American Board of Internal Medicine (ABIM) Certification Practice Exam

What urine phosphate excretion level indicates renal phosphate wasting?

1. Less than 50 mg/d

2. Greater than 100 mg/d

3. Equal to 80 mg/d

4. Greater than 200 mg/d

The correct answer is: Greater than 100 mg/d

To determine renal phosphate wasting, it is essential to understand the role of the kidneys in maintaining phosphate balance. Normally, the kidneys filter phosphate, and a significant proportion is reabsorbed in the renal tubules. When there is an impairment in phosphate reabsorption, the result is increased phosphate excretion in the urine. A urine phosphate excretion greater than 100 mg/d typically indicates renal phosphate wasting. This level suggests that the kidneys are unable to reabsorb phosphate effectively, leading to higher levels of phosphate in urine. In conditions that cause renal phosphate wasting, such as certain types of renal tubular acidosis or genetic disorders like Fanconi syndrome, the urine phosphate levels can exceed this threshold, reflecting the kidneys' inability to retain phosphate. In contrast, levels less than 50 mg/d or around 80 mg/d would be more indicative of either normal phosphate handling or potentially even renal conservation related to phosphate deficiency. A threshold greater than 200 mg/d is indicative of excessive phosphate wasting, but the commonly accepted level for confirming renal phosphate wasting is above 100 mg/d, giving the clearer and more practical marker for clinical evaluation. Thus, greater than 100 mg/d is the relevant measure for identifying renal phosphate wasting.