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Useful Formulae
1 3
•
FE
Na
<1% in prerenal states, early acute tubular necrosis, contrast
or heme-pigment nephropathy, and acute glomerulonephritis.
•
Not valid when diuretics have been given.
•
See “Acute Kidney Injury” section in Chapter 12, Top 10
Workups.
FRACTIONAL EXCRETION OF UREA
FE
urea
= [(U
[urea]
× P
[Cr]
)/(P
[urea]
× U
[Cr]
)] × 100
(
all units in mg/dL
)
•
FE
urea
<35% in prerenal states.
•
Not affected by diuretics.
•
See “Acute Kidney Injury” section in Chapter 12, Top 10
Workups.
MEAN ARTERIAL PRESSURE
Mean arterial pressure = [SBP + (2 × DBP)]/3
MODEL FOR END-STAGE LIVER DISEASE
MELD = (3.78 × Ln[bilirubin]) + (11.2 × Ln INR) + (9.57 × Ln[SCr]) + 6.43
(
[bilirubin] and [Cr] in mg/dL
)
•
Who remembers what a nature log is? Rhetorical… Only included
to provide the interpretation (Table 4-1).
OSMOLALITY (SERUM, ESTIMATED)
Calculated serum osm = (2 × [Na
+
]) + ([glucose]/18) + ([BUN]/2.8)
(
[Na
+
] in mmol/L; [glucose] and [BUN] in mg/dL
)
•
To correct for methanol + ([MeOH]/3.2).
•
To correct for ethanol + ([EtOH]/4.6).