Investigations

• CT or ultrasound scan ^a

Look for hydronephrosis (collecting system dilatation): most patients with hydronephrosis have obstruction

Why?

Ultrasound and CT can help rule out hydronephrosis

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
acute renal failure a (pre-test probability: 14%)	hydronephrosis (excretory urography)	CT	11 (5.9 to 19)	64%	0.0 (0.0 to 0.17)	0%
		ultrasound scan	6.2 (4.1 to 9.2)	64%	0.0 (0.0 to 0.13)	0%

Note:

• 9% of CTs and ultrasound scans are non-diagnostic ^a
• Absence of hydronephrosis does not absolutely rule out obstruction. 
• If clinical suspicion is high, consider antegrade/retrograde urography (reference standard).