Osmolality

Question 1

Define osmolality

Answer 1

A measure of the number of moles of solute present per kilogram of solvent

Question 2

What is the principle behind osmolality?

Answer 2

When solutes are dissolved in a solvent, the freezing-point of the solvent is depressed in proportion to the number of molecules of solute

Measurement of the freezing point temperature thus determines the osmolality

Question 3

Explain how the osmometer works

Answer 3

Freezing point depression
The sample is super cooled several degrees below its freezing point and then agitated to produce rapid crystallisation
During the freezing process the released heat of fusion causes the temperature of the solution to rise to its freezing point

Question 4

What sample types are used on the osmometer?

Answer 4

Serum
Urine

Question 5

What conditions could cause a high reading in urine?

Answer 5

Addison’s disease
Congestive heart failure

Question 6

What conditions could cause a high reading in serum?

Answer 6

Dehydration
Toxic ingestion of chemicals such as ethylene glycol, aspirin, ethanol, methanol, isopropyl alcohol etc

Question 7

What conditions could cause a low reading in serum?

Answer 7

Overhydration
Hyponatremis

Question 8

What conditions could cause a low reading in urine?

Answer 8

Diabetes insipidus
Damage to kidney tubule cells
Overhydration

Question 9

What are the osmo units used in the MMUH?

Answer 9

mmol/kg

Question 10

Give some examples of situations whereby osmolality would be carried out on a sample

Answer 10

High anion gap
Pseudohyponatremia
Presence of osmotic agents e.g. glucose/glycine
Metabolic acidosis of unkown origin where there is a possibility of ethylene glycol intoxication
Ingestion of ethylene glycol, propylene glycol or methanol