Acute renal failure: fractional excretion of sodium was useful in the diagnosis of prerenal azotaemia in selected patients.

Clinical bottom line (level 4)

  1. In patients with acute renal failure, a fractional excretion of sodium <1% (LR+4.4) , a urine sodium <10 mmol/l (LR+3.5) , or urine osmolality >500 mOsm/kg (LR+3.1) made pre-renal failure slightly more likely.
  2. A fractional excretion of sodium > or = 1% made pre-renal failure much less likely (LR+0.0) .
Espinel and Gregory: Clinical Nephrology 1980; 13 (2): 73-77
Expires July 2004

The study

Setting: university hospital, USA

87 patients (aged ?, ?% male) acute renal failure defined as an 'abrupt increase' in creatinine to > OR = 176 µ mol/l

Excluded if
  • chronic renal failure
  • malignant hypertension
  • adrenal insufficiency
  • interstitial nephritis
  • papillary necrosis
  • glycosuria
  • diuretics within 24 hours



    • Independent ?blinded reference standard, applied in all patients from a consecutive appropriate spectrum.
    Reference standard:
    • prerenal azotaemia defined as improvement in urine output and fall in creatinine in response to improved cardiac function, diuretics or treatment for shock (as appropriate)
    Diagnostic test:
    • urine fractional excretion of sodium ([urine Na x plasma creatinine]/[plasma Na x urine creatinine] x 100)
    • urine sodium
    • urine osmolality

    The evidence

    pre-test probability of pre-renal failure: 24%, (95% CI: 15% to 33%)

    diagnostic test pre-renal failure no pre-renal failure LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    fractional excretion of sodium <1% 21 15 4.4
    (2.8 to 6.9)     		58% 0.0
    ( to )     		0%
    total 21 45


    diagnostic test pre-renal failure no pre-renal failure LR
    (95% CI)    		 post-test probability
    urine sodium <10 mmol/l 10 9 3.5
    (1.6 to 7.4)     		53%
    urine sodium 10 to 20 mmol/l 5 11 1.4
    (0.56 to 3.6)     		21%
    urine sodium >20 mmol/l 6 46 0.41
    (0.20 to 0.82)     		12%
    total 21 45


    diagnostic test pre-renal failure no pre-renal failure LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    urine osmolality > or = 400 mOsm/kg 5 5 3.1
    (1.0 to 9.8)     		50% 0.82
    (0.64 to 1.1)     		21%
    total 21 45

    Comments

    1. Patients without prerenal azotaemia had a variety of diagnoses (acute tubular necrosis, glomerulonephritis, obstruction).
    2. This study included patients with acute glomerulonephritis in the population. All these patients had Fe Na <1% and it is there patients that account for the poor specificity of the test. This is not well known outside the world of renal physicians.
    3. Values at the extremes are of most diagnostic value. Unfortunately, most patients have intermediate values. Excluding patients with 'false' causing diagnoses ((e.g. acute glomerulonephritis or obstructive uropathy) prior to interpreting this test may increase its utility.

    Citation

    1. Espinel CH, and Gregory AW: Differential diagnosis of acute renal failure. Clinical Nephrology 1980; 13 (2): 73-77
    Search Terms: kidney failure, acute; sodium/urine or osmolality; sensitive strategy for studies of diagnostic tests (Haynes)
    Contributor: Catherine Clase, Chris Ball and Clare Wotton, April 2000
    Reviewer: Mohammad Saklayen

    Clinical Question.
    Patient acute renal failure
    Intervention or Exposure urine fractional excretion of sodium, urine sodium, urine osmolality
    Comparison improvement in urine output and fall in creatinine
    Outcome diagnosis of prerenal azotaemia