Renal threshold refers to the plasma concentration at which a substance begins to appear in urine. Which statement correctly reflects this concept?

Renal threshold refers to the plasma concentration at which a substance begins to appear in urine. Which statement correctly reflects this concept?

• A. True, for example glucose threshold is about 180 mg/dL
• B. False, thresholds are when urine stops forming
• C. True, but only for water
• D. False, thresholds refer to plasma urine ratio

Answer: (A)

The renal threshold concept hinges on the point at which filtered substances begin to appear in urine because the tubules’ reabsorptive capacity is saturated. For glucose, the proximal tubule can reabsorb it up to a maximum, and once plasma glucose exceeds that capacity, glucose spills into urine. That threshold is commonly cited around 180 mg/dL, so the statement is true and the example is helpful. The idea that thresholds occur when urine stops forming isn’t correct—thresholds describe the plasma concentration at which a substance first appears in urine, not a cessation of urine production. Thresholds aren’t specific to water; water reabsorption is controlled by hormones and osmotic forces rather than a fixed plasma level. And thresholds don’t refer to a plasma–urine ratio.

The renal threshold concept hinges on the point at which filtered substances begin to appear in urine because the tubules’ reabsorptive capacity is saturated. For glucose, the proximal tubule can reabsorb it up to a maximum, and once plasma glucose exceeds that capacity, glucose spills into urine. That threshold is commonly cited around 180 mg/dL, so the statement is true and the example is helpful.

The idea that thresholds occur when urine stops forming isn’t correct—thresholds describe the plasma concentration at which a substance first appears in urine, not a cessation of urine production. Thresholds aren’t specific to water; water reabsorption is controlled by hormones and osmotic forces rather than a fixed plasma level. And thresholds don’t refer to a plasma–urine ratio.