OSMOLALITY
1
OSMOLALITY
Definition - When a solute is dissolved in a solvent the colligative properties of the original
fluid change.
As solute is added:
Decrease Freezing Point
Vapor Pressure
Increase
Boiling Point
Osmotic Pressure
Changes in Osmolality are based on NUMBER of particles in solution and do not depend on
weight, electrical charge, or shape of dissolved particles.
One mole of a substance contains 6.023 x 1023 (Avogadro’s #) molecules and when dissolved
in one Kg of H20 decreases the freezing point by 1.86C
One osmole of a substance is equal to the molecular weight of the substance  by the # of
freely moving particles each molecule liberates in solution.
A one osmolar solution contains one osmole of solute in a total of 1000 mL of solution.
Ex.
Glucose
Does not ionize
GFW is 180
A solution containing 180 gm of Glucose per Liter of solution contains 1 mole or 1 osmol and
has an Osmolarity of 1
NaCl
Ionizes into 2 particles GFW is 58
A solution containing 58 gm of NaCl per Liter of solution contains 1 mole or 2 osmol
and has an Osmolarity of 2
Osmol = Molarity x # particles it ionizes into
A milliosmole = 1/1000 of an Osmole
Ratio of Urine osmolarity / serum osmolarity
Range is about 0.2 -4.7
should exceed 1
Normal
should exceed 3
Overnight fast
below 1
Diabetes Insipidous or large H2O overload
Convert Specific Gravity of urine to Osmolarity
Approximately 40 mosmol for one unit Specific Gravity
1.010 = 400
1.020 = 800
1.030 = 1200
Calculated Urine Osmolarities are higher than actual especially alkaline urines
OSMOLALITY
2
Osmometer or Cryoscope
Consist of:
1. Refrigerated Bath Hold Supercool Stir and Freeze
2. Electric Thermister Probe Changes resistance with temp and is sensitive to 0.001 C
3. Wheatstone Bridge and galvanometer Translates temp changes to calibrated units
mOsm/Kg H2O
Principle (Note Our Osmometer can use 0.2 mL but recommends 0.25 mL)
0.2 to 2 mL of specimen is rapidly supercooled (i.e. below Freezing to neg. 7 C)
Sample is vibrated with stirrer producing Heat of Fusion as crystallization occurs.
Temp of Fluid reaches a plateau
Temp is registered by Thermister
Specimen
Serum avoid stasis No anticoagulant
Remove from clotted cells ASAP
Serum stable for 3 hours at RT, 10 hr refrigerated and 3 days frozen
Urine 24 hr specimen well centrifuged
Random specimens not recommended due to wide variations
Normal Values
Serum
280 - 300 mOsm/Kg H2O
Urine
Random
600 mOsm/Kg H2O
12 hr Fl Restriction 900 mOsm/Kg H2O
Urine Osmolality depends on diet and fluid intake
Males
392 - 1090
Females
300 - 1100
Measured Osmolality from osmometer
Calculated Osmolality from formula
Calc Osmol = 2(Na) + Gluc + BUN
18
2.8
Osmolal Gap is the difference between the calculated and measured Osmolality
Osmol Gap = Measured - Calculated
In most people the calculated Osmol will be slightly lower than the measured.
Significant Difference (40 Osmol or More) indicates presence of high concentration of solutes
which do not normally contribute (Poison like Alcohol)
Toxic Metabolic Byproducts - lymphoma, metastatic cancer, liver failure, MI hemoralgic shock
Infection Intoxication from things like Salicylate or Ethanol Lactic Acid
Major Clinical Uses
1. Detection of unmeasured substances in the plasma
2. Assessment of Renal Concentrating ability
OSMOLALITY
3
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?
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
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
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.
5.
What is the approximate osmolality of urine from a healthy patient with a
specific gravity of 1.015?
6.
Explain the principles of operation of an osmometer.
A. Freezing Point Depression
b. Vapor Pressure