Diagnosis of ^BUN/Cr
                        ====================

þ Classification of ^BUN/Cr:
 - oliguric (<400 cc/24hrs)
 - non-oliguric (usually from nephrotoxins")
 - polyuric (usually from partial obstruction)
 - spurious (nonrenal):
  - ^ Creatinine from rhabdomyolysis
  - false + creatinine assay by older assays from high bilirubin levels,
    cefoxitin

þ FENA:  Fractional Excretion of Na+
 - is the ratio of kidney's sodium clearance to the creatinine clearance
 - >1% suggests ATN
 - <1% suggests "prerenal azotemia" (or obstruction or acute
   glomerulonephritis)
 - equation:

         U   * V                        Where
          Na+
         --------                        U    is urine Na+ concentration
            P           U   * P           Na+
             Na+         Na+   Cr
FeNa+ = ----------  =  -----------       P    is plasma Na+ concentration
         U   * V        P   * U           Na+
          Cr             Na+   Cr
         --------                        (and similarly for Cr, creatinine)
            P
             Cr                          and V is the Volume of the sample

þ Multi-factor analysis for ^BUN/Cr
+--------------+--------+------+-------+------+-------------------------+
|              | BUN/Cr | U    | U/P   | FENA | UA                      |
|              |        |  Na+ |    Cr |      |                         |
+--------------+--------+------+-------+------+-------------------------+
|     Prerenal | 20/1   | <20  | >40   | <1%  | normal or protein casts |
+--------------+--------+------+-------+------+-------------------------+
| Renal (?ATN) | 10/1   | >=40 | <20   | >1%  | "dirty brown casts"     |
+--------------+--------+------+-------+------+-------------------------+