Mon22November1200AM 10

A 24-year-old male is admitted to the Emergency department having collapsed at the finishing line of the London Marathon.

The following biochemistry results are obtained.

Na 144 mmol/l 137 - 144 mmol/L

K 6 mmol/l 3.5 - 4.9 mmol/L

Cl 113 mmol/l 95 - 107 mmol/L

Bic 25 mmol/l 20 - 28 mmol/L

Creatinine 310 umol/l 60 - 110 umol/L

Urea 9.5 2.5 - 7.5 mmol/L

Glucose 3.5 mmol/L 3.0 - 6.0 mmol/L

Which one of the following range of values approximates best to his serum osmolarity (mOsm/l)?

(Please select 1 option)

321-333 Incorrect answer selected

295-299

314-320

300-313 This is the correct answer

288-294

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 main osmotically active cations in plasma are sodium and potassium. Other significant molecules that contribute to osmolarity are glucose and urea. If one assumes electical neutrality the equation doubles the cation activity to account for the anions.

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

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

an alternative equation used in the UK excludes the potassium ion as follows:

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

2 (144) + 9.5 + 3.5 = 301mOsm/l

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.

Answer Statistics

1

5%

2

9%

3

11%

4

67%

5

10%

Times answered: 265