Physiological range of insulin secretion and concentration
The physiological insulin secretion (suppression) and thus insulin levels in serum by means of complex regulatory mechanisms of glucose homoeostasis effectively prevent fasting or spontaneous hypoglycemia, e.g. the lowering of blood glucose concentrations below the critical level of 50 mg/dL, required for "normal" function of the central nervous system, the brain.
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Normal suppression of insulin secretion
Adaequate suppression of insulin secretion during prolonged fasting is achieved when the insulin concentration in serum samples drops below 5 µU/mL ( < 30 pmol/L ) during normal blood glucose levels in the range of 50 mg/dL. In case of adaequate suppression of insulin secretion the concentration of proinsulin should be measured in the range of a few pmol/l close to the detection limit: < 5 pmol/l Adaequate suppression of the C-peptide concentration is achieved with parallel levels < 0.6 ng/ml If the blood glucose concentration during assured fasting (positive acetonuria) definitely is > 50 mg/dL (55-70 mg/dL), the insulin levels may be higher than 5 µU/mL (6-10 µU/mL / 36-60 pmol/L).
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Pathological suppression of insulin secretion
Patients with an insulinoma demonstrate a lack of the physiological and adaequate suppression of insulin concentrations after meals when blood glucose is falling to basal pre-meal levels.
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During the late postprandial phase ( 3 to 15 hours after glucose ingestion) inadaequate suppression of insulin secretion will result in hypoglycemia, which is regularly found in patients with an insulinoma with reproducible low blood glucose concentrations below 40 mg/dL (< 2.2 mmol/L). |
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Hypoglycemia sometimes occurs as late as 24-30 hours after the last meal, rarely in 2-3% of the patients as late as more than 48 hours. Thus, claims to reduce the duration of fasting tests from 72 to 48 hours would result in a loss of diagnostic accuracy. |
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Insulin concentrations > 6 µU/mL ( > 36 pmol/L) have to be considered as pathologically elevated if the simultaneous blood glucose concentration is < 40 mg/dL (< 2.2 mmol/L). This reflected by simultaneously elevated proinsulin concentrations of > 5 - 10 pmol/L, and C-peptide concentrations > 200 pmolL = > 0.6 ng/ml. However, concentrations 10 to 25 times higher within the range of 60 - 150 µU/mL (360 - 900 pmol/L) may be measured.
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