Acute renal failure: urinary indices were useful in distinguishing prerenal from intrinsic renal failure.

Clinical bottom line (level 4)

  1. In patients with acute renal failure, fractional excretion of sodium <1% (LR+) , or a urine sodium <20 mmol/l (LR+17) was strongly suggestive of prerenal azotaemia.
  2. A low urine osmolality, high urine sodium or fractional excretion of sodium >1% made pre-renal azotaemia less likely.
Miller et al: Annals of Internal Medicine 1978; 89: 47-50
Expires July 2004

The study

Setting: three university hospitals, USA

64 patients (aged mean ~62 years, ~75% male) adults with acute renal failure defined as an acute elevation of serum creatinine from normal levels ( = 1.4 mg/dl) to >2.0 mg/dl

Excluded if
  • chronic renal failure
  • on diuretics
  • glycosuria or bicarbonaturia (pH7) on urinalysis
  • adrenal insufficiency, hepatic cirrhosis, advanced renal failure (creatinine = 880 µ mol/l)
  • non-oliguric acute tubular necrosis, acute glomerulonephritis or urinary tract obstruction



    • Independent ?blinded reference standard, applied in all patients from a consecutive inappropriate spectrum.
    Reference standard:
    • prerenal azotaemia defined as:
      • oliguria (<500 ml urine/day)
      • development of renal failure in the context of hypovolaemia, congestive heart failure or transient hypotension
      • return of renal function to normal within 72 hours of correction of precipitating factors
      • absence of casts on urinalysis
    Diagnostic test:
    • urine osmolality
    • urine sodium
    • urine urea:plasma urea ratio
    • fractional excretion of sodium ([urine Na x plasma creatinine]/[plasma Ma x urine creatinine] x 100)

    The evidence

    pre-test probability of prerenal failure: 33%, (95% CI: 22% to 44%)

    diagnostic test prerenal failure acute tubular necrosis LR
    (95% CI)    		 post-test probability
    urine osmolality >500 mOsm/kg H2 0 15 5 6.0
    (2.5 to 15)     		75%
    urine osmolality 350-500 7 11 1.3
    (0.57 to 2.9)     		39%
    urine osmolality <350 2 32 0.13
    (0.033 to 0.48)     		6%
    total 24 48


    diagnostic test prerenal failure acute tubular necrosis LR
    (95% CI)    		 post-test probability
    urine sodium <20 mmol/l 18 2 17
    (4.1 to 66)     		90%
    urine sodium 20-40 12 25 0.88
    (0.52 to 1.5)     		32%
    urine sodium >40 0 28 0.0
    (0.0 to 1.9)     		0%
    total 30 55


    diagnostic test prerenal failure acute tubular necrosis LR
    (95% CI)    		 post-test probability
    urea ratio >8 12 6 3.6
    (1.6 to 8.2)     		67%
    urea ratio 3-8 8 23 0.63
    (0.34 to 1.2)     		26%
    urea ratio <3 1 9 0.20
    (0.027 to 1.5)     		10%
    total 21 38


    diagnostic test prerenal failure acute tubular necrosis LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    fractional excretion of sodium =1% 27 1 22
    (3.2 to 150)     		96% 0.10
    (0.036 to 0.31)     		12%
    total 30 24

    Comments

    1. Highly selected group of patients- all comparison patients had acute tubular necrosis. The inclusion of only these patients in this comparison may have improved the properties of the test, and makes the results less certain.

    Citation

    1. Miller TR, Anderson RJ, Linas SL, et al: Urinary diagnostic indices in acute renal failure: a prospective failure. Annals of Internal Medicine 1978; 89: 47-50
    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:

    Clinical Question.
    Patient acute renal failure
    Intervention or Exposure urine chemistry
    Comparison clinical diagnosis
    Outcome prerenal azotaemia