours later for routine processing is a common cause of mildly low glucose levels in general chemistry panels.
Children's blood sugar levels are often slightly lower than adults'. Overnight fasting glucose levels are below 70 mg/dL (3.9 mM) in 5% of healthy adults, but up to 5% of children can be below 60 mg/dL (3.3 mM) in the morning fasting state. As the duration of fasting is extended, a higher percentage of infants and children will have mildly low plasma glucose levels, usually without symptoms. The normal range of newborn blood sugars continues to be debated. Obvious impairment may not occur until the glucose falls below 40 mg/dL (2.2 mM), and many healthy people may occasionally have glucose levels below 65 in the morning without apparent effects. Since the brain effects of hypoglycemia, termed neuroglycopenia, determine whether a given low glucose is a "problem" for that person, most doctors use the term ''hypoglycemia'' only when a moderately low glucose level is accompanied by symptoms or brain effects.
Determining the presence of both parts of this definition is not always straightforward, as hypoglycemic symptoms and effects are vague and can be produced by other conditions; people with recurrently low glucose levels can lose their threshold symptoms so that severe neuroglycopenic impairment can occur without much warning, and many measurement methods (especially glucose meters) are imprecise at low levels.
Diabetic hypoglycemia represents a special case with respect to the relationship of measured glucose and hypoglycemic symptoms for several reasons. First, although home glucose meter readings are often misleading, the probability that a low reading, whether accompanied by symptoms or not, represents real hypoglycemia is much higher in a person who takes insulin than in someone who does not. Second, because injected insulin cannot be "turned off", diabetic hypoglycemia has a greater chance of progressing to serious impairment if not treated, compared to most other forms of hypoglycemia. Third, because glucose levels are often above normal for long periods of time (hours, days, or months) in persons with diabetes, hypoglycemic symptoms may sometimes occur at higher thresholds than in people whose blood sugar is usually normal. For all of these reasons, higher meter glucose thresholds are often considered "hypoglycemic" in people with diabetes.
Purpose of definition
For all of the reasons explained in the above paragraphs, deciding whether a blood glucose in the borderline range of 45–75 mg/dL (2.5-4.2 mM) represents clinically problematic hypoglycemia is not always simple. This leads people to use different "cutoff levels" of glucose in different contexts and for different purposes. Because of all of the statistical and measurement variations listed above, the Endocrine Society recommends that a diagnosis of hypoglycemia as problem for an individual person be based on the combination of a low glucose level and evidence of adverse effects.
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