Plasma glucose that is filtered into the urine is reabsorbed by the renal tubular epithelium until a certain level, after which it spills over into the urine. This extra osmotic load can result in diuresis.
The renal threshold for glucose (plasma level at which the reabsorption is saturated and glucose starts to appear in the urine) is around 160 to 180 mg/dL.
The theoretical renal threshold (at a glomerular filtration rate of 125 mL per minute) is higher (around 280 to 300 mg/dL, 16 mmol/L).
If the patient shows glucosuria at a lower plasma level, then there may be a renal tubular defect or an error in the urinalysis.
If a normal patient is receiving a dextrose infusion, then the maximum rate at which it can be infused without glucosuria is 0.5 g per kg body weight per hour. This works out to be the glucose load required to raise a normal plasma glucose level to the renal threshold level. If the patient is hypoglycemic then a higher infusion rate is needed; if the patient is hyperglycemic then a lower infusion rate is needed.
where:
• 0.5 g per kg per hour equals 8.33 mg per kg per minute.
• The normal plasma volume is around 40 mL per kg (0.4 dL per kg).
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