Osmolality and Osmolarity Flashcards Preview

Electrolytes and Acid-base disorders > Osmolality and Osmolarity > Flashcards

Flashcards in Osmolality and Osmolarity Deck (13)
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1
Q

What is osmolarity?

A

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

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

2
Q

What is osmolality?

A

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

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

3
Q

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

A

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)

4
Q

Serum Osmolarity and Osmolaity >>> normal value

A

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.

5
Q

Osmolar gap formula

A

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

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

6
Q

Osmolar gap >>> normal value

A

10 to 15

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

7
Q

Major causes of raised osmolal (osmolar) gap

A

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
8
Q

Osmolality value is independant of - ?

A

Temperature & Pressure

9
Q

Laboratory instrument to measure osmolality

A

osmometer

10
Q

What does an osmometer use?

A

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

11
Q

How is osmolar gap elevated?

A
  • 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
12
Q

Criteria of a substance to elevate osmolar gap

A
  • Low molecular weight +
  • Uncharged +
  • Present in a form and concentration (measured in mmol/l) sufficient to elevate the osmolar gap
13
Q

Name some substances that can elevate osmolar gap

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