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Urine Chemistries and Osmolarity




    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
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- 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



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Thursday, September 6, 2012 10:14 am