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Three Part Question

In [adults (18+) presenting to the A&E with acute renal failure] is [fractional excretion of urate] better at [differentiating between those patients with renal parenchymal disease and those patients with poor renal perfusion who are in need of further fluid resuscitation] than fractional excretion of sodium.

Clinical Scenario

A patient arrives in your emergency department that has signs of acute renal failure,. You suspect this may be due to their fluid imbalance, causing the kidneys to be inadequately perfused, but you cannot completely rule out parenchymal disease. You have never been completely convinced of the reliability of the fractional excretion of sodium to differentiate between the two, but you wonder if fractional excretion of urate is more effective.

Search Strategy

Two separate searches were performed using the OVID interface, with the following databases: Medline 1950- July 2007, Embase 1980 – July 2007, ACP Journal Club 1991 to May/June 2007, Cochrane Central Register of Controlled Trials 3rd Quarter 2007, Cochrane Database of Systematic Reviews 2nd Quarter 2007, Database of Abstracts of Reviews of Effects 2nd Quarter 2007.
({[exp kidney failure, acute OR acute kidney failure.mp OR kidney failure.mp OR exp kidney failure/ OR acute renal failure.mp OR renal failure.mp] AND [Sodium/ur]} LIMIT to English AND human)
and
([fena.mp OR fractional excretion of sodium.mp] AND [feun.mp OR fractional excretion of nitr$.mp OR fractional excretion of ur$.mp] LIMIT to English AND human)

Search Outcome

298 Papers were found of which 2 were relevant.

Relevant Paper(s)

Author, date and country	Patient group	Study type (level of evidence)	Outcomes	Key results	Study Weaknesses
Carvounis et al. 2002 USA	102 patients in acute renal failure, who were sub classified into three groups: Group A - prerenal azotaemia (50 patients) Group B - prerenal azotaemia treated with diuretics (27 patients) Group C – acute tubular necrosis (25 patients)	Prospective study	Group A - prerenal azotaemia (50 patients)	FENa: 0.4±0.1 FEUn: 28±2	Small patient group
			Group B - prerenal azotaemia treated with diuretics (27 patients)	FENa: 2.1±0.6 FEUn: 24±2
			Group C – acute tubular necrosis (25 patients)	FENa: 8.9±2.2 FEUn: 59±3
Fushimi, K et al. 1990 Japan	65 chronic renal disease patients, 8 of whom had prerenal azotaemia, 7 had acute renal failure, 50 had chronic renal failure. All had uric acid levels measured over 24 hours and then compared.	Clinical trial	Prerenal azotaemia	FEUA decreased compared to chronic renal failure patients; FEUA significantly lower than in acute renal failure (P<0.001)	Small patient group. Translated paper. Tested uric acid and not urea.
			Acute renal failure	FEUA was higher than in chronic renal disease patients

Comment(s)

The fractional excretion of urea can be worked out using the formula [(urine urea nitrogen/ blood urea nitrogen)/(urine creatinine/plasma creatinine)] x100. Using this method, FEUN appears to provide a reliable and sensitive discrimination test for acute renal failure and prerenal azotaemia. The test is reasonably easy and quick to do, and presents no risk to the patient. Christos P et al. state that in a well hydrated patient, FEUN should be between 50 and 65%.

Clinical Bottom Line

Fractional excretion of urate should be considered in all patients where there is the possibility of fluid depletion giving an azotaemic picture.

References

1. Carvounis et al. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure Kidney Int 62(6): 2223-9
2. Fushimi, K et al. Decreased fractional excretion of urate as an indicator of prerenal azotemia Am J Nephrol 10 (6): 489-94