OSMOMETRY

Question 1

Differentiate Molarity vs Molality; Osmolality vs Osmolarity

Answer 1

Molarity: mol/ L
Molality: mol/ kg

Osmolarity: number of particles/ L
Osmolality: number of particles/ kg

Question 2

Describe the four colligative properties

Answer 2

Increased osmolality:
1. Increases boiling point
2. Decreases vapour pressure
3. Increases osmotic pressure
4. Decreases freezing point depression

Question 3

Describe the mathematical relationship between freezing point and osmolarity

Answer 3

• “freezing point depression”
• freezing point of a solution decreases by -1.86°C for each Osm of particles/kg

[(1000 mOsm/kg)/ 1.86°C] = [(x mOsm/kg)/ measured freezing point of sample]

Question 4

Describe the steps of freezing point depression measurements

Answer 4

• 20 μl of sample is placed in cooling chamber via syringe injection
• “supercooling” occurs; temp drops below expected freezing point (-7°C)
• “seeding”; a physical shock initiates formation of ice crystals
• sample begins to solidify and releases heat
• thermistor detects decrease in resistance as temp increases
• true freezing point (mOsm/kg) = where temp plateaus

Question 5

Identify the 4 parts of a freezing point depression osmometer

Answer 5

1. Cooling mechanism
2. Thermistor; measures temp
3. Mechanism for “seeding”
4. Display

Question 6

Discuss calibration of a freezing point osmometer

Answer 6

• calibrated using NaCl of known concentrations
• must be performed at same temp as patient samples

Question 7

Analyze osmolality results and correlate results with clinical conditions

Question 8

Ref: Serum Osm

Answer 8

280 - 300 mmol/kg

Question 9

Ref: Osmol Gap

Answer 9

<10 mmol/kg

Question 10

What is an osmole (Osm) ?

Answer 10

The amount of solute that dissociates to produce 1 mole of particles in a solution

Question 11

Osm of NaCl

Answer 11

2 Osm; NaCl dissociates into individual ions

Question 12

Osm of CH4N2O

Answer 12

1 Osm; urea does not dissociate

Question 13

Define osmotic pressure

Answer 13

The hydrostatic pressure caused by a difference in the amounts of solutes between solutions that are separated by a semipermeable membrane

Question 14

Vapour pressure is measured __ by measuring the __ of a solution; the temperature at which condensation of water from a vapour state occurs.

Answer 14

Vapour pressure is measured INDIRECTLY by measuring the DEW POINT of a solution; the temperature at which condensation of water from a vapour state occurs.

Question 15

Why is freezing point depression the most common method to measure osmolality ?

Answer 15

• detects volatile substances in biological substances; does not denature proteins
• not sensitive to variation in atmospheric temperature

Question 16

Sources of Error: Freezing point osmometer

Answer 16

• variation in pipetting sample volumes
• air bubbles in sample
• preparing sample too early and evaporation occurs
• contamination can initiate seeding = inaccurate results

Question 17

Formula for Osmolal Gap

Answer 17

Osmolal gap (mOsm/kg) = Measured osmolality (mOsm/kg) - Calculated osmolality (mmol/L)

Question 18

Formula for APL Unaccounted Osmolal Gap

Answer 18

Unaccounted osmolal gap (mOsm/kg) = Measured osmolality (mOsm/kg) - [Calculated osmolality (mmol/L) + (1.25 x Ethanol (mmol/L)]

Question 19

The osmolal gap is used to screen for exogenous substances such as __, __, __ and __.

Answer 19

The osmolal gap is used to screen for exogenous substances such as ethanol, methanol, isopropanol and ethylene glycol.

Question 20

Formula for Calculated Osmolality

Answer 20

[2Na+] + [glucose] + [urea] = mmol/L

Question 21

List clinical utility of osmometry

Answer 21

• evaluate ability of kidney to concentrate urine
• monitor kidney disease
• monitor fluid and electrolytes therapy
• aid in diagnosis of hypernatremia and hyponatremia
• evaluate secretion and renal response of ADH

Question 22

Which of the following can be assessed by measuring urine osmolality?

a.
filtering capacity of glomeruli

b.
renal blood flow

c.
renal concentrating ability

d.
renal tubular secretion

Answer 22

c.
renal concentrating ability

Question 23

Which of the following is an advantage of osmometry when compared to specific gravity?

a.
each solute contribution is proportional to its molecular weight

b.
all solutes contribute equally regardless of molecular weight

c.
only the contribution of ionic solutes is measured

d.
only the contribution of non-ionic substances is measured

Answer 23

b.
all solutes contribute equally regardless of molecular weight

Question 24

What component in an osmometer that measures the heat of fusion?

a.
barometer

b.
thermometer

c.
thermistor

d.
manometer

Answer 24

c.
thermistor

Question 25

What is the mOsm equivalent of 2 mol of sodium chloride in 1kg of water? a. 1 b. 2 c. 2000 d. 4000

Answer 25

d. 4000

Question 26

Which of the following warms the sample in a freeze point osmometer? a. heat of vaporization b. heat of fusion c. heat of condensation d. heat of sublimation

Answer 26

b. heat of fusion

Question 27

Which of the following parts is the temperature sensitive probe found in an osmometer? a. thermometer b. thermistor c. thermostat d. thermocoupler

Answer 27

b. thermistor

Question 28

Calculate the change in freezing point (°C) when 1g of sodium chloride is added to 1kg of water? a. -1.86°C b. +1.86°C c. -3.72°C d. -0.93°C

Answer 28

c. -3.72°C NOTE: [(1000 mOsm/kg)/ 1.86°C] = [(x mOsm/kg)/ measured freezing point of sample]

Question 29

Which of the following can falsely decrease osmolality reading when using a freezing point osmometer? a. ethylene glycol b. dust c. serum stored at room temperature d. hemolysis

Answer 29

b. dust NOTE: particulate matter can induce ice seeding prematurely = falsely decreased

Question 30

Which of the following can be assessed by measuring urine osmolality? a. glomerular filtration b. renal tubular secretion c. renal blood flow d. renal tubular function

Answer 30

d. renal tubular function NOTE: glomerular filtration is measured by clearance