Investigations

• the fractional excretion of sodium:  ^c
  • [Urine Na x Plasma creatinine]/[Plasma Na x Urine creatinine] x 100

• the fractional excretion of chloride  ^c
  • [Urine Cl x Plasma creatinine]/[Plasma CL x Urine creatinine] x 100

• no previous diuretics
• previously normal renal function

Why?

A low fractional excretion of sodium or chloride makes pre-renal failure more likely

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
acute renal failure c (pre-test probability: 31%)	pre-renal failure (possible cause and response to pre-renal therapy)	fractional excretion of sodium ≤1%	10 (3.9 to 26)	82%	0.16 (0.055 to 0.45)	7%
		fractional excretion of chloride ≤1%	22 (5.8 to 87)	91%	0.050 (0.0073 to 0.34)	2%

A high urine osmolality makes pre-renal failure more likely

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
acute renal failurec (pre-test probability: 24%)	pre-renal failure (response to pre-renal therapy)	urine osmolality > 500 mOsm/kg	7.9 (1.6 to 38)	71%	0.79 (0.62 to 1.0)	20%