Although evidence of volume contraction cannot confirm prerenal ARF, as this can progress into intrinsic renal failure, it suggests that prerenal factors are contributing. In prerenal ARF, specific gravity is usually > 1.020 and sodium concentration is < 10 mmol/L. The fractional excretion of sodium relates sodium clearance to creatinine clearance and is more sensitive than direct measurements of sodium excretion. In prerenal azotemia, sodium is avidly resorbed from glomerular filtrate, but not in intrinsic renal azotemia because of tubular epithelial cell injury. Creatinine is resorbed less efficiently in both conditions. Therefore, the fractional excretion of sodium is <1% in prerenal azotemia (often much less) whereas it is >1% in intrinsic renal azotemia. Fractional excretion of sodium(%)=UNa/PCrxPNa/UCrx100.