Diagnostic Indexes in Acute Renal Failure
PreRenal
PostRenal
Renal
Acute GN
------------------------------------------------------------------------
U/P osm
>
1.5
1.0 - 1.5
1.0 - 1.5
1.0 - 1.5
Urine Na, mEq/l
< 20.0
> 40.0
> 40.0
< 30.0
FE: Na
<
1.0
>
4.0
>
2.0
<
1.0
RF index
<
1.0
>
2.0
>
2.0
<
1.0
-------------------------------------------------------------------------
- Fractional Excretion of Na:
- percentage of filtered sodium that escapes reabsorption;
- is a more precise method of differentiating ATN from pre-renal azotemia;
- calculation of the FE Na requires simultaneous measurement of serum
sodium and
Creatinine and urine sodium and
Creatinine
- note the problem with use of FEna
- radiocontrast induced ATN is assoc w/ FEna values < 1 percent;
- FE = Excreted Na/Filtered Na = (Una x Pcr) / (Pna x Ucr)
- under normal circumstances, the FEna is less than 1 per cent of the
filtered load;
- ARF from established tubular injury or obstruction is associated
with FEna values greater than 3 percent;
- Sodium:
- hypoperfused kidney is sodium avid, & low urine sodium concentration
is characteristic of prerenal azotemia;
- in contrast, when actual tubular injury has occurred, there is
diminished renal sodium reabsorption;
- thus, pts w/ established acute tubular necrosis or prolonged urinary
obstruction usually have a higher urine sodium, specifically above
40 mEq per liter;
- Osmolarity:
- urine osmolarity is very close to the serum osmolarity in ATN
and hence patients that have ATN will have U osm of 300;
- patients with pre-renal azotemia will have U osm of > 500;
- urine to plasma osmolarity ratios < 1.10 are consistent with ATN
and ratios greater than 1.25 are more consistent with Prerenal;
- Chloride < 20mEq/L:
- Chloride Sensitive
metabolic alkalosis (Vomiting, Excessive Diuretics),
& Volume Depletion;
- Potassium <10mEq/L:
-
hypokalemia, Potassium depletion, Extrarenal loss;
