What is the renal threshold?

• A. The concentration at which a substance in the blood that is not normally excreted by the kidneys begins to appear in the urine
• B. The concentration at which any substance is always excreted in urine
• C. The amount of waste produced per day
• D. The rate of urine formation per minute

Answer: (A)

Renal threshold is the plasma concentration at which a substance starts to appear in the urine. In the kidneys, substances are filtered but often reabsorbed back into the blood. When a substance is reabsorbed completely at low concentrations, none is lost in urine. As blood levels rise and approach the transport maximum for reabsorption, the tubules become saturated and cannot reabsorb any more. The excess, now filtered and not reabsorbed, is excreted in urine, so you begin to detect the substance in urine at this threshold. This explains why glucose, normally reabsorbed, appears in urine once blood glucose passes about a certain level (around 180 mg/dL in many individuals). The other options don't describe the point at which urine begins to contain that substance, or they refer to unrelated urinary metrics.