Medicine Newbie: Osmolarity vs. Osmolality and Osmotic Gap

Monday 19 May 2014

Osmolarity vs. Osmolality and Osmotic Gap

OSMOLARITY

Osmolarity:
- is an estimation of the osmolar concentration of plasma
- is proportional to the number of particles per litre of solution
- is a calculated value (from a formula or an equation)
- is derived from the values determined for Na+, K+, urea and glucose concentrations
- is expressed as mmol/L
- is unreliable in various conditions:
- hyperlipidaemia in nephrotic syndrome (pseudohyponatraemia)
- hyperproteinaemia

Calculation of osmolarity:

The following equations can be used to calculate osmolarity:

Equation 1:

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

The doubling of sodium accounts for chloride, its main associated anion.
The exclusion of potassium is because K+ values are too small compared to sodium.

Equation 2:

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

The doubling of sodium accounts for the negative ions associated with sodium (mainly chloride).
The exclusion of potassium is because the values are too small compared to sodium.

Normal values for osmolarity:

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OSMOLALITY

Osmolality:
- is an estimation of the osmolar concentration of plasma
- is proportional to the number of particles per kilogram of solvent
- is expressed as mOsmol/kg (the SI unit is mmol/kg but mOsmol/kg is still widely used).
- is measured by clinical laboratories using an osmometer. There are 2 types:
- a freezing-point depression osmometer
- a vapour-pressure depression osmometer

Normal values for osmolality:

Normal osmolality of extracellular fluid (ECF) is 280-295 mOsmol/kg.

Uses of osmolality:
- it provides a snapshot of the number of solutes present in the blood (serum), urine, or stool
- it is ordered to help evaluate the body's water balance
- it is ordered to find out its ability to produce and concentrate urine
- it is used to help investigate low sodium levels (hyponatremia)
- it is used to detect the presence of toxins such as methanol and ethylene glycol (see osmotic particles)
- it is used to monitor osmotically active drug therapies such as mannitol, used to treat cerebral edema.
- it is also ordered to help monitor the effectiveness of treatment for any conditions found to be affecting a person's osmolality.

OSMOTIC PARTICLES

Osmotic particles attract or "pull" water:

Sodium, glucose, and urea account for the majority of the osmotically active particles in the blood.
Extraneous osmotically active substances are ethanol, ethylene glycol, mannitol, methanol, or other toxins.

OSMOTIC GAP

Osmotic gap:
- is also called osmolal gap
- is an arbitrary measure of the difference between the actual osmolality (measured by the laboratory) and the calculated osmolarity

Normal values for osmotic gap:

Osmotic gap is normally less than 10-15 mOsmol/kg (refer local laboratory for range).

Increased values for osmotic gap:

Where the osmotic gap is increased, it indicates the presence of other osmotically active solutes which are not taken into account in the calculated osmolality - eg, in methanol or ethylene glycol ingestion. Victims commit suicide by drinking methanol. Ethylene glycol is radiator fluid for cars. Victims accidentally drink ethylene glycol.

Typical causes of an increased osmolar gap:

acetone, decreased serum water, ethanol, ethylene glycol, glycerol, hyperlipidemia, hyperproteinemia, isopropyl alcohol,laboratory error, sorbitol

OSMOMETER

So far we have used the Gonotec Osmomat 3000 osmometer (freezing-point depression). It requires a small amount of sample (50 microlitres). There are tubes for smaller volumes (15 microlitres).