FENa Calculator

About this calculator

This calculator computes the fractional excretion of sodium (FENa), FENa = [(urine Na × plasma creatinine) / (plasma Na × urine creatinine)] × 100, a bedside test that estimates what fraction of filtered sodium the kidneys are excreting rather than reabsorbing. A FENa below 1% typically points to a pre-renal cause of acute kidney injury (the kidneys are working normally but under-perfused, so they aggressively reabsorb sodium), while a FENa above 2% suggests intrinsic renal damage such as acute tubular necrosis, where the damaged tubules can't reabsorb sodium properly.

Nephrologists, emergency physicians, internists, and medical students use FENa at the bedside — usually from a single spot urine and blood sample — to help distinguish the two most common causes of acute kidney injury and decide on fluid resuscitation versus further renal workup. Enter urine sodium, plasma sodium, urine creatinine and plasma creatinine to calculate FENa.