Tabulate the differences between Acute tubular necrosis and pre-renal failure with respect to the following :

a)  Urea/creatinine ratio

b)  Urine microscopy

c)  Urine osmolality

d)  Urine sodium concentration

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College Answer

ATN

Pre-renal

Urea/plasma 
creatinine ratio

Normal in ATN

May be greater

Urine microscopy

Urinalysis in ATN reveals muddy brown granular and epithelial cell casts and free epithelial cells.

However, the absence of these urinary findings does not exclude ATN.

Normal or near normal in prerenal
disease; hyaline casts may be seen
but these are not an abnormal finding.

Urine sodium 
concentration

High in ATN (>40 meq/L) due in part to the tubular injury.

Low in prerenal disease (<20 meq/L) in an appropriate attempt to conserve
sodium

Urine osmolality

Low, because of loss of concentrating ability. Below 450
mosmol/kg in almost
all cases and usually being below
350 mosmol/kg

High because of preserved
concentrating ability. Osmolality
above 500 mosmol/kg is highly suggestive of prerenal disease

Discussion

The college answer is difficult to improve upon.

One can only attempt to add explanations to the brief responses we have been supplied.

Some additional indices can be excavated from Chapter 61 of Critical Care Nephrology, and these are included in the table below.

A Comparison of Findings in Pre-Renal and Intra-Renal Failure

Intra-renal Failure

Pre-renal failure

Urine osmolality

Less than 400-450 mOsm/kg: concentrating ability is lost

More than 450-500 mOsm/kg: concentrated urine is being passed.

This demonstrated that concentrating capacity is preserved,
which is unlikely in ATN.

Urine sodium 
concentration

High in ATN (>40 meq/L) due in part to the tubular injury.Injured tubules cannot concentrate urine or appropriately reabsorb sodium.

Low in prerenal disease (<20 meq/L) in a (sometimes) appropriate attempt to conserve sodium. Pre-renal failure may also include various low-output or decreased renal blood flow states such as cirrhosis and CCF. 

Urea/ creatinine ratio

Normal in ATN

May be greater. In dehydration, urea is disproportionately elevated (indicating a loss of total body water).

The ratio is calculated from US units, rather than the usual units. In the US, your creatinine is not 500μmol/L, its 0.5mmol/L. Urea remains in mmol/L. Thus, urea/creatinine gives you the ratio. Anything above 100 is considered abnormal (ie. too much urea and not enough creatinine).

Urine/serum creatinine ratio

More than 40

Less than 20

Urine/serum osmolality

More than 1.0

More than 1.5

Fractional excretion of urea

More than 25%

Less than 25%

Fractional excreton of sodium

More than 2% (demonstrating a failure of sodium resorption)

Less than 1% (demonstrating a tendency to conserve sodium, as if in a state of hypovolemia)

Urine microscopy

ATN:

  • muddy brown granular casts
  • epithelial cell casts
  • free epithelial cells
  • Nothing, or hyaline casts (which are non-specific)

 

 

References

UpToDate have an excellent summary of this topic for the paying customer.

 

Bagshaw, Sean M., Christoph Langenberg, and Rinaldo Bellomo. "Urinary biochemistry and microscopy in septic acute renal failure: a systematic review."American journal of kidney diseases 48.5 (2006): 695-705.

 

Sanjay Subramanian, John A. Kellum, and Claudio Ronco "Oliguria" in: Critical Care Nephrology by Ronco, Bellomo and Kellum (2009) pp. 341