Osmolality and Osmolarity

Question 1

What is osmolarity?

Answer 1

It is the measure of solute concentration, defined as >>>

the number of osmoles (Osm) of solute per litre (l) of solution (Osmol/l).

Question 2

What is osmolality?

Answer 2

it is also a measure of solute concentration defined as >>>

the number of osmoles (Osm) of solute per kilogram (Osm/Kg).

Question 3

Serum Osmolality (mOsm/kg) or Serum Osmolarity (mOsm/L) >>> formula

Answer 3

2 x [Na + K]mmol/L + (glucose/18) + (BUN/2.8)

(when, glucose and BUN is in mg/dL)

OR if all units are in mmol/L >>> 2x[Na + K] + [Glucose] + [Urea]

• Ref. Magrabi*
• In UptoDate, 2x[Na] is given instead of 2x[Na+K]*

(practically, same is for serum osmolarity)

Question 4

Serum Osmolarity and Osmolaity >>> normal value

Answer 4

For osmolarity: 275 to 295 mOsm/kg

For osmolarity: 275 to 295 mOsm/L or mmol/L

(Practically same is also referred as serum osmolarity with unit ‘mOsm/L’ or ‘mmol/L)

Osmolarity can be affected by temperature and pressure and for a given solution, this calculated variable is less than the osmolality.

Question 5

Osmolar gap formula

Answer 5

Osmolar gap= Calculated Osmolarity (Osmol/L) – Lab Osmolality (Osmol/Kg)

Also, Osmolar gap= Osmolality − osmolarity (Unit: mOsm/Kg)

Question 6

Osmolar gap >>> normal value

Answer 6

10 to 15

(But may be increased in the presence of unmeasured ‘abnormal’ osmotically active ions in the plasma)

Question 7

Major causes of raised osmolal (osmolar) gap

Answer 7

An elevated osmolar gap provides >>> indirect evidence for the presence of “an abnormal solute that may be present in significant amounts”

• Raised anion gap metabolic acidosis (& all of its causes)
• Low osmolarity IV contrast infusion
• Pseudohyponatraemia

Question 8

Osmolality value is independant of - ?

Answer 8

Temperature & Pressure

Question 9

Laboratory instrument to measure osmolality

Answer 9

osmometer

Question 10

What does an osmometer use?

Answer 10

the colligative properties of a solution such as depression of freezing point or vapour pressure

Question 11

How is osmolar gap elevated?

Answer 11

• It is elevated when an ‘abnormal solute’ is present in significant amounts
• this substance needs to have >>> a low molecular weight + uncharged + present in a form and concentration (measured in mmol/l) sufficient to elevate the osmolar gap

Question 12

Criteria of a substance to elevate osmolar gap

Answer 12

• Low molecular weight +
• Uncharged +
• Present in a form and concentration (measured in mmol/l) sufficient to elevate the osmolar gap

Question 13

Name some substances that can elevate osmolar gap

Answer 13

• Ethanol
• Ethylene glycol (anti-freeze)
• Acetone
• Methanol