Sat27November0445PM 31

A 64-year-old man is scheduled for orthopaedic surgery. He is normally fit and well. His routine biochemistry results are reviewed and are within the normal range.

Which one of the following pairs contributes most toward his plasma osmolarity?

(Please select 1 option)

Chloride and bicarbonate anions

Sodium and potassium cations Correct

Glucose and urea molecules

Albumin and globulin molecules

Calcium and phosphate ions

Explanation

Osmolarity is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (Osmol/L). The calculated serum osmolality assumes that sodium salts (chloride and bicarbonate), glucose, and urea nitrogen are the primary solutes in the serum.

Calculated osmolarity = 2 (Na + K) + Glucose + Urea (all in mmol/L).

2 (144 + 6) + 9.5 + 3.5 = 313 mOsm/L

Clearly the sodium and potassium ions contribute most towards the plasma osmolarity. Glucose and urea less so.

Normal serum osmolarity is 285-295 mOsm/L. Osmolarity can be affected by temperature and pressure and for a given solution, this calculated variable is less than the osmolality.

Osmolality is also a measure of solute concentration but is defined as the number of osmoles (Osm) of solute per kilogram (Osm/kg). The value is independent of temperature and pressure. It is measured in the laboratory using an osmometer. Osmometers use the colligative properties of a solution such as depression of freezing point or vapour pressure.

The osmolar gap (OG) is typically calculated as:

OG = measured serum osmolaLity - calculated osmolaRity

The difference can result from the presence of excess alcohols, lipids and proteins in blood.

Answer Statistics

1

6%

2

62%

3

24%

4

10%

5

1%

Times answered: 271