Fractional excretion of sodium < 1 is seen in -
The fractional excretion of filtered sodium (FENa) has been demonstrated to be a reliably discriminating test between prerenal azotemia and acute tubular necrosis. One paicular urinary index cannot be expected to reliably discriminate between prerenal azotemia and acute renal failure in all cases Prerenal azotemia Oliguria (urine volume < 500 mL/day) or anuria (< 100 mL/day), high urine specific gravity (>1.015), normal urinary sediment, and low urinary sodium (< 20 mEq/L; fractional excretion of sodium < 1%) are seen Ref Harrison20th edition pg 267
Pre renal azotemia
Acute tubular necrosis
Acute ureteral obstruction
Interstitial nephritis
Kidney