Investigations

• urine sodium ^c
• urine chloride ^c
• urine creatinine
• urine osmolality ^c

These indices are only valid predictors of pre-renal failure if

• no previous diuretics
• previously normal renal function

Why?

A low urine sodium makes pre-renal failure more likely

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability
acute renal failure c (pre-test probability: 24%)	pre-renal failure (response to pre-renal therapy)	urine sodium < 10 mmol/l	3.5 (1.6 to 7.4)	53%
		urine sodium 10-20 mmol/l	1.4 (0.56 to 3.6)	31%
		urine sodium > 20 mmol/l	0.41 (0.20 to 0.82)	12%

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability
acute renal failure c (pre-test probability: 31%)	pre-renal failure (response to pre-renal therapy)	urine chloride < 10 mmol/l	- (6.9 to infinity)	100%
		urine chloride 10-20 mmol/l	3.6 (1.3 to 9.7)	62%
		urine chloride > 20 mmol/l	0.21 (0.088 to 0.52)	9%