The kidney has a limited number of glucose transporters in its tubules, and this limit, called the transport maximum, decides at what blood glucose level glucose starts leaking into urine.
- 160-180 mg/dl: This is the commonly cited renal threshold, the blood glucose level at which the tubular transporters become saturated and stop reabsorbing all the filtered glucose, allowing the extra amount to pass into the urine.
- 120-140 mg/dl: Blood glucose in this range is still comfortably below the transporters' saturation point, so essentially all filtered glucose is still reabsorbed.
- 130-150 mg/dl: This range is also below the saturation point of the transport system, so it is not high enough to cause glucose to appear in urine.
- 100-120 mg/dl: This is close to typical fasting glucose levels, far below the point of transporter saturation.
Because the tubular transporters only become overwhelmed once blood glucose passes about 160-180 mg/dl, this is the threshold linked to glycosuria.
Therefore, the correct answer is 160-180 mg/dl.