Clinical Chem Lab: Osmolarity

Is a measure of the number of dissolved particles of solution.

Osmolality

What is osmolality dependent on? What doesn't affect it?

- number of particles in solution
- size or charge of the ion

What does one osmol of a substance equal to?

- the gram molecular weight divided by the number of particles or ions into which the substance dissociates in solution

Since glucose molecules don't dissociate into aqueous solutions, osmol of glucose=?

1 mole= 180g

For NaCl, which dissociates into 2 ions of aqueous solutions, 1 osmol= ?

58.5 mole/2= 29g

A solution containing 1 osmol of solute per kilogram of solvent has a concentration of _____________.

1 osmolal

The electrolytes Na+, Cl-, and HCO3-, contribute to over ______ of serum osmolality. Why?

- 92%
- because they are in high concentrations in the ECF & they are monoionic

What are responsible for the remaining 8% of serum osmolality?

glucose and urea

What is the formula that is used to convert serum solute concentration to osmolality?

Osmolality= 2(Na) + (Glucose/18) + (BUN/2.8)

T or F: directly measured values are more significant than osmolality calculation.

true

Urine osmolality varies depending on the state of ______________.

hydration

__________________ corresponds well with urine-specific gravity in non-disease states.

Urine osmolality

Correlation between urine osmolality and urine-specific gravity is poor in _________ disease states. Owing to the greater contribution of high molecular weight substances to ___________ compared to _________________.

- renal
- specific gravity, osmolality

How can osmolality be estimated?

by measuring the freezing point depression

How is osmolality measured using freezing point depression? (what is the process) (4)

1) instrument super-cools the solution in an insulated freezing bath
2) mechanical agitation induces crystallization
3) as crystallization occurs, the heat of fusion increases
4) the sample temperature rises to a plateau where a liquid/solid equilibrium occurs, slightly below the freezing point.

How are the temperature changes measured?

by a thermistor and converted to mosmol/kg H2O

The sample was introduced into the cooling chamber and the Start button pressed. The sample begins to cool rapidly. The digital display reads a cooling sample.

Fast cool

When the temperature of the sample reaches 0 C the display begins counting up and the cooling rate slows.

slow cool

The display reading becomes constant as the sample temperature reaches equilibrium at the freezing point.

Plateau

What is an alternate (not commonly used method) used for measuring osmolality?

vapor pressure or dew point

What are the differences between vapor pressure depression osmometers vs, freezing-point depression osmometers? (2)

- vapor pressure depression osmometers use smaller volumes
- dew point instruments cannot detect the presence of alcohols in solution, freezing point instruments can

Why are osmolality measurements performed?

to evaluate body's water balance

Abnormal shift of water between the intravascular and extravascular compartments will result tending to produce either ______________ or ___________.

dehydration or edema

What are clinical conditions that increase osmolality in serum? (7)

- dehydration, sepsis, fever, sweating, burns
- diabetes mellitus
- diabetes insipidus
- uremia
- hypernatremia
- ethanol, methanol, ethylene glycol
- mannitol therapy

What are clinical conditions that increase osmolality in urine? (7)

- dehydration
- syndrome inappropriate ADH secretion (SIADH)
- adrenal insufficiency
- glycosuria
- Hypernatremia
- high protein diet
- congestive heart failure

What are clinical conditions that decrease osmolality in serum? (3)

- excess hydration
- hyponatremia
- syndrome inappropriate ADH secretion (SIADH)

What are clinical conditions that decrease osmolality in urine? (4)

- diabetes insipidus
- excess fluid intake
- acute renal insufficiency
- glomerulonephritis

What is the specimen type used for osmolality testing? (2)

- serum
- urine

What are some collection requirements for osmolality testing? (4)

- samples should be free of particulate matter
- particulate matter can cause premature crystallization
- centrifuge the sample if turbid
- serum & urine should be collected within an hour of each other

What are the reagents and equipment for the osmometer? (7)

- Advanced micro-Osmometer
- 20 uL-sampler
- Sampler tips
- Chamber cleaners
- Kimwipes
- 290 mOsm/kg reference solution
- Controls: urine/serum

What are the 2 calibration points for the osmometer?

- 50 mOsm/kg
- 850 mOsm/kg

Samples of what reference solution are run daily on the instrument.

Clinitrol 290

What is the purpose of running a reference solution on the osmolarity machine? (4)

- Check instrument operation
- Confirm calibration
- Check the repeatability and accuracy of the instrument
- To avoid erratic results

What is the acceptability of the reference solution?

290 mOsm/kg +/- 5

What other controls can be run on the machine?

2 levels of serum and urine control

If you have more solutes in solution, what happens to the four colligative properties? (4)

- freezing point is decreased
- boiling point is raised
- osmotic pressure is raised
- vapor pressure is decreased

As the sample freezes, this asymptotically warms the sample towards its freezing point. The display counts down very quickly, then slows as the sample temperature approaches equilibrium at the freezing point.

Heat of Fusion

When the sample temperature is determined to be constant on the freezing-point plateau, the sample osmolality is locked in and the display reads, "Osmolality xxx mOsm"

Readout

This is when the sample has been sufficiently supercooled.

freeze