Internal Medicine,  Nephrology

Renal Formulas

Fractional excretion of sodium (FENa)(urine sodium x serum creatinine) / (serum sodium x urine creatinine)<1% suggests pre-renal and other causes. Useful in assessing oliguria or acute kidney injury
Fractional excretion of urea (FEUrea)(urine urea x serum creatinine) / (serum urea x urine creatinine)<35% suggests pre-renal and other causes. Useful in assessing hyponatremia
Urine Anion Gapsodium + potassium – chlorideUseful in assessing a normal anion gap metabolic acidosis. A + UAG suggests renal cause of acidosis
Transtubular potassium gradient (TTKG)(urine potassium x serum osmolality) / (serum potassium x urine osmolality)A high TTKG suggests renal potassium loss. Useful in assessing hypokalemia.

Fractional Excretion of Sodium (FENa)

A FENa <1% in a patient with oliguria suggests that the cause is prerenal. The sodium is being reabsorbed suggesting that the tubules are not damaged, rather that the problem is above the kidney level. If there was damage to the renal tubules, then the sodium in the urine would be increased. Causes for FENa <1% also include dehydration, cirrhosis, heart failure, and prerenal azotemia.

A FENa >1% suggests and intrinsic cause, and a FENa >4% suggests a post-renal cause.


  • The Hospitalist Website ( – How should urine electrolytes be ordered and interpreted in acute kidney injury and electrolyte abnormalities? available here