Osmolality
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Osmolality
• 1. A patient has a urine osmolality of 110
mOsm/kg water and a serum of 297
mOsm/kg water.
• Calculate the urine/serum osmolality ratio.
• Is this normal?
• What is a possible clinical diagnosis?
• 1. A patient has a urine osmolality of 110
mOsm/kg water and a serum of 297
mOsm/kg water.
• Calculate the urine/serum osmolality ratio.
110/297 = .370
Is this normal?
NO
• What is a possible clinical diagnosis?
Diabetes Insipidus
• 2. What is the ethanol content in mg/dl of a
serum specimen that has a calculated
osmolality of 280 mOsmo/kg water and a
measured osmolality of 340 mOsmo/kg water
assuming in this situation that the difference
between the measured and calculated results is
due to the ethanol only?
•
M.W. of ethanol is 46
• 2.
What is the ethanol content in mg/dl of a serum specimen
that has a calculated osmolality of 280 mOsmo/kg water and a
measured osmolality of 340 mOsmo/kg water assuming in this
situation that the difference between the measured and
calculated results is due to the ethanol only?
•
M.W. of ethanol is 46
• 340 – 280 = 60 mOsm/Kg ethanol
• 46mg = ? mg
1mOsm
60mOsm
• 2760 mg/L or 276 mg/dL or .276 %
• Assume mOsm is Kg of water or 1 Liter
• 3. How much would a sodium salicylate level
of 50 mg/dl contribute to a patient's serum
osmolality?
•
M.W. of sodium salicylate is 160
Sodium salicylate ionizes into 2 particles
• 160 = 80 mg of Na Salicylate in 1 mOsm
2
1mOsmo
80mgNaSal
50 mg
dL
x 50 mg Na Sal
dL
x 1 mmole
160 mg
x
10 dL
1L
= 6.25 mOsm
L
x 10 dL x 2 mosmo =
1L
mmole
• 5. What is the approximate osmolality of
urine from a healthy patient with a specific
gravity of 1.015?
• To estimate the urine osmolarity of a
patient from the urine specific gravity
multiply the last two digits by 40
• 40 x 15 = 600
• 4. Calculate the osmolality of a serum
specimen with a Na of 138 mEq/L,
Glucose of 90 mg/dl, and a BUN of 20 mg/dl.
• 2(Na) + Glucose + BUN
18
2.8
• 2(138) + 90
+ 20
18
2.8
•
288
• Freezing Point Depression
• When 1 mole of a solute is added to 1 Kg of
H20 the Freezing Point is depressed by 1.86
degrees C. The osmometer measures the heat
released by the freezing fluid and relates it to
the freezing point of the fluid.
• Vapor Pressure
• Measures dew point depression of the vapor.
Vapor in equilibrium with the solution being
measured. The more dissolved particles
present the less the Vapor Pressure of the
aqueous component of the solution.