Fractional Na+ excretion is a clinical description of renal tubular Na+ reabsorption

All physiology textbooks indicate that the renal tubules reabsorb >99% of the filtered Na+ load. Clinical tests, however, focus on the inverse, and indicate that the fractional excretion of Na+ is <1% of the filtered load. Fractional Na+ excretion is used to distinguish between renal failure from tubular Na+ reabsorption defects vs. renal failure from pre‐renal diseases. For example, the damage to the tubular epithelia in acute tubular necrosis causes an impaired Na+ reabsorption across the tubular epithelia. This disease is characterized by a fractional Na+ excretion of > 2%, corresponding to a reduction in Na+ reabsorption from > 99% to < 98%. In contrast, renal failure from hypotension is characterized by intact epithelial transport, and since epithelial Na+ transport is not diminished, the fractional Na+ excretion remains normal at < 1%. Addition of the clinical term "fractional Na+ excretion" to the tubular transport of the medical physiology texts will allow students to more easily connect the diagnostic tests to the underlying mechanisms of diseases.